Fill Flash in Sunlight
Direct sun is a harsh light for portraits. 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. Early or late is better when possible. If no flash, the norm is to face subject into the sun. If using flash, face them somewhat away from the sun. But it can be 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. Unless it is balanced flash mode, the flash will require some manual compensation. There are a couple of major considerations:
- If we attempt a proper exposure for the bright ambient, then shutter sync speed is limited for the flash. Flash in bright sunlight probably will require aperture to be up near f/11 (ISO 100) or f/16 (ISO 200) in bright sun (Sunny 16), because shutter speed cannot exceed Maximum Shutter Sync Speed.
Camera A mode has to use the aperture you specify, so then you're the one that must know to stop the aperture down to an allowable shutter speed. If you go outside from an indoor session with A mode and f/5, A mode ain't gonna work in sunlight with flash until we move up around f/11 (because the shutter speed cannot exceed Maximum Sync Speed).
But camera P mode can adjust both shutter and aperture, and it is very good about understanding all of this about flash in sunlight. If you've been using A mode at f/5.6 indoors, that is not going to work if you move into bright sun. Wedding photographers call P mode "Professional" mode because of this. Nikon calls it Programmable mode that the camera can do.
- Adding another light always adds more light to any scene, a concern if the subject was already exposed properly. So if using TTL flash mode, correctly metering both ambient and TTL flash is two correct exposures, which are added, and the sum totals 2x, or one stop overexposure. We can underexpose the flash power (fill flash), or underexpose the ambient exposure a stop or two (highlighted flash).
- If using balanced flash mode, you need to realize that balanced flash mode tries to compensate the fill flash automatically (reduces TTL flash to fill level). Plain unbalanced TTL flash mode does not — you have to do it yourself. So the combination of camera P mode and flash balanced flash mode can be nearly point&shoot fill flash in bright sun. Or camera A mode and flash TTL mode works fine too, so long as YOU know to stop down the aperture enough to allow a usable shutter speed, and also know to compensate the flash about -1 2/3 EV compensation, more or less.
- Or, if you just gotta have f/2.8 too, you can add HSS FP mode for wide apertures (but power is limited to short distance range). Nikon tends to call it Auto FP mode.
But another advantage of camera P mode (in this fill flash in bright sun mode, and if you forgot to turn Auto FP mode off) is that camera P mode will go to f/22 or f/32 first (if the lens provides it), BEFORE it will allow a shutter speed that would switch Auto FP into FP mode, to surprise you with insufficient flash power.
All three pictures are Nikon D800, camera mode A, ISO 100, f/8, 1/125 second at six feet. SB-800 speedlight flash mode, straight TTL mode, here meaning the default TTL BL Balanced flash mode was overridden here to instead be plain non-balanced TTL flash mode. Cropped off-center to show more of the distant ambient.
No flash
Unacceptable result. Late afternoon, side lighting. Portraits in bright sun simply have harsh dark shadows, in various degree. It was chosen for a worst case. It's better to use your flash for fill.
At right was a direct hot shoe TTL speedlight, with manual compensation. The camera popup flash can work too, having probable range for fill up to about 7 or 8 feet. But the Nikon internal flash defaults to TTL BL balanced flash mode.
TTL mode Fill, -1.33 EV Flash Compensation
Better, but not best. Maybe -1.33 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.
The distant background is not affected, the flash power doesn't reach that far. In bright sun, we are likely at f/11 or f/16
TTL mode Fill, -2.0 EV Flash Compensation
Opinions always vary, but mine is this is perpaps a bit too little flash, but still a huge improvement. We have choices, and in between these two at -1.67 EV flash compensation has historically been a popular choice. But adjust it as needed.
Balanced flash mode does this fill compensation automatically, but flash compensation should still tweak it.
The basic idea is that two proper exposures, from flash (as metered by TTL) would also be proper alone at night or indoors, and one from the Sun too, is one too many (which is overexposure). So you have to back off on one of them to just a fill level. Backing off on the daylight will darken the background too, so the flash is the obvious choice, its effect is just fromthe distance. It's called Fill flash. In the shade is a bit different, the daylight is reduced there, but the camera automation cranks up exposure more for it (ambient settings are determined before the flash fires). So the ratio of flash ought to drop down a bit more too then.
We do have choices about fill amount, but -1 2/3 EV fill (-1.67 EV, in between these two) is a popular TTL value in bright sun. Or, we can use TTL BL (balanced) mode, which does automatic fill compensation, and is often a good choice with camera P mode in bright sunlight, so we may not need to handle compensation ourself. Balanced flash mode automatically reduces TTL flash level to be a more appropriate fill level in bright ambient.
