Basics of Flash Photography
Four Fundamentals we must know

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4.  Flash pictures are double exposures

Any flash picture is effectively a double exposure, once from the flash, and once from any ambient continuous light that might be present. These two effects work differently, with different rules. The ambient may be an insignificant level indoors, or it may be overwhelmingly significant when using fill flash in bright sunlight. We might be able to ignore the ambient indoors, but generally must match it in daylight. There are always these two effects to be considered in any flash picture. Note again, they have different rules.

The camera system measures ambient and TTL flash separately and independently. The camera meter, and settings it may choose, indicate only the continuous ambient light it sees (it is never about the flash), and camera automation sets the camera settings appropriately. Then later, the TTL flash system meters the preflash to set flash power level for the aperture setting it discovers in effect at the time. But we don't see this TTL meter - all we see is the picture result on the camera rear LCD. The flash of course affects the picture exposure, but it does not affect the exposure of the separate ambient light.

The TTL flash metering doesn't ever change automatic camera settings, however the flash presence does have a few effects. If the camera is aware that the flash is present, then:

But these are merely limits, which are not about actually metering anything about flash. TTL flash has its own metering system which is used at time of preflash, which adjusts the amount of flash power that is used to match the camera settings it discovers are in effect then. TTL flash is automatic flash exposure, regardless of any camera settings (including camera M mode, where we can set whatever we want).

Shutter speed can keep out continuous light, or allow it in

OK, so knowing that instantaneous flash is not affected by shutter speed (see Part 2), and knowing any continuous ambient light is so affected, we can use that fact. For example, indoors, using incandescent modeling lights with studio lights, we can use maximum shutter sync speed (without affecting the flash exposure at all) to reduce that ambient continuous so that the unwanted light does not affect our picture. One purpose could be to prevent orange incandescent light in our flash picture. Or to prevent normal room lights from affecting our carefully lighted studio setup. Studio flash surely always uses camera M mode and maximum shutter sync speed, for this reason. But in a normal room snapshot setting at low ISO, we might intentionally use a slow shutter speed to show and maximize the contribution of that continuous room light, for a warmer "ambient" look. Caution though, at high ISO, without incandescent white balance and a CTO filter on the flash, you surely get excessive orange.

Normally it is our choice, to control the continuous ambient light with shutter speed, to block it out or allow it in, independent of the flash exposure. We only need to know that any flash picture is a double exposure of the two situations. With flash, we always have two decisions to make - proper aperture so flash power level is possible, and indoors, and shutter speed about if we want the ambient included or not (outdoors in daylight, we are probably going to have to account for the sunlight).

The flash exposure will not care about whatever shutter speed we use, but we have choices, and our choice ought to be intentional. When we use the camera's A or P exposure mode indoors, when we turn the flash on, we see the automatic shutter speed increase from "slow" (perhaps 1/4 second metered in the dim room?) back up to 1/60 second. Not for any reason other than this is the lower limit set by the camera when using flash - we do not need a slower shutter speed if we are using flash. Many DSLR models have a menu for this called Flash Shutter Speed (often Nikon menu E2, D90 E1, default 1/60 second). There are ways around this limit - camera Manual mode allows any permissible shutter speed, or Slow Sync or Rear Curtain Sync option ignores this limit and uses whatever slow shutter speed the camera actually metered for ambient. This minimum lower shutter speed limit with flash (often a 1/60 second default for minimum shutter speed with flash) is simply because we might be able to hand hold 1/60 second in case there is some degree of ambient light. But you may prefer to keep out the continuous ambient, because incandescent is orangeish in color. Or if you are using the very fast flash for the purpose to stop motion, a 1/60 second shutter may allow dim ambient to blur what the fast flash stopped. If you noticed the metered light reading BEFORE you turned on the flash, it was surely slower than 1/60 (if dim indoors - but in sunlight, you get the actual that it meters). So indoors (with insignificant ambient), it seems much better to always use camera Manual M mode with flash, which allows us to set the shutter speed as we wish, either slow to allow the continuous ambient in, or fast at maximum sync speed to keep it out. It is our choice, but the automation will choose 1/60 second, possibly not for reasons useful to us. The Speedlight flash power used in TTL mode will depend only on the aperture and ISO values, and the TTL flash exposure is still fully automatic even in camera M mode.

