The Sub-Menu (a few pages):
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, flash and ambient have some different rules about exposure.
The camera system measures ambient and TTL flash separately and independently. The camera meter, and the 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 and ISO settings 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 affects the picture exposure, but it does not affect the exposure of the separate ambient light.
The exceptions we see in camera A or P modes are if Slow Sync is selected, it has the purpose to "Ignore any Minimum Shutter Speed with Flash", and use whatever slow shutter speed the dim ambient metering actually meters, no matter how slow. A tripod is likely necessary then. Rear Curtain sync typically only has use for a slow shutter that will cause motion blur. Rear Curtain Sync to makes the blur trail follow the action, instead of lead it.
In contrast, camera M mode generally can set any shutter speed, except it will not exceed Maximum shutter sync speed if the camera is aware the flash is present. Also, camera requirements for Manual flash mode are Auto ISO OFF and camera mode M or A (manual flash units cannot respond to aperture or ISO changes).
But these are merely limits, which are not about actually metering anything. 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 settings we want).
My point is to encourage the wisdom of turning Auto ISO off with flash, certainly for any important studio situation.
Everything has exceptions, and (speaking of Nikon) Nikon DSLR models (D3000) introduced in 2009 did allow full increase of Auto ISO with flash, which caused an adverse commotion when introduced. Internal flashes still do that today, but thankfully external flashes cause Auto ISO to stay at minimum. Older Nikon DSLR models (D90 and D700 and earlier) always disabled Auto ISO with any flash (to Minimum ISO). Relatively recent models (since 2010) will now allow Auto ISO with external flash to increase only up to 2 EV, like from ISO 100 minimum to 400 maximum. Bounce flash can benefit from ISO 400. We can always turn Auto ISO off, at least in A, S, P, M modes.
Later changes: The Nikon D850 and D500 models added two menus, E3 and E4 (OK, the D810 also had E3, but called it E4 there because E3 was already used for its internal flash). Since ambient and flash is all that can be metered, my interpretation is that Nikon's words Entire frame means both flash on subject and ambient on background, where Subject Only means flash, and "Background Only" means ambient. Nikon Exposure Compensation previously affected both ambient and flash (on Canon, it only affects ambient, and both brands also provide separate Flash Compensation). Now Nikon E3 selects the choice if Exposure Compensation still affects both or only ambient. E4 determines if Auto ISO affects both ambient and flash, or only flash. I am told this E4 Flash Only mode will first raise TTL flash power, and if not enough power, will then raise ISO. The default is Both, which won't raise ISO if flash is present. I have not used these models to verify this, but it seems good, keeping the old default but offering the choice.
Automation seems a difficult choice for manufacturers. Typically automation tries to help beginners in every possible way to save the picture, but experienced users (who have learned to see the actual scene requirements) typically will have their own better choices.
For DSLR, there can be several options with various modes to use flash, several modes on the flash itself, and also modes on the camera. Some top flash models have most modes, but some low-end may only have one mode, Manual or TTL, or maybe only a couple of modes.
Menus: The flash menu in the camera that selects TTL, Manual, or maybe Commander, only applies to the internal flash when its popup door is open. Then this camera menu has no effect when the popup door is shut. External flashes have their own menu (however Nikon SB-300 and SB-400 are exceptions, they have no menu and must share the camera internal menu). In the automated camera Scene modes, the internal flash may pop up and fire when automation thinks it is needed. But in camera A, S, P and M modes, the internal flash never pops up to fire, but it will always fire if you open it yourself, and a hot shoe flash will always flash with shutter if it is connected and turned on (unless maybe it is incorrectly set to some slave mode).
Auto ISO does still operate in Manual mode. It cannot change any settings itself, but does automatically change manual camera ambient exposure with ISO. But with Manual flash, Auto ISO must be Off, it is absolutely out of the question with manual flash, since the manual flash power cannot respond to any automation changes. Any exposure Compensation controls are ignored by Manual flash, instead you adjust results by setting a different flash power level (or by adjusting aperture or distance or ISO, but NOT Auto ISO).