You should practice this a time or two. Try a couple of different situations and different fill levels, see what it does. Fill flash in bright sun is an important big deal in anything near a portrait situation. There's not much that is more important.
The same procedure is also needed in any case of a photo of subject in shade, but with full bright sun is lighting the background (under a tree, etc.) This is two lighting situations, and no one exposure can be a suitable compromise for both. So that is a classic case when, if you want that picture, then flash is required, to illuminate the shaded foreground subject to the same level as the background in sun. This is not actually fill flash, the flash will be the major illumination of the darker subject. This is actually pretty simple to do with the balanced flash mode. Take your flash with you outdoors. Even the little internal flash can sometimes help, when the subject is close enough.
The Nikon system has two TTL flash modes, called TTL and TTL BL (balanced). Flash models like SB-600, SB-800, SB-900 (all discontinued now) and the SB-910 have a menu to select either choice. The other flash models, like SB-400, SB-500, SB-700, the internal flash, Commander flash, and almost all third party flashes (except a Metz model), 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 camera metering default. Purists may prefer TTL for the more absolute manual compensation control, but TTL is disappearing, the default is TTL BL. Selecting Nikon Spot Metering mode will switch TTL BL mode to be TTL Mode (which does not do Spot Metering for flash, but it is only Spot Metering for ambient). Spot Metering does not make the selected spot be "correct" exposure, it only makes it be middle tone exposure, regardless of what that color it should be. Which with balanced flash becomes quite different outdoors, usually needing compensation. Indoors, ambient is likely too dim to matter what it does (so Spot Metering probably does not matter then). Users never seeing the two TTL menus may not grasp that there are two distinct modes, but I am speaking of the two TTL modes here.
Straight TTL vs Balanced 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 Balanced flash 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). 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, so 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). HSS flash is the exception allowing faster shutter speeds, but substantially less flash power.
This is a narrow window, with few choices. Camera P mode and Balanced 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 (HSS is not going to be allowed in P mode.) 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 can 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.
Cameras that have HSS High Speed Sync mode (see Part 2B, HSS, Nikon Auto FP) allow a faster shutter speed with HSS flash, to do maybe 1/1000 at f/5.6, or even 1/4000 at f/2.8 (ISO 100). This does work, but the HSS flash power drops drastically, and fast shutter drops HSS exposure even more drastically, and the range falls (to less than half at most, best case). HSS allows the high speed shutter in sunlight situations (with ambient properly exposed, but indoors, regular speedlight flash mode runs circles around HSS, the flash is faster than the shutter). The HSS flash itself is modified to generate a continuous series of smaller flash pulses to mimic continuous light, so the focal plane shutter can work. HSS (Auto FP on many Nikon 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 Nikon 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, and Lighting Ratio calculation
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, those 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 (often near negligible, which is why we're using flash), but the opposite is true of normal outdoor fill flash.
And ANY extra light added to an already perfect exposure will be some degree of overexposure.
Here's a calculator to show result of various combinations of TTL flash compensation with ambient levels (and also standard studio style Main/Fill lighting), and shows the percentage of TTL flash each includes, and the overexposure over ambient with the total sum with added flash. In these calculations, the TTL ambient light plays the same role as the studio Main light, with the assumption the situations are often similar (both are frontal fill).
For the Ambient and TTL fill flash mode, the calculator's definitions are:
- Caution: The chart is about manually selected TTL Flash Compensation. Balanced flash mode will implement its own plans and cannot be controlled.
Any two lights added are brighter than the one brightest light.
If the ambient compensation is 0 EV, any significant amount of TTL flash adds to overexposure.
Or, if flash TTL is at 0 EV compensation, any significant amount of ambient adds to over exposure.
- Automation of default Balanced TTL flash mode automatically selects and reduces the flash in bright ambient, often to around -2 EV if in bright sun (this EV is variable). So this calculator does NOT apply to the cameras default Balanced mode automation (which is doing its own thing), meaning while its level is probably good, you don't know what the flash compensation actually is (it varies with ambient). For the difference in balanced and not balanced flash, see below, and more on the previous page.
- Ambient at 0 EV compensation is assumed to be metered at 100% of the proper full ambient exposure (like a light meter in sunlight).
- TTL at 0 EV flash compensation is assumed to be metered at 100% of the proper full flash exposure (like indoors or at night, in dim ambient).
- If both bright ambient and flash use, then in ideal normal situations, Exposure Compensation of 0 EV is likely a proper exposure of ambient, like in sunlight. And Flash Compensation of 0 EV is a likely proper TTL flash exposure, like indoors or at night (with minimal ambient). But two proper exposures (like sunlight and full flash) are twice too many proper exposures. This sum is 200% exposure, or one EV overexposed at subject.