To say the obvious: There is no camera intelligence that can treat one area of the picture different than any other area. It may choose one area to meter with preference, but the camera settings can only use ONE aperture setting and ONE shutter speed setting. The flash can only use ONE power level. These single settings affect everything in the picture (there is no magic). ISO, aperture and shutter speed affect the continuous ambient light exposure. ISO and aperture and flash power level affects the flash exposure. TTL automation tries to determine some of those settings, but the one big thing important to realize is that our manual Flash Compensation also affects the flash (power level and exposure), as we see fit. If TTL automation does not give the result you want, then you can moan, or you can simply fix it. :) Flash compensation is a main trick for getting great flash exposures. Flash compensation should be considered (if not used) for every TTL flash picture, and certainly in any new situation.

Maybe this is overly simplified, but regardless how we might reach the final choices, remember the exposure goal is in fact simple: No matter how mixed and complex the scene, our photo can only set one aperture, one shutter speed, one ISO value, and one flash power level (in each flash unit anyway). Maybe we specify some ISO and aperture, like f/5. If we want to expose the ambient properly, we set shutter speed to do it. If we want to expose the flash properly, we set the flash power to do it. There are choices of course, but these two exposures (ambient and flash) is all there is to do. Sometimes these values are a compromise, sometimes a tool, or maybe we ignore a very underexposed ambient, but each parameter has just one value. Look at your results, and when the one value seems wrong for your purpose, simply just fix it. That is your job. Just do what you see you need to do. Flash Compensation is the tool we use to adjust what the automatic TTL flash is doing. I am suggesting we remember to approach it as a simple problem, instead of as incomprehensible magic. Learn a few basics, and it becomes very understandable (honest, it really does).

Flash Compensation only affects relative TTL flash power levels. Exposure Compensation (on Nikons) affects Both ambient exposure and TTL flash exposure. And these methods all add for a final TTL flash value.

Understanding these few fundamentals will greatly simply and explain about everything you see happening with flash.

First, a quick section on "Modes".   TTL BL vs TTL modes towards page end below.

Flash modes on the Flash

Organization is a big issue here, sorry. Some material (like this flash mode section) are basics, you might skim past pretty fast. But IMO, other sections below contain substantial meat, so I hope skimming catches it all.

There are many options, and various modes to use flash, several modes on the flash itself, and also modes on the camera. Some top flash models have every mode, but some low-end may only have one mode, Manual or TTL, or maybe only a couple of modes.

Menus: The camera flash menu that selects TTL, Manual, or maybe Commander, only applies to the internal flash when its door is open. This menu has no effect when the popup door is shut. External flashes have their own menu (SB-300 and SB-400 are exceptions, using the camera internal menu).

A quick summary of these modes on the flash menu itself are these:   (Nikon nomenclature, SB-800 and SB-910 have all of this).

Camera Exposure modes for flash

Note the camera light meter scale that we see is only about ambient light... the TTL system has its own invisible light meter system that it uses for the flash. TTL flash is automatic, point&shoot flash, metering and responding automatically to whatever aperture and ISO that either you or the ambient metering has set (TTL simply responds to it). However we can control TTL flash using Flash Compensation, to tweak it in.

These camera modes affect how the ambient light is metered. Then the flash modes above affect how the flash power level is adjusted. Camera A mode is very popular for flash, but I would suggest camera P mode for fill flash in bright sunshine, and camera M mode indoors.

D7100 manual page 123, Internal flash in P mode
D7100 manual page 307, External flash in P mode

We specify camera mode appropriately for the property important to our current picture, to ensure it is what it needs to be. Maybe we need wide aperture for more effective flash power capability or a blurred background, or stopped down for more depth of field. Or a fast shutter speed to stop motion in ambient light, or a slow shutter to blur a waterfall. Whatever we think it needs, we have choices.

A Summary - Things We Need to Know about Nikon TTL BL vs. TTL flash modes

The Nikon iTTL flash system (controlled by the camera metering) includes TTL and TTL BL metering modes. Some iTTL flashes (SB-600, SB-800, SB-900/SB-910) have a menu to select which TTL or TTL BL mode for the camera metering to use. The other iTTL flash models (SB-700, SB-400, camera internal flash, and Commanders) have a menu that says TTL, but these are TTL BL by default. Most non-Nikon-brand iTTL flash models for Nikon cameras also default to TTL BL mode, because Nikon is a TTL BL system. The camera controls the metering, and the flash just flashes at the programmed level.

The camera light meter that we see only meters the ambient (the flash meter is invisible to us). The camera meters the ambient, and sets aperture and shutter speed and Auto ISO for the ambient level. The flash presence can impose minimum and maximum shutter speed limits (shutter speed does not affect flash exposure). These settings are not about the flash (we can manually choose f/stop and ISO to favor the flash however). In many camera models, Auto ISO is certain to use very high ISO in dim places, and the flash has to work into whatever ISO and settings it discovers to be in place (exceptions are the older DSLR, and a few most recent new cameras). So turning Auto ISO OFF is an extremely fine idea when using flash.