Experienced users can consider Manual control to be a plus, as it allows their choice of settings, particularly for indoor flash. The settings starting point if often done by experience, about the same power level you remember using in a previous similar situation. The proper settings are determined manually by the user, either by trial and error experimentation (easy for one flash), judged by eye or by watching histogram, or by using a handheld light meter (the light meter is highly recommended for multiple studio flash units, to control the lighting ratio with known precision). If using a radio trigger or external optical slave, the flash would be in this same Manual mode. See triggering methods for off-camera flash. Most studio flash units are Manual mode only, which is optimum for a fixed portrait setting, since you do NOT want TTL to be varying power continually.
For the TTL flash mode, the camera mode used can be Manual or automated. The user sets the Manual camera settings as desired, but if automated, the camera meters the ambient and sets up the camera settings accordingly for the ambient (aperture and possibly Auto ISO). The TTL flash will have its power adjusted to match those camera settings that are currently in place. Again, TTL is automatic metered flash. No matter how you set Aperture or ISO, the automatic metering will try set the TTL flash to respond to it, and you still get the same TTL flash exposure (if it can, if it has sufficient power to achieve it). So the way you adjust results of the TTL flash automation is with Flash Compensation, for ± flash exposure. TTL requires communication with the camera metering, and is either on hot shoe (or hot shoe extension cord), or there are camera systems (Commanders) to provide multiple remote TTL operation (individually metered), which is wireless only (using light signal transmission, no cables). Some radio triggers simulate TTL too.
Some opinions: Auto ISO with flash indoors seems bad news indoors, since the dim ambient will greatly increase ISO, and incandescent lighting is typically orange, but flash is white (daylight), so white balance is a problem. The flash then needs to be balanced mode, which just becomes fill flash to the ambient. And since indoor ambient levels are typically insignificantly low (needing flash), probably resulting in grossly inflated metered camera settings attempting it, indoor flash users often choose Manual camera mode with TTL flash indoors, so they can set more appropriate camera settings. Shutter speed does not affect the flash, so ISO and maximum sync speed can be used to keep out any orange incandescent ambient, or slower shutter can be chosen to let some of it in, as desired.
There are normally two camera metering choices provided for the TTL flash.
Canon calls this TTL mode "Average" mode, and has a camera menu to select it. Basically, this mode is the "Not Balanced" flash mode. Canon calls balanced flash mode "Evaluative". Balanced means the metered flash power is reduced to Fill Flash level, which prevents the subject overexposure from two added metered exposures (ambient and flash).
Nikon calls not-balanced flash mode to be TTL, and calls Balanced mode TTL BL. However, the menus generally say the generic TTL regardless of which mode will be metered (default today is TTL BL balanced). The Nikon Exif data (Manufacturers section) says which mode was used. Some Nikon iTTL flashes (SB-600, SB-800, SB-900/910) do have a menu to select which TTL BL (balanced) or TTL (non-balanced) mode for the camera metering to use. Only the SB-910 is still in production, so this TTL vs TTL BL menu is disappearing (Balanced flash is the default). The way to select Nikon not-balanced TTL flash mode in the Nikon DSLR cameras is by setting camera Spot Metering mode. Camera Spot Metering changes the default flash metering system from TTL BL to instead be TTL mode. Don't misunderstand, the flash system never does Spot Metering, the flash metering always reads its own center area, but the bright ambient metering certainly can do Spot Metering. Spot Metering can strongly affect metering of bright outdoor ambient. So this is awkward, you will quickly learn to NOT leave it in Spot Metering mode when you go back outdoors into bright ambient. 😊 You very definitely do need to know what you're doing with Spot Metering. Balanced flash is improved today, but I'd still much prefer a camera menu to select non-balanced flash without involving Spot Metering.
