If the flash unit for Nikon cameras does not have a menu for both TTL and TTL BL modes, then the one generic TTL menu is actually metered as TTL BL mode, Balanced Fill Flash mode (unless Spot Metering .. which is mentioned many times on the previous page). I routinely always start with maybe +2/3 EV or +1 EV flash compensation for TTL BL indoors, and adjust from there as necessary.
You should realize that even in flash mode, the camera metering sets the exposure settings (f/stop, shutter speed, and Auto ISO) based only on the existing ambient light. There are a few ifs and buts, but in a dim situation (where you plan to use flash), your camera metering will necessarily provide a wide aperture, slow shutter speed, and if Auto ISO, also a high ISO. Because that is what the ambient is, and these settings are suitable if using no flash, and the flash plays no factor in this.
You should do this quick simplest test to be sure you understand. With camera P mode (automatic aperture and shutter speed), and at ISO 100 (Not Auto ISO, so Not Auto mode), and normal sync mode (Not Slow sync or Rear Curtain sync), and with the flash unit initially turned off (or shut internal flash door), take any indoor picture where it is normal indoor brightness, not excessively bright. At ISO 100, the lens will be wide open, with a slow shutter from 1/4 second to 2 seconds (much more dim than bright sun). Then reach up and turn the flash on, and repeat. Still ISO 100, still near wide open (perhaps a stop less, P mode Maximum, previous page), but the shutter speed will be 1/60 second. This 1/60 second is Not metering anything, the ambient will be seriously underexposed, but we are using flash instead, and this is just an arbitrary system Minimum Shutter Speed with flash, simply because we are using flash, and don't need the excessively slow shutter.
The point is that the camera metering always sets choices for the ambient. Even if the ambient is unusable level, and we have no plans to try to use the ambient. TTL flash has its own metering system, and the automatic flash has to play into whatever settings situation it discovers present at shutter time, which then sets its own TTL flash power level accordingly.
If we are using flash, we can normally choose better settings in camera A or M modes (especially ISO, see more details below). Note a few important things we should know:
TTL modes set the power based on metering the TTL preflash. In brighter ambient (sunlight), the camera is already trying to give a proper exposure based on ambient. A proper TTL level and a proper ambient exposure is two proper exposures, which add to 2x proper exposure, to be one stop overexposure (of the near subject). TTL BL (i.e., Balanced flash) understands that and backs off on the flash power level, which is great outdoors in the sun, but indoors, while it reduces overexposure, but high ISO will force a picture mostly of ambient light (with a little bit of fill flash). Of course, we might have selected things better to favor the flash.
Repeating, speaking of a dim ambient, when the flash is not enabled, metering will of course give a rather slow shutter speed (which is why we are using flash). Reach up and enable the flash, and then the same scene will change to a 1/60 second shutter speed (the default, in dim ambient situations). This is NOT a metered value, it is Not about the flash exposure (flash is not sensitive to shutter speed, but continuous ambient is). The system just thinks that since you are using flash, you don't need slow (like 1/4 second, for example), and 1/60 will be better. The wide-open aperture will not change for flash. The high ISO may or may not change, depending on your camera model (but probably never for the internal flash). But a change from 1/4 to 1/60 second shutter will underexpose the ambient by four stops (extreme, but often a good thing). And normally we don't care, because we are providing flash instead. Often we prefer to use camera Manual mode indoors, so we can set an even faster shutter speed (1/200 second), to better keep the orange incandescent ambient out of the picture. The TTL flash is still always automatic flash exposure, even in camera M mode.
High ISO with flash indoors only has the (usually bad) effect of bringing the dim and orange ambient incandescent light way up brighter, to be the dominant light source, and be a real white balance problem.
The tiny internal flash may need high ISO to have any distance range, but for a hot shoe flash, I would turn Auto ISO off, and set ISO low. Bounce flash may need ISO 400, but rarely more. If using flash, then use the flash. Camera Auto mode will always be Auto ISO. Indoors, I'd use camera A or M mode, so I can turn Auto ISO off.