And the sum of any two lights is brighter than the brightest one, so adding any light to an already properly metered situation is some degree of overexposure, but a bit can usually be tolerated. Half of a third EV is 1/6 EV, or 0.017 EV, and is a bit, not much. A very common practice is that -1 2/3 EV is a reasonable flash compensation in daylight, and automated Balanced Flash in sunlight will often be near that. -1 2/3 EV is 24% flash if ambient is 0 EV. Maybe at least set first try to -1 2/3 EV TTL flash compensation. A little practice will help.
Note that Flash Compensation has no effect on Manual flash exposure, same as Exposure Compensation has no effect on Manual exposures. On many Nikons, Exposure Compensation also affects Flash Compensation by the same amount, and so reduces the fill flash too (more recent model Nikon DSLR have a menu E4 (E3 if no internal flash) to allow changing that behavior).
- In normal indoor situations, the normal room light level is dim compared to sunlight, and at normal camera settings, the indoor ambient light is usually well underexposed (depending on your normals — except Auto ISO likely puts ISO sky-high which might be reconsidered. Auto ISO can be turned Off.)
In carefully planned portrait situations, the ambient light shutout can be maximized with low ISO (100), fast shutter speed (maximum sync speed), and stopped down aperture (like f/8, speaking of DSLR class cameras). Portraits like the soft light from near umbrellas, so you will have enough flash power. And then you won't even have to turn all of the normal room lights off. That first test picture with all flash disabled is hopefully all near jet-black (meaning you successfully eliminated any ambient from bothering exposure, white balance, or your planned lighting setup).
We need flash indoors, and the weak ambient then will not add overexposure of the flash picture. But in brighter ambient, we need to back off on the flash (to be fill level), or back off on the ambient exposure. The Balanced fill flash modes try to do that automatically.
- Power Ratio and Lighting Ratio is discussed below (and more for Main/Fill also here). These are the ratio of the (Ambient or Main light) : (TTL or Fill light). The TTL fill flash calls its level -EV Flash Compensation (relative to metered ambient), but the ratios can be computed the same way, treating the Ambient to be the Main light... TTL does meter the frontal face in the ambient light, but this ratio does assume the ambient Sun is near a similar Main light angle .
- The calculator allows the lights to be declared even brighter than EV 0, but of course overexposure is not desirable. The numbers +6 EV to -6 EV are allowed, but the reasonable mode is that the Ambient or Main light is called EV 0, which means that fill flash power that meters to match the main light would be called 0 EV fill. It probably already shows all you want to see.
TTL flash: Again, note that if Balanced TTL 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 the cameras default Balanced 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 Balanced TTL), 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 Balanced mode is different, flash in a dark room stays strong, but automation reduces flash near two stops in bright sun (fill flash level).
The Purpose of Balanced TTL flash mode is automation to reduce the metered TTL flash level, to minimize the overexposure when added to the other lighting, and to simply provide lower fill level too, and NOT to provide the major illumination done by the major light (ambient or the key light). The user must control this ratio for manual flash or unbalanced TTL mode. Balanced flash mode has become the norm today, and unbalanced TTL mode is disappearing, but there are still advantages in understanding how things work.
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 Balanced 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 handheld 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 meter system. The meter uses Incident metering, always a good thing. Another big difference is that the handheld light meter Manually meters for Both camera and flash (both are in Manual mode), vs. automation metering the TTL flash. There is a little trial and error in manual flash, we adjust flash power or flash distance until it meters the percentage desired. The meter meters ambient and flash combined, and tells us the camera aperture to set. We adjust the flash (manual power level or flash distance) to be metered at the desired flash percent, either 20% or 30% (the meter can only indicate 10% steps, cannot show 25%). The handheld flash meter does meter and show the correct total sum camera exposure, which along with incident metering, is the major advantage. Assuming we metered at the subject, this is correct for the near subject, but it can underexpose at the background (where the flash doesn't reach). If the meter does not offer flash percent, still simple, the total with flash on should meter brighter than ambient meters alone.
- TTL system. The camera meter is Reflective, affected by scene colors. Automation meters the TTL flash and sets flash power accordingly. We can use either camera manual or automatic modes, but the TTL flash is automatic either way. The system meters ambient and TTL flash individually. If automatic modes, the camera is set for the metered ambient. If a manual camera, ambient uses the manual settings we chose. Either way, TTL compensation sets flash power and exposure automatically to be the EV level as we specified for it to do. The flash power is set to be a proper flash exposure level for the flash compensation we set.