But indoor ambient is typically too dim to much matter (unless ISO is too high). Meaning, indoors, Spot Metering has little if any exposure effect with the flash, ambient is generally too dim to show up much, which is why flash is needed. If Spot Metering is set, the camera just changes from TTL BL to be TTL mode metering, and indoor ambient is generally too low and underexposed to matter (not without exception). This selection method may seem odd and awkward, but 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 in Spot mode. Which is what TTL mode is, just regular flash, exposed as metered, no balancing (meaning, no reduction of flash to a fill flash level). When it might matter, Flash Compensation is then used to optionally reduce TTL mode to fill flash level.
In contrast to balanced TTL BL, TTL mode comes ahead on, using full metered power, flash as metered, regardless if any significant ambient is present or not. The flash is Not reduced or balanced. So then for TTL mode, for fill flash in bright light, then you have to know to set about -2 EV flash compensation yourself, otherwise TTL will give an overexposed subject (when the two full exposures add). Setting TTL Flash Compensation for fill works great (and offers control), you just have to know it is required. Whereas, TTL BL does that flash reduction automatically, point&shoot fill flash, called Balanced. And TTL BL is generally the default mode, unless you set Spot Metering mode.
It is the camera that meters and controls all of this. The flash simply responds to camera metering commands about what flash power is to be used. The Exif reports which flash mode was used (in the Camera Manufacturer data section).
Several Chinese flashes (like Yongnuo for example) have built-in optical slave triggers, and call that mode S1. Then an off-camera Manual flash with optical slave mode will fire in sync (at the preset manual power level) when it sees any other flash. They also have a S2 mode that is the same, except it ignores one first TTL preflash, which allows it to be used with simple cameras that don't provide Manual flash mode.
In other situations in bright sun outdoors, some users instead reduce the bright ambient exposure maybe 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 (if the flash reaches that far). 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, I like to routinely start balanced flash mode near +⅔ EV or +1 EV Flash Compensation in the general case.
The next six pictures are about shutter speed choices to control the ambient vs flash, to show what happens. Important tools are: Balanced flash to prevent overexposure of sum of ambient and flash. Camera M mode allows any shutter speed (not exceeding maximum sync speed), specifically faster will keep out off-color indoor ambient to protect flash white balance. And Slow Sync mode allows slower (actually metered) shutter speed, to include dim ambient.
This day was overcast and raining (Not bright). Without flash, the less bright ambient required camera mode A selection of 1/13 second at the selected f/5.6 ISO 400 settings (late afternoon, -8 EV from Sunny 16 bright sun). All pictures were taken within about a minute (in same ambient). #4 is camera M mode, all others are camera A mode. All are bounce flash except #1 is no flash. Flash is non-balanced TTL except only #3 is Balanced flash. All are regular front curtain sync except #5 and #6 are Slow Sync. Nikon D300, 12-24 mm lens at 22 mm. This is just quick point&shoot (but on a tripod).
All six photos are f/5.6 and ISO 400 (not Auto ISO). Again the point is, shutter speed only affects ambient, and NOT the flash, so shutter speed offers some control of ambient (which is an extremely important tool for photographers). Again, all are camera A mode (allowing automatic shutter speed), except #4. The bounce flash does not reach to the fence outside.
With flash, the camera still first meters the ambient light, and those same settings are what it will still use for flash. The TTL flash must adjust its power level for whatever ISO and aperture settings it discovers to be in place then. We might consider choosing the settings differently to help the flash though.
The Exception is that with flash in A or P mode (automation which adjusts shutter speed), the shutter speed is not permitted to be slower than the "Minimum Shutter Speed With Flash", which is menu E2 on many Nikon DSLR models, which allows a Minimum Shutter Speed setting as fast as 1/60 second (default of most cameras is 1/60 second minimum if using flash). Camera models without this menu will simply use a similar minimum with flash, likely 1/60 second. This is an arbitrary limit (probably is about handholding shake, but the thinking is we don't need slower if we are using flash anyway). It is a minimum limit, NOT about metered or a proper exposure. The flash exposure is not even affected by shutter speed, but the ambient light exposure is.