TTL BL is automatic balanced flash (if in bright ambient). Balanced with ambient just means the metered flash power level is reduced, to serve as fill level, but specifically reduced because the sum of a proper ambient exposure and a proper flash exposure adds to be two proper exposures (of the near flash subject), which is 2x proper exposure, which is one stop overexposed, by definition. So, TTL BL mode backs off on the flash power, which becomes a reasonable fill level. Typically TTL BL flash in bright sun is reduced nearly two stops less than the ambient... automatically, pretty much perfect for fill flash in bright sun. That is what TTL BL is.

Using camera P mode for the bright ambient, and using TTL BL flash mode (default on SB-700, SB-400, and the camera internal flash), normally gives very good point&shoot fill flash results on human portraits in bright sun. Camera A or M modes will do the same if you set the same aperture that P mode selects, which is just saying that you must know to set up near f/11 (bright sun, Sunny 16, ISO 100) to keep shutter speed within maximum sync speed - otherwise you just see a flashing Error - wider aperture cannot give correct exposure since the shutter speed cannot go faster than sync speed. But P mode can adjust aperture, so it knows how to do this.

TTL mode can be selected on any of the flash models by setting camera Spot metering mode. Camera Spot metering changes the flash metering system from TTL BL to instead be TTL mode. Don't misunderstand, the flash never does Spot metering, the flash metering always reads a center area - but the ambient can do Spot. If Spot metering is set, the camera just changes from TTL BL to be TTL mode metering. It is the camera that meters and controls this. The Exif reports which flash mode was used. This metering mode change is necessary, because Spot metering is only about the ambient spot, with no concept of the background or total frame exposure, so meaningful balancing of flash is not possible. Which is what TTL mode is, just regular flash. no balancing.

In contrast to balanced TTL BL, TTL mode comes ahead on, using full metered power, regardless if any significant ambient is present or not. Not reduced or balanced. For fill flash in bright sun, then you have to know to set about -2 EV flash compensation yourself, otherwise TTL will give an overexposed subject (if the two full exposures add). Setting manual TTL flash compensation for fill works great (and offers control), you just have to know it is required. Whereas, TTL BL does that automatically, point&shoot fill flash, called Balanced. And TTL BL is often the default mode, unless you set Spot metering mode.

TTL mode is very good for indoor flash, when we want full metered flash level. And since the ambient is normally low level indoors (dim, where we need flash), usually low enough to ignore , then we can set Spot metering to get TTL mode with flashes that otherwise default to TTL BL. A little tricky to explain, but Spot metering implies different things with flash indoors. It sets TTL mode of course, but also, if the ambient indoors is too low to be significant (unless we have Auto ISO ON), then Spot metering means nothing for an ambient level too low to care about. Camera A or P mode will set a Minimum shutter speed for flash anyway (often 1/60 second, overriding slower Spot choices in dim light). Or camera mode M works great indoors with flash, and Spot metering can not change settings if camera M mode (the TTL flash is still automatic flash in camera M mode). So there may be no Spot metering with flash indoors, but Spot will change the flash metering to be TTL mode - giving full flash level as metered, no balanced adjustments. Again, flash does not do Spot metering - it always meters its own center area regardless (not exactly same as Center Weighted metering, but reasonably similar).

In other situations in bright sun outdoors, some users instead reduce the bright ambient exposure about two stops, and then do full TTL flash level. This makes the subject stand out against the darker background, and it helps stop subject motion in fast sports. You would not call it fill flash however.

Cameras use reflective metering (metering is affected by subject colors, so sometimes not precise). Therefore to achieve better results, we may need to use Flash Compensation to tweak what the automation does with flash power level, in either TTL or TTL BL mode. Simply watch the flash exposure result in either mode, and then do what you see you need to do for a perfect flash result. This is key for great flash pictures. Flash is never fully point&shoot, we have to watch and help a little. It is quite easy.
Hint: Scene situations vary, but indoors in normal dim ambient, you may like routinely starting TTL BL mode near +⅔ EV or +1 EV Flash Compensation in the general case. Or you can instead use Spot metering mode to use TTL mode indoors. You will quickly learn to NOT leave it in Spot metering mode when you go back outdoors into bright ambient. :)

Again, Flash Compensation only affects relative TTL flash power levels. Exposure Compensation (on Nikons) affects Both ambient exposure and TTL flash exposure. And these methods all add for a final TTL flash value.