In camera mode A, the shutter speed is surely always 1/60 second indoors in dim ambient (Exceptions, speaking Nikon DSLR - unless Slow Sync or Rear Curtain sync - 1/60 Minimum is default in E2 menu of many Nikon DSLR). This 1/60 second is not about ambient and is not about flash. It's just an automation "It's OK, a Minimum shutter speed with flash, good enough, and really doesn't matter much". Flash is not affected by shutter speed, but you don't want to hand hold slower. So you will notice that all your indoor flash pictures in A mode are 1/60 second, regardless. It is a usable choice, but not really about anything, and not necessarily the best choice. If you want faster shutter speed in camera A mode, then you have to go outside into brighter ambient that will meter higher. But in camera M mode (where metering the too-dim ambient is unimportant), you can choose any shutter speed you want, either fast (1/200 second) to keep the orange incandescent light out, or slow (1/30 second) to allow some of it in. It becomes your choice. The TTL flash is still automatic flash in camera M mode.
Aperture is set if in A or M mode... Maybe around f/5.6 or f/8 for direct flash, and maybe f/5 for bounce flash. But whatever you choose.
Check the first flash shot, and if not perfect, simply just fix it, then and there. You're the photographer, and a bit of +EV or -EV flash compensation can work wonders. Fix the first shot in a series, and the rest are probably good (in the same situation). Simply do what you see is necessary. Why not? This is easy stuff that every photographer learns to deal with. If some cannot be bothered to try, then sorry, that is sad.
Bounce and direct flash are rather different situations for TTL metering. The TTL system does automatically compensate for the longer path of bounce (metering simply measures the preflash it sees that arrived via whatever route), and you will see the same reflected meter's averaged middle gray result for both (see How Light Meters Work.), but there are scene differences in bounce and direct flash.
The Inverse Square Law describes how light falls off with distance, therefore direct flash backgrounds are often dark (the room behind the subject). For example, a background that is as far behind the subject as the flash is in front (background is 2x farther than subject) will therefore be two stops darker than the subject (influencing metering, since dark scenes do overexpose). The TTL reflective meter sees and meters the preflash on scene that way too, sometimes lots of dark is seen, which then direct flash frequently overexposes a foreground subject as a result of the reflective meter helping in its way. So to direct flash, a distant background is seen like a black paper background, influencing exposure to be brighter. Also direct flash on near backgrounds can cause also dark shadows, generally hidden behind the subject, but in some cases are seen below, and metered by preflash too.
For bounce flash, most of the room is more nearly about the same distance from the ceiling (via flash path), so (within reason) the rest of the (normal size) room simply lights up. Preflash meters the scene that way too, not so dark back there. Less contrast, no dark shadows, no dark distant walls, which is generally easier to expose satisfactorily. However, then maybe white walls are well seen, which the reflective meter tends to underexpose due to the lighter objects seen.
Indoor flash, normally sees weak ambient, which is why we need flash. TTL automatic flash mode. Camera A mode is common, but it likely always uses 1/60 second shutter with flash indoors, This is just an arbitrary Minimum Shutter Speed with Flash, every time in every indoor situation. Which can be OK, but 1/60 second indoors is certainly NOT about any calculated exposure. So maybe better indoors, camera M mode lets you control shutter speed too (see below). The TTL flash is still automatic flash exposure in any camera mode. Bounce flash at every opportunity, hot shoe flash head aimed up, pull out bounce card out, standing under a 8 or 10 foot white ceiling. Don't stand too close for bounce flash, but zoom in all you wish. Set ISO 400 and f/5 (assuming a regular full-powered TTL speedlight), and have at it. With the automatic flash, this generally will work great for most reasonable situations.
My own strong preference for one decent-powered hot shoe flash indoors: I use TTL bounce flash and camera M mode at every opportunity (not always possible). Manual mode is no problem then since the TTL flash is still automatic exposure, and the indoor ambient is low and insignificant anyway. Typically f/5 and ISO 400 under ten foot ceilings. A full powered flash model probably can do f/7.1 (standing, under a ten foot ceiling), but f/5 provides a good safety margin for power, and a fast recycle. For flash, Manual mode is same as camera A mode, except it also allows my choice of high or low shutter speed, as I choose desirable (regarding orange colored incandescent ambient - to keep it out, or let a little in).
If my flash has a TTL/TTL BL menu choice, I choose TTL mode indoors. That menu is fairly rare though, so if no such menu choice, the TTL menu means TTL BL in the Nikon system. So then (indoors), I can choose camera Spot Metering to force the TTL BL system to be TTL. Indoors, this TTL shift is more or less the equivalent of near +1 EV flash compensation - do either one, but sometimes we might need both TTL and +1 for bounce, or maybe more. This is easy, you can see the image on your rear LCD screen, so just do what you see you have to do. Learn what you have to do.