If we had wanted to use the actual slower metered shutter speed for the ambient, we could use Slow Sync mode for the flash, or some cameras can set a slower Minimum. Or more commonly, we could choose Manual camera mode, using any shutter speed settings we wished, which is often a great plan for indoor flash. Or camera S mode will set shutter speed too, but then in a dim situation, camera mode S would likely always use wide open aperture in dim ambient.
Slow Sync ignores the 1/60 second Minimum shutter limit with flash, and uses the slower shutter speed actually metered for the dim ambient. But the sum of any two lights added is brighter than the brightest, so one issue then (Slow Sync or in any properly metered light) is that if we assume properly metered ambient is 100% of a correct exposure, and properly metered TTL flash is 100% of a correct exposure, that sum of both is 200%, which is 1 EV overexposed on the near subject. We see that in #5, where the red and yellow flowers are overexposed (and the sky is overexposed much worse, but it is not considered any issue in this picture). This compensation percentage has a calculator on next page. The definition of Balanced flash mode is that it reduces TTL flash exposure to minimize this overexposure of the sum, but non-balanced TTL does not bother. I still prefer using non-balanced flash, having learned to control the compensation myself, so I'm not always thinking about balanced flash mode, but it has become hard on todays gear to even select non-balanced flash. But indoor flash typically instead forces a 1/60 second Minimum shutter speed which probably underexposes ambient unless ISO is very high, or we choose Slow Sync. My own notions are that Slow Sync mode, camera mode S for flash, or either Auto ISO or High ISO for flash Indoors, are very special cases that I rarely have use for. But they do exist when and if needed.
Balanced flash mode will normally reduce the TTL flash level to minimize overexposure (and here, in #3, due to the f/stop and ISO I selected, the ambient was reduced by the Minimum Shutter Speed With Flash limit). But as always, if ever TTL flash is too bright or too dim, use Flash Compensation to get what you want. Or for Manual flash, if result is not right, correct it's power level (or the f/stop if indoors). Digital cameras make this easy to see and correct instantly, then and there.
And another limit with flash, the shutter speed is not permitted to be faster than the Maximum Shutter Sync Speed, typically about 1/200 or 1/250 second on many cameras, which does require attention in bright sun (camera P mode in sunlight is good about knowing all of that), but the presence of the limit is rarely any concern in dimmer light or indoors (the flash is faster than the shutter speed).
The way Auto ISO is handled with flash can vary with camera model.
Anyway, back to the idea in the six pictures above... So the camera did arbitrarily increase the shutter speed to the 1/60 second Minimum if any flash was detected present, and the ambient was simply underexposed by that shutter speed increase. But still, we are using flash instead.
Look through your old photos if you have Exif data. In A or P or Auto modes, every indoor speedlight flash picture will surely all use this 1/60 second Minimum Shutter Speed With Flash, which is good reason to instead consider camera M mode indoors. If you are setting Camera A mode and f/5.6, and the camera is going to use 1/60 every time (in indoor lighting, maybe faster in stronger light), which is the same as manual mode f/5.6 1/60. Except then you don't have to choose 1/60, it was not about exposure. The 1/60 was Not a metered value. You can arbitrarily select shutter speed to control the ambient as you wish (next section a few lines below). But the TTL flash is still automatic flash even in camera M mode, which still uses the same aperture you'd set in A mode, but it also allows control of ambient with shutter speed.