Is it required to turn Auto ISO OFF for flash?  

Some optimum choices for flash

Regardless of your choice, remember it is always important to monitor your TTL results, and use Flash Compensation as you see necessary to get exactly the result you want.

Fill Flash in Sunlight

Often there are choices, if we think about it. Try to find some shade if you can. Or make some shade if you can. Under a tree is good, but out near the edge of the shade line is better than back in the deepest shade. Cloudy days are good. If in the sun, avoid the hours around noon. Sunrise or sunset is better (but not always possible). If no flash, the norm is to face subject into the sun. If using flash, face them away from the sun. But it's better if the Sun is a little to one side, not fully front or back. Fill flash will be quite important. Even the popup flash if that's all you have. The flash will require some compensation. There are a couple of major considerations:

All three pictures are camera mode A, ISO 100, f/8, 1/125 second at six feet, late afternoon.

No flash   (side lighting)

Unacceptable result. Portraits in bright sun simply have harsh dark shadows, in various degree. Use your flash for fill, even the popup.   At right was a direct hot shoe speedlight.

TTL mode Fill, -1.33 EV Flash Compensation

Better, but not best. Maybe -1.3 EV is slightly too much flash (nearly 30% flash, noticeably present), which makes the subject "stand out" a bit too much. Some subtle (desirable) gradient shadow tonal shading is still left, but certainly any more flash would be too much.

TTL mode Fill, -2.0 EV Flash Compensation

Opinions always vary, but mine is that the lesser fill helps greatly, because it is not obviously recognized present as such. Still a huge improvement, but more invisible, and a more natural look. We have choices.

We have choices about fill amount, but -1.67 EV fill (in between these two) is a popular TTL value in bright sun. Or, we can use TTL BL mode, which does automatic fill compensation, and is often a good choice with camera P mode in sun, so we may not need to handle it ourself.

You should practice this a time or two. Fill flash in bright sun is an important big deal.

The Nikon system has two flash modes, TTL and TTL BL. Flash models like SB-600, SB-800, SB-900 (all discontinued now) and the SB-910 have a menu to select either choice. Flash models like SB-400, SB-700, the internal flash, Commander flash, and almost all third party flashes, only offer a generic TTL menu, which in fact is TTL BL by default (Nikon is a balanced flash system). Very few third party flashes offer that menu, and TTL BL is default. Purists may prefer TTL for the more absolute control, but TTL is disappearing, the default is TTL BL. Selecting Spot Metering mode will switch TTL BL mode to be TTL Mode (which does not do spot metering for flash, but it is Spot for ambient, which becomes quite different outdoors). Users never seeing the two menus may not grasp that there are two distinct modes, but I am speaking of the two modes here.

TTL vs TTL BL flash mode

Again, note if you are using fill flash in bright sun, the amount of Flash Compensation needed will depend on which flash mode you use. We must understand that TTL BL default tries to do automatic fill flash compensation, and that TTL mode does Not.

We might choose to underexpose the ambient (to make our flash subject stand out more), but usually we must meter the continuous ambient daylight (it being extremely significant and cannot be ignored). Maximum shutter sync speed for flash makes this be a little harder. For example, using fill flash outdoors in the sunshine is pretty important, but sometimes fairly difficult. The daylight exposure is Sunny 16, so for ISO 200, we expect 1/200 second at f/16 in direct sunshine. The fill flash must match that (more or less, we likely want the fill to be a stop or so less - a fill is less level than the main light). But f/16 at say 10 feet requires Guide Number of (16x10) = 160 at ISO 200 (divided by 1.414 to convert to ISO 100 is equivalent GN 113 in the ISO 100 chart). This is possibly close to our flashes capability, at least at wide zooms at more distance. And 1/200 second is very near our shutter's maximum sync speed - so with flash, we cannot go faster - we are unable to convert to Sunny 16 equivalent of say 1/800 second at f/8 (to gain flash power or wider aperture). We are limited to the 1/200 second maximum sync speed, and Sunny 16 says f/16 at 1/200 in bright sun (if ISO 200).

This is a narrow window, with few choices. Camera P mode and TTL BL flash mode understand all of this, so that combination is point&shoot settings for fill flash in sunlight. For this reason, wedding photographers joke to call P mode: Professional Mode. :) But P mode flash can adjust automatically when moving outdoors in sun, and then back indoors. But in contrast, indoors in dim light where we simply need flash, P mode simplicity gives up some options (maximum aperture - also Auto FP is not going to be allowed.) Indoors, there is nothing better than camera A or M mode, and flash TTL mode. However, camera A mode easily allows us to set f/4 (for bounce indoors), and then if we walk out into bright sun without thinking, the camera will simply fuss and refuse with error warning HI, simply because the shutter sync speed will not go to 1/3200 second with flash.