Note that selecting Spot metering is NOT doing Spot metering here. The details are below and elsewhere here, but it is like this: The flash system does NOT use Spot Metering. The flash system uses its own central area to meter. Ambient does use Spot Metering, but with low ISO indoors, the ambient is low and insignificant and is well underexposed, so it has no effect (it's why we are using flash). Even if the weak ambient could have some effect, we are using camera Manual mode, so metering cannot change the camera settings anyway. The point is, at low ISO indoors, all Spot Metering can do is to switch the flash metering from TTL BL to be TTL mode (usually an increased better exposure indoors).
The only issue with using Spot metering THIS WAY, is that we forget to turn it off when we go back outside. That will get your attention, ambient will use it, and Spot can have strong effect in brighter ambient, and requires that you know what you are doing then. Also TTL flash is Not balanced with ambient like TTL BL is. An alternative is to usually add near about +1 EV flash compensation for a TTL BL flash indoors, which has no remaining effect after you turn the flash off.
Outdoor fill flash, normally sees very bright ambient. Use direct flash to fill dark harsh shadows in bright sun: (previous page) Strong ambient makes it be a different story. Hot shoe TTL direct flash, low ISO. If you use camera A or M mode, you have to realize that the bright sun will require aperture up around f/11 (ISO 100) or f/16 (ISO 200) to be compatible with the maximum sync speed. Camera P mode is smart here (it knows this, for both ambient and flash). Flash in bright ambient will require compensation (because a proper sunlight exposure and a proper flash exposure add to be 2x overexposure). Plus we want fill to be lower anyway. If the flash does not have an overt TTL BL vs TTL menu, then the Nikon system is TTL BL mode when in camera metering modes Matrix or Center Weighted. Significance is, TTL BL flash mode does automatic fill flash level compensation, but actual TTL mode will require about -2 EV manual flash compensation (in bright sun).
For every case, the major trick to know: Adjust flash compensation when and if and as you see it is needed. Get the result that you want.
The rest below is about the factors that might require attention.
It may seem that flash needs compensation more often than ambient, but flash is more intensely local, with more factors like distance, so the contrast varies more, but it is still about these same metering concepts, about the scene in front of the lens. Compensation is about the scene, and is NOT about calibrating the camera gear (although there are other factors too, like TTL vs. TTL BL).
Bottom line is, (camera reflective) metering, and also compensation, are entirely about the scene in front of the lens. We always need to watch the TTL results, and stand ready with Flash Compensation, to get the picture that we want. You being the photographer, this is your job to do. Simply realizing that this is your job is the first full step ahead.
Spot, Center Weighted, or Matrix metering choices only affect metering the ambient. The flash has its own system, which does not use Spot metering (said again, the flash exposure does not use Spot Metering, but the ambient metering does if selected). However, TTL BL "balances" the flash exposure with the ambient (TTL BL reduces flash level to be fill flash level, to not overexpose the sum with ambient - but TTL mode does not back off, it comes ahead on). But if Spot metering, only the small spot area matters (to ambient), so there is no concept of background ambient to balance the TTL BL flash with - so the flash system switches to TTL mode if ambient is doing Spot metering. Spot metering is a way to switch the flash to TTL mode - however it may also greatly affect your ambient if ambient is normally bright (and meterable).
Incorrect D-lens distance data (in wide zoom lenses) is one common cause of underexposure in TTL BL direct flash. D-lens data is only used by TTL BL direct flash. Bounce flash (tilting the flash head) does not use (ignores) the D-lens data. TTL mode does not use D-lens data. Commander remotes do not use the D-lens data. Or, if your camera model (those with Commander) has the FV Lock function, FV Lock will simply ignore the D-lens data, and will use the direct TTL BL metered and balanced value.
Serious Caution: A TTL BL non-tilted flash head (esp including an umbrella) on a hot shoe extension cord (Nikon SC-17, SC-28, SC-29) can seriously limit flash exposure with the D-lens data, when the TTL BL flash is at a very different distance than the lens. Nikon advises to switch hot shoe extension cord flashes to TTL mode (but they no longer provide the obvious way to do that). For a SB-700, that is done with Spot Metering (and again, the flash exposure itself does not use Spot Metering). Or an external flash can simply rotate the body 90 degrees to the side, and then rotate the flash head back around to the subject.