For formal portraits with flash, it can be important for white balance to use low ISO and to stop down a bit, and to use Maximum Sync shutter speed (camera M mode). Stopping down needs more flash power, but it improves depth of field, and these actions better prevent any stray orange incandescent ambient from affecting your white balance. At ISO 100, typically 1/60 f/4 will let a little of the ambient in, and 1/200 f/8 will generally keep it all out (again, indoors). Faster shutter reduces ambient, but does not affect flash. Low ISO and stopping down more reduces both, but the flash can increase power level to stay the same. Using the same formal settings, you can turn all flashes off and snap off a shot to verify that sees a totally black frame (no ambient leakage).
This case above (1/13 sec changed to 1/60) leaves the ambient underexposed 2 1/3 EV (pretty dark, and indoors is often more difference than that). This is NOT a metered value, it is simply an arbitrary limit (because we're using flash, and wouldn't normally want to handhold 1/13 second). But this 1/60 Minimum with flash is rarely about a proper exposure (not unless it also might happen to properly expose the ambient).
So one big point about flash is, a faster shutter speed is a way to keep the ambient light out of the exposure of the indoor flash picture. A little bit of incandescent ambient (that 1/60 second might provide) can "warm" casual pictures that some consider pleasant, but it doesn't take much to spoil formal portraits.
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 of using ISO 100 and fast Maximum Sync Speed shutter with indoor flash would 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 low ISO and maximum shutter sync speed, for this purpose. And you will want to test your setup and exposure once, say ISO 100 f/8 1/200 second, with all normal continuous room lights on, but all flash disabled, to ensure you do still get a pretty black picture (without effect from the continuous lights). 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 handhold 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 orange 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/5.6 and ISO 100. 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 always choices, 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 much that you see happening with flash. See a homework page about trying this concept.
Then, first a quick section on "Modes". (Balanced TTL vs TTL modes).
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 M mode indoors, and maybe camera P mode for fill flash in bright sunshine.
However, for flash indoors, since the ambient is dimmer, defaults are that camera A mode will likely always see a 1/60 second Minimum Shutter Speed With Flash (exceptions are brighter ambient reads the higher shutter speed actually metered, or Slow Sync and Rear Curtain Sync modes allow the slower shutters actually metered by the dimmer ambient). In dim light where we need flash, this 1/60 second is not a metered choice, simply a default choice, just to prevent even slower speeds. Shutter speed does not affect the flash, but it does affect the ambient (but indoors, the ambient is likely too dim to matter with flash). So camera M mode is very popular for flash indoors, allowing shutter speed to be set as desired (typically slower to allow some special ambient to register, or faster to totally keep out the ambient to prevent messing up our flash white balance).
Three very important things to know about camera M mode:
In dim light, P Mode is normally near widest aperture too. Or it does if no flash, but P mode also enforces a Maximum aperture with flash. At lowest ISO 100, a hot shoe flash limits to typically f/4 or f/5 maximum aperture, and internal flash is typically f/2.8 or f/3.5 maximum aperture. Which gets far worse at higher ISO. In P mode with flash, this maximum aperture limit is stopped down 1/2 stop per each stop of higher ISO. I realize that's the relation of Guide Number to ISO, but don't know why it's needed here? A and S modes don't need this, and the one stop more restrictive limit for a hot shoe flash would be a pretty small hot shoe flash. But your f/1.4 or f/2.8 lenses won't be much help with flash in P mode. This is counterproductive with flash, because if you really need high ISO, the lens is stopped down more, unable to use much of it. Camera P mode and Auto ISO seem an especially poor combination for indoor flash (but camera P mode is often a great choice for fill flash in bright sun).
For P mode, many Nikon camera model manuals have two charts of this, internal and external flash (shown here). This is the D7100, representative of the others.
EDIT: The latest models appear to have abandoned this difference, with one P mode chart starting at f/2.8 Maximum at ISO 100 for either flash.
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. Whereas other than exposure, automation never understands the situation.