Shady conditions or overcast days are not so extreme (unless of course, you also try to include areas of bright sun), but Sunny 16 says flash with bright sun is going to need more like f/16 at 1/ISO shutter speed, which cannot exceed maximum sync speed. Which means, at ISO 100, equivalent exposures of f/11 1/200 second will work, but f/5.6 at 1/800 second cannot work with flash.

Fill flash in bright sun is a difficult problem, so we may need to seek some shade for our picture. Open shade or overcast is soft light anyway. A neutral density filter, or a slower ISO setting, can help with a wider aperture regarding depth of field. Both of these affect flash and sun equally, but we still cannot exceed maximum shutter sync speed. However, if extra flash power is available, we of course turn the flash power back up. We cannot turn the sun back up, so turning up the flash creates a ratio change of flash to sun (with ISO or ND filter). The easier solution for flash power is either a closer flash distance, or a more powerful flash. A large studio light used outdoors can overpower the sun at reasonable distances, but cannot overcome the maximum shutter sync speed issue.

So then cameras that have it might try Auto FP High Speed Sync mode (see Part 2B, HSS Auto FP) for a faster shutter speed to do the 1/800 at f/8, or even 1/6400 at f/2.8. This does work, but the FP flash power drops drastically, and range falls to less than half. The FP flash itself is modified to generate a continuous series of smaller flash pulses to mimic continuous light, so the focal plane shutter can work. Auto FP mode (on many camera models with focal plane shutters) will allow any shutter speed faster than maximum shutter sync speed with flash, but at much reduced power level from the flash, which greatly limits the distance range of the flash. The rear LCD on the SB-700, SB-800, SB-900 and SB-910 flashes will show that maximum distance range currently in effect.

Compensated TTL Fill Flash Percentage Calculator

As opposed to TTL flash compensation, some Outdoor photographers use manual flash with a handheld Sekonic meter model that shows the percentage of manual flash in the total exposure, including ambient. When they discuss using 20% or 30% fill flash in daylight, their values compare to TTL fill at -2 EV or -1.33 EV TTL compensation. Note that ambient is normally significantly underexposed when using indoor flash (why we're using flash), but the opposite is true of normal outdoor fill flash.

Here's a calculator to show result of various combinations of TTL flash compensation with Outdoor ambient levels, and then show the percentage of TTL flash each includes, and the overexposure over ambient that the total sum with flash adds. It's definitions are

TTL Flash Percentage Calculator

  The Ambient Exposure  

Please report ( Here ) any problems with the calculator, or with any aspect of this or any page. It will be appreciated, thank you.

Note that if Nikon's default TTL BL flash mode (balanced flash), the system is already doing its own invisible automatic fill flash compensation (automation, typically already near -2 EV flash if in bright sun). This calculator is NOT for default TTL BL mode, but is instead for the actual TTL mode (non-balanced mode), where the user does the compensation (more below).

In TTL mode (as opposed to TTL BL), compensation is measured from the metered flash level considered proper and fully exposed. This is independent of ambient, and is NOT measured from the existing ambient level, and this is the case applicable to the table above. However, default TTL BL mode is different, flash in a dark room of course stays strong, but automation reduces flash near two stops in bright sun (fill flash level).

Flash Ratio above is as compared to the specified ambient level. You can see that 20% flash level is a lighting ratio of -2 EV flash, and 28.4% is -1.33 EV flash (flash difference from the ambient level in the Ratio column). The EV ratio might be easier and more meaningful than the percentage (it's the same concerns as we would set up in a portrait studio). And you have third stops too. The chart numbers can be useful, but it seems to me that the importance is the concept it shows. We can always use that, without the chart. The camera meters the ambient, and 0 EV ambient implies a proper 100% exposure of ambient. The TTL system meters the flash, and 0 EV TTL flash implies the proper 100% exposure of flash. The difference in them is 0 EV (ratio), and flash is 50% of it.

However, when at 0 EV Exposure Compensation of ambient and 0 EV Flash Compensation, the sum adding two proper exposures of 100% + 100% is 200%, or one stop overexposed (in the area that both flash and ambient fully illuminate). The sum of two sources is always greater than the brightest, in some degree. So that's one reason why in bright ambient, we learn to compensate the flash (to a lower flash level, or TTL BL mode does this automatically). And then, even ambient at - 1/3 EV can make a difference in that sum too.