But adding fill flash to the metered ambient may overexpose the near subject slightly. One proper ambient exposure plus one proper flash exposure is 2x proper exposure. No big deal if we know to expect it (and we compensate the flash accordingly). We frequently choose -1.67 EV Flash Compensation in bright sun, popular and which works well, but even its sum still adds about 0.4 stop to the total metered exposure (adds at the near flash subject, but not at the background).
Even -1/3 EV on bright sun ambient can make a difference in the added overexposure.
On most Nikons, the Exposure Compensation control affects BOTH ambient and TTL flash. If you set Exposure Compensation to - 1/3 EV, this also reduces Flash Compensation by - 1/3 EV too (which is not visible in the Flash Compensation menu, but Exif shows it). However, the newer Nikon models (from D4, D810, D850, D7200) offer a new E4 menu choice (E3 if no internal flash) to separate them so it can affect ambient only, like Canon does), allowing ambient to easily be underexposed -1/3 EV, or whatever, without affecting TTL flash. Saying that otherwise (the older combined way without this new E4 menu), Exposure Compensation of -1 EV and Flash Compensation of -1 EV results in ambient at -1 EV and flash at (-1 -1) = -2 EV (EC is added to FC on Nikons without E4). The newer way, both are individually at -1 EV then. There are pros and cons of the two methods. If you simply wanted to raise the overall exposure level of both, then the combined menu easily does that (adjusts all exposure), but the separated method would require individual attention to do it then.
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 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 are from the calculator above for any two lights (frontally metered). Except 1.5 EV technically requires log₂(3) = 1.585 stops to compute the 3:1 power and 4:1 lighting. 1.5 EV has just been conventionally said historically to smooth the numbers.
The EV Fill compensation is relative to the Main, but the ratios show the Main relative to the Fill (Main:Fill). If the Main light is assumed 100% exposure, then the Fill light entered as -EV difference from it will compute the ratio and the total. Numerically, the 1.5 stop rule of thumb really should be log₂(3) = 1.585 stops to be the 3 power or 4 lighting, however convention has called it 1.5 EV.
Overexposure: The Overexposure column is the amount the sum of the two lights exceed the brightest. That may be important when the brightest is the ambient with TTL fill flash (and represents the cameras metered settings for that ambient), but for studio Main/Fill situations, we simply meter exposure again from the two lights together after ratio has been set up.
Contrast: Greater ratio is contrast (darker shadows, greater shading. Instead of dull flat lighting, 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 (often uses 1 EV, or 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 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 may have to read closely to see which way the numbers 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 power in our flashes — it is what we can directly control. 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.
Lighting Ratio = (Main + Fill) : Fill is the actual lighting we get then — the intensity result at each part of the subject, which is somewhat theoretical since we don't actually measure each area. The concept is that the Fill light is very frontal, on the entire face surface, but the Main light is to one side, more or less lighting that side of the face. So the highlight areas see both main and fill light, but the shadows see only the fill light. That ratio is contrast (color also adds contrast).
However, the same EV Power Ratio of 2:1 is considered to be a "Lighting Ratio" of 3:1. If Fill is 1x and Main is 2x, the main's side sees the 3x light sum. By definition, this 2:1 power computes 3:1 lighting, because 2/2 main + 1/2 fill is 3/2 on the highlights (main plus fill), and only 1/2 on shadows (fill only), which is called 3:1 lighting ratio. We find it easy to meter both lights, but harder to try to meter the specific highlight and shadows on the face.
We may see either ratio being called lighting ratio, but which are different numbers, so pay attention to what the writers say they do (their procedure), and not to what they call it. Some will call a one stop difference to be 3:1 (lighting ratio), and more will call it 2:1 (power ratio). You can 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 about lighting).
More about Nikon TTL BL Balanced Flash mode
None of that percent calculation stuff applies to Balanced 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. Balanced 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 Balanced 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 Balanced 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 Balanced: "for natural balance between main subject and ambient background". Basically, Balanced turns the flash down. TTL does not.
Note again: Spot metering (which is about ambient) always ignores the background, which turns Balanced off to become TTL mode — which is the only way SB-400, SB-700, camera popup flash, or Commander can choose TTL mode (otherwise they are Balanced). 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 Balanced 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 Balanced 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, Balanced 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 Balanced 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 Balanced 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. Balanced 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 Balanced 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 Balanced with auto compensation) 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 Balanced 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, Balanced 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 Balanced.
One More Time:
Nikon's system is a Balanced system (their TTL BL), 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 if Nikon). Manual flash mode is whatever level you set.
Balanced 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. Balanced 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 Balanced mode (Balanced 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