That is speaking of a normally exposed ambient, like flash outdoors in sunshine, or Slow Sync, or indoors with outrageously high ISO. Those will need flash compensation (still speaking of non-balanced TTL). But indoors with low ISO is a different story if in camera A mode (if Not using Slow Sync), because the Minimum Shutter Speed With Flash (probably 1/60 second) will underexpose the ambient instead, no longer 100%, no longer 200% overexposed.
I'm an old timer, from before the era of balanced flash, which is a bit newfangled to me. Flash compensation was quite important before, and being old-fogy, I still prefer to do it myself rather than to trust the automation. But today, cameras usually default to balanced flash mode (e.g., Nikon TTL BL or Canon Evaluative) which tries to automate it, and it has gotten better today. It certainly is a big help for those unwilling to think about what they are doing to help out themselves. So it's also very important to realize that cameras today probably default to balanced flash TTL mode, and what that means is that balanced flash mode tries to take care of this situation itself, automation computing what needs to be done. You can still compensate balanced flash to change the result it is getting, but that is a different idea.
Exposure Compensation adjusts ambient exposure of the camera auto modes, like A, S, P, and Auto modes. It does Not change camera Manual mode exposure, however, it does still affect what the light meter reads and shows, perhaps guiding you to adjust manual exposure yourself.
Flash Compensation adjusts metered TTL flash exposure. It does not affect manual flash. Manual is manual. But if the automatic TTL flash exposure is not coming out just right, too dark or too bright, then a bit of plus or minus EV Flash Compensation does the trick.
Auto ISO does affect exposure, including camera Manual mode exposure, and does affect TTL flash metering. Manual flash cannot react to Auto ISO changing, and Auto ISO will stay at Minimum ISO if camera recognizes a manual flash is present.
There may be multiple places flash compensation can be specified (on flash or on camera for example, or in Commander menu if used), and they all add to a total.
On Nikons, Exposure Compensation also adds to Flash Compensation so that TTL flash reacts with the sum total. However, newer models, i.e, D7200 and D810, have a new E4 menu to either does that add (EC + FC), or not. Canon models for example, do not. Pros and cons either way, individual control or collectively. Individually, we can think of Exposure Compensation as affecting the ambient in the background, and Flash Compensation as affecting the flash subject in the foreground. Or collectively, we can change the entire frame with one control.
Nikon likes to discuss flash picture exposure in those terms (background, subject, entire frame), but I think it is more clear in terms of ambient or flash. The flash power cannot reach to the distant background, therefore in general, the background is only ambient light, and the near foreground subject is flash exposure.
The camera internal flash, the SB-700, and the SB-400, are TTL BL mode by default. Others have a selection menu on them.
Anytime I say -1.67 stops, I mean -1 2/3 EV on the camera dial ( -1.67 is easier to type and reads more clear.)
Camera mode A lets you set the aperture, which is a concern for flash power requirement. But really, if insignificant ambient, camera M mode obviously offers the most choice. Camera mode A and M can set the same aperture. TTL flash is always still fully automatic flash exposure in any camera mode, including camera mode M. If you set camera M mode, and also set 1/60 second shutter speed, then this is exactly the same as camera A mode, which would have set 1/60 second speed too (that is all the camera A mode does with flash in dim light, it sets the 1/60 second Minimum Shutter Speed with flash — Rear Curtain sync and Slow sync are exceptions allowing slower.) We have just seen that the flash does not care about shutter speed. You can still set the same aperture for both A or M mode, both are same thing then, maybe f/5 for bounce flash. The TTL flash is automatic flash exposure either way.
But camera M mode allows setting any faster shutter too, up to maximum sync speed, for control of the ambient indoors (to shut it out, or to show it stronger). There are two flash modes, TTL and balanced flash. TTL flash mode is arguably best indoors, or balanced TTL outdoors. Sometimes balanced just needs a little more compensation, but either may need compensation, so it may be a difference without a distinction (speaking of flash indoors).
Continued - Fill Flash in Sunlight