The other reason is because desirable lighting is a ratio, NOT flat uninteresting light. So if we know ambient is at -1/3 EV, and the TTL is at -2 EV, then we already know (in our head) that the lighting ratio is the -1 2/3 EV difference. That's what we want to know. We could look up that this is 24% flash, but the ratio is what we need to know and judge (and EV is what our meters measure). This ratio of two lights (Ratio here is the flash level relative to ambient level) is called Lighting Ratio, and is a strong factor in our pictures. So the idea is about how we can use compensation for a useful result. However indoors (barring high ISO with flash), the ambient level is normally too low to be an issue.

TTL users can compare their fill flash compensation to percent if they choose, but ratio is about EV. Instead of remembering the numbers 20% or 30% fill, all we have to remember is -2 EV or -1.33 EV ratio of compensation. Same thing as 20% or 30%, except 10% steps are a bit coarse, closer to full stops, but we can set TTL to third stops.

The hand held meter is the only option for manual flash, but TTL also uses its own meter. So just one opinion, but TTL seems an advantage, and the stops of compensation seem natural, since stops is what we use to control it. The two methods (EV or percent) are not precisely comparable because the camera and flash work in third stops, and the meter percentage shows only 10% steps (bigger steps, closer to a full stop).

Other differences are:

Handheld flash meters cannot be used with TTL flash, the camera uses its own meter. The two systems are different, and they each do what they can do. The handheld meter is an incident meter, which is always an advantage. It meters the fixed power flash at the subject, including the ambient effect, and it tells us camera settings to use. The camera meter is a reflected meter, but it can simply aim the camera at the subject. Shutter speed only affects ambient, but handheld flash meters can take shutter speed into account, i.e., changing shutter speed, or moving into shade, changes ambient exposure with respect to a fixed manual flash, so flash EV difference and percentage also change. They have to start all over when ambient changes.

But in the TTL compensation method, we simply dial in the desired settings at the camera, without having to meter at the subject. It is reflected metering however, not incident. The ambient is metered, which sets the camera settings for the ambient. Then the TTL also meters the near subject, and sees what the handheld meter sees, except via a reflected meter. It sets flash power level for the metered reading and the existing camera settings, including flash compensation. But if in shade, TTL also sees dim, so flash compensation EV remains a constant difference from however ambient metered it. The flash and ambient are both metered, so in that sense, the flash can be said to follow any ambient changes. TTL percentage stays constant, independent of shutter speed or shade. If we specify -1.67 EV flash, we get -1.67 EV flash. This seems a plus. :)

However there are of course exceptions when you may want to specify a different flash compensation. For example, a bright window in the background, or if the subject is in deep shade under a big tree, but the distant background in full sun is very bright, the ambient automation will underexpose the subject. Help from the photographer is required in such situations. The TTL flash will meter the near subject, but will still be this -1.67 EV less than TTL actually meters. The ambient still adds a little to that, but -1.67 EV flash compensation would not be the right value for that situation. Neither method handles this situation without the photographer doing something to earn his money.

Lighting ratio

These percentage numbers above work the same way for any two lights. We know the standard lighting ratios for portrait lights are:

Main light
Relative to Fill
EV, Main Relative to FillEV, Fill Relative to Main
Total, Percent, Over FillTotal, Percent, Over Main
0 stops1:12:1  0 EV200%50%1 EV  0 EV200%50%1 EV
1 stop2:13:1+1 EV300%67%1.58 EV-1 EV150%33%0.58 EV
1.5 stops3:14:1+1.5 EV380%74%1.93 EV-1.5 EV135%26%0.43 EV
2 stops4:15:1+2 EV500%80%2.32 EV-2 EV125%20%0.32 EV
3 stops8:19:1+3 EV900%89%3.17 EV-3 EV113%11%0.17 EV

The calculator above shows the right side of this table. Numerically, the 1.5 stop rule of thumb really should be 1.6 stops to be 400%.

Contrast: Ratio is contrast (darker shadows, greater shading. The shading better defines subject shape and curves). This is desirable in some degree, but it can be too much. IMO, 1.5 EV is plenty of ratio for color portrait work (and less for softer photos). Do realize that ratio is a tool, a control that your lights can easily change. Colors can generally provide the contrast, but B&W grayscale needs and can use more ratio and contrast. Ansel Adams said (of his grayscale work) that pictures always need some area of true black, and also some area of true white (this contrast range really aids grayscale). But too much tonal contrast can be less pleasing for color work.

Many lighting writers today say Lighting Ratio, but use and mean Power Ratio (because that's how we set the lights). We really have to read closely to see which they mean when they say ratio.

Power Ratio = Main : Fill is convenient and popular, it's how we setup the lights. I call it Power, because we set that - it is what we can measure. It's the ratio of lights themselves. We double the flash power to increase the light by one stop. If the main light is set to meter f/8 and the fill light is metered to be f/5.6, that is power ratio of 2:1, or one stop. It is considered to be a "lighting ratio" of 3:1 (by definition, this 2:1 power computes 3:1 lighting, because 2/2 main + 1/2 fill is 3/2 on the highlights, and only 1/2 on shadows, which is called 3:1 lighting).

Lighting Ratio = (Main + Fill) : Fill is the actual lighting we get then. The highlight areas see both main and fill light, but the shadows see only the fill light. We may see either ratio called lighting ratio, but which are different numbers, so pay attention to what the writers say they do (their procedure), not to what they call it. Some will call it 3:1, and some call it 2:1. You can of course see this ratio result in their pictures they show, and will quickly learn to see the difference (beginners are advised to stop and think, to look and notice - ratio is a very major point).

More about Balanced Flash mode

None of that percent stuff applies to TTL BL fill flash mode. Planning goes much better when we realize what is happening. Nikon's TTL BL flash mode is mainly for balancing the fill flash to the daylight. TTL BL mode tries to automatically match reduced flash to a bright ambient (outdoors), whereas the TTL mode ignores ambient and makes flash match aperture. But note that the best fill flash fills subtly, adequately, but without even looking like flash was used, so for this fill flash in bright sun, if in TTL mode, we normally want to specify about -1.67 EV (stops) flash compensation, so the flash does not overwhelm. Specifically note that TTL BL mode helps out by doing this fill reduction itself, automatically. This is a big difference you should know and expect.

Different opinions guess about how TTL BL works, what it actually does. No one but Nikon actually knows, and they don't say much about details. Nikon used to say more words in the manuals of cameras and flashes over the last twenty years, which does not conflict with what they say today, but all they say today is this:

Nikons manuals (SB-600 page 33, SB-700 page C-2, SB-800 page 37 , SB-900 page D2) say the modes do this:

TTL BL - Automatic Balanced Fill-Flash:
The flash output level is automatically adjusted for a well-balanced exposure of the main subject and background.

TTL - Standard TTL flash:
The main subject is correctly exposed regardless of the background brightness. This is useful when you want to highlight the main subject.

This actually reads pretty coherently if you interpret background as ambient, and subject as flash (what else could they mean? These are the things that are metered), and if you understand that any two lights add to be brighter than the brightest. Some of the camera manuals (section about internal flash) add the word ambient for TTL BL: "for natural balance between main subject and ambient background". Basically, TTL BL turns the flash down. TTL does not.

Note again: Spot metering (which is about ambient) always ignores the background, which turns TTL BL off to become TTL mode - which is only way SB-400, SB-700, camera popup flash, or Commander can choose TTL mode (otherwise they are TTL BL). The SB-600, SB-800, SB-900, SB-910 have a menu to force TTL (if connected to hot shoe). Except for Spot Metering mode, the only current flash model still in production that offers TTL override is the SB-910. Otherwise, the Nikon system default is TTL BL mode. Except, Spot metering is always TTL mode (there is no concept of background when spot metering, it is only about the spot).

Commander: The Nikon Commander controls multiple wireless remote flashes, including TTL (but which is TTL BL mode). See More Here.

Implications which we should realize, flash in bright sun

Speaking of pictures of people in bright sun, the sun's shadows are dark and harsh, and fill flash makes dramatic improvements to smooth them. Any Fill flash ought to be weaker than the main light, down a stop or two, to fill dark shadows somewhat, partially but not fully filled. Specifically, towards this end, TTL BL mode reduces flash exposure automatically, especially so in bright sun.

Otherwise, adding TTL mode flash gives TWO correct exposures, one from ambient and one from flash, which is two correct exposures, which add to overexpose the subject 2x, or by one stop. TTL mode does this by default (adds another correct exposure), which is what "without regard for background brightness" means. But TTL BL mode backs off (automatic compensation, for "well balanced exposure"), trying to prevent this by default. Indoors (insignificant ambient), this does not matter, TTL tries to get it right (and TTL BL might come out a little low). But for fill in bright sun, we know TTL ABSOLUTELY MUST use Flash Compensation of about -1.67 EV or so. Or if the flash is to be the main light (to make subject stand out from background, as opposed to fill), many users will manually underexpose the bright ambient a stop or two, and let TTL do its thing. We never want TWO correct exposures worth of light.

Two choices to prevent 2x TTL mode exposure in bright sun

Reduced Ambient - One alternative is to underexpose the bright sun ambient a couple of stops, and then let TTL meter the flash. This causes a darker background, and the flash will make the subject stand out strongly against it (a bit like a spot light on a stage). It is an interesting novelty look for some of your pictures. Due to maximum sync shutter speed, shutter speed is probably limited to ballpark of 1/200 second. But underexposing bright ambient a couple of stops helps prevent the ambient from blurring the fast sports action the speedlight can stop (flash is not often allowed, it interferes with the players). It does require more flash power (to override the sun), so is easier indoors, like in gymnasiums. This is not fill flash then. It highlights the subject.

Reduced Flash - Or, we can expose the sunlight normally, but then reduce the flash power exposure a stop or two, to provide a regularly lighted background and subject, but with subtle fill flash to slightly fill the dark shadows, for a very natural look. TTL BL flash mode attempts to do this automatically by default. TTL mode must be manually compensated to do it. -1.67 EV flash compensation is a good value for this in bright sun.

Or 3, use TTL BL mode instead (a variation of reduced fill flash), which uses automatic compensation, but you may still want to control by tweaking flash compensation.

For TTL mode,with manual flash compensation, -1 2/3 EV (-1.67 stops) is a very common fill flash preference in bright sun. It is a preference, the way you want your picture to look, and normally, anything is better than nothing. But TTL -1.67 EV (or TTL BL) really works wonders for pictures of people in bright sun, to help lighten harsh dark shadows, looking natural, without looking obvious. But yes, you really should want to adopt fill flash. Compare same picture with and without fill flash, at a couple of levels, so you can truly appreciate the difference. Even the camera popup flash, which does not have great range, but if not too far, and that's all you have with you, it can really help for fill in bright sun. Try it.

The term Balanced Fill Flash does not mean flash level equal to ambient. It means sufficient fill flash level to help, but not enough to mess it up, or be identifiable as such. You can always add Flash Compensation also to TTL BL to adjust its results, but automation is a moving target, it does things on its own. If you are going to specify your own flash compensation, then why not use TTL mode instead? It won't be the same number, so what? But then you have a zero base line, more predictable with only one cook salting the broth, no one to fight you. The difference is, in bright sun, TTL BL mode does this fill reduction automatically, and doesn't need much extra compensation, but TTL mode definitely will need maybe the full -1.67 stop flash compensation. Just don't misunderstand and try both -1.67 EV and TTL BL.

One More Time:

Nikon's system is a TTL BL system, and the camera's internal popup flash, the SB-400, the SB-700, and any Commander remote flash, are always TTL BL mode (when automatic flash, unless Spot Metering, which becomes TTL). You may need to realize that. A few other flash models have a mode switch to force TTL mode. If you don't have menu choices showing both TTL and TTL BL, then your system default is TTL BL (unless Spot metering). Manual flash mode is of course whatever level you set.

TTL BL mode typically is for fill flash outdoors in daylight, when there is enough ambient light to give a good regular exposure. The fill flash is added and matched (reduced) to balance (not overexpose) that background exposure. The camera meters the background for proper exposure, and the flash is balanced (reduced) to that. The flash power is typically reduced a good stop, maybe more, as needed in sunlight. TTL BL often seems slightly underexposed indoors, but all auto flash needs to be watched for the need of compensation. Some people resort to Spot metering (forces TTL mode) to bring the usual underexposure of Commander TTL BL back up. Understand that Spot metering is not a general purpose metering method. It just makes that one spot come out middle gray (should that spot even be middle gray?), instead of making the overall image average be middle gray. Seems better in general to address the situation directly, and just use maybe + EV flash compensation, if needed. It often needs to be done anyway, and will have greater effect. Flash compensation is something we MUST learn.

TTL mode is typically intended more for indoors, for places when flash is considered to be the only light source. In TTL mode, the flash intensity is not matched to anything, nothing else is considered except the flash exposure. The flash output is simply what it should be if flash is the sole light source, which is usually true indoors. It assumes no background light exists to be balanced. If there is bright ambient, it may well overexpose TTL mode. Fill flash in bright sunlight is the classic case for TTL BL mode (TTL BL flash mode and camera P mode knows how to do this, is point&shoot), but TTL mode with about -1.67 EV flash compensation works great too.

Continued - Flash Indoors, and Auto ISO

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