There are a few things that cause inconsistent metered exposures of TTL flash. If using manual flash mode, we control flash exposure ourself, but TTL is automation, and things can happen (not entirely point and shoot, but easily correctable, just watch results, and do what you see you need to do with flash compensation). These variations are due to the scene, or the camera metering/automation, and are Not normally caused by the flash unit itself. TTL and TTL BL are metering modes performed by the camera controlling the flash. Don't misunderstand - TTL metering is a very close starting point for flash exposure, same as it is great for ambient. However flash can be affected by a few more factors than affect ambient:
Note that TTL BL is the only mode available on many flashes (unless the flash menu offers both TTL and TTL BL choices). TTL mode on the Nikon camera internal flash, the Commander, the SB-700, the SB-300/400/500, and almost all third party flashes do TTL BL mode. The SB-600, SB-800, and SB-900/910 offer a choice.
So TTL mode assumes we are using flash because we need flash as the main light, whereas TTL BL mode will be reduced to fill flash in a bright situation. Bottom line, TTL BL indoors will often need some +EV flash compensation, commonly up to + 1EV. TTL mode, less so indoors. But TTL in bright ambient will need the opposite, up to -2 EV compensation in bright sunlight. At a simple level, we can say TTL mode is great for flash indoors (immune to many problems), and TTL BL is great for fill flash in bright sun (automatic self compensation).
However, two factors can apply (does the flash result vary with different lenses? It does.)
Spot Metering mode changes TTL BL metering mode to be TTL mode (because a Fill flash concept is not compatible with a Spot metering concept). Therefore no D-lens reduction and no fill flash reduction affects the flash. So Spot is sometimes intentionally desired with indoor flash, to achieve TTL metering mode so this becomes true.
Note that the flash system does Not actually do Spot metering. Matrix, Center Weighted, and Spot metering is about ambient. The flash system has its own metering system (not unlike Center). Spot mode certainly can change things for flash though, converting TTL BL metering to TTL mode.
Is that what's troublin' you?
There are a few possible factors of flash exposure, and it's normally Not about the flash, the camera does all of this. Before blaming the flash, we have this list of things to rule out first.
But really, this is no big deal. It's easy to watch flash results, and just always simply do what you see you need to do (flash compensation). And with just a little experience, we quickly learn to recognize a white background or subject, or a bright ambient, or a distant dark background behind direct flash, so we already know what to expect, and quickly learn to compensate in advance. "Experience" means "expect about the same as last time in a similar situation". Flash is pretty easy, but it does involve thinking about it a bit. Flash compensation is the tool used to control what automatic TTL flash does. The least we can do is to learn to watch our results, and then simply do what the situation needs (flash compensation). It is just how it works. If flash exposure is not right, simply fix it, then and there. Flash does have more reasons than ambient about why compensation is often necessary, but ambient has ample opportunity too. Only a beginner imagines his meter always ought to be right (see How Light Meters Work).
The actual fundamental about TTL BL mode is that the system reduces BL flash power in bright ambient (balanced fill flash in sunlight, automatic flash compensation), so as not to overexpose the sum of the two light sources (ambient and flash). In indoor situations, the ambient is normally much weaker than the flash, with less effect of any "sum". However, direct flash often suffers dark backgrounds, which flash metering sees, and tends to overexpose the subject. But the D lens distance info can be a safeguard, which can hold back TTL BL direct flash (can use Guide Number to protect direct flash against overexposure), as shown below (but D lens distance does not affect bounce flash).
To demonstrate some of this:
This picture below is black on black (does not reflect light well), intentionally chosen here to be a problem. We already knew when we first walked up that this scene needed at least -2EV compensation (but I didn't). We expect the reflective meter to overexpose this, trying to make the black appear middle tone, or gray. This is D800, SB-800 direct flash on hot shoe. 105 mm f/2.8 VR macro lens at five feet. All are ISO 100, f/8, 1/125 second. The idea is that this situation should be easy for anyone to duplicate same thing. We tend to believe it, and be able to use it, after we see it ourselves. :)
First, two examples above of a "correct" exposure", specifically meaning, neither of which methods are affected by reflective camera meters (both incident and GN readings are Not reflected, i.e., independent of the subject and its colors).
Left above: Flash power level was metered and set with a hand-held incident flash meter (SB-800, 1/16 + 1/3 stop power level for f/8). May be slightly overexposed, because the background paper ought to be very black. Incident meters meter the light source directly, and are not affected by the reflection of subject colors (see more).
Right above: Using flash GN flash mode (5.2 feet entered here), which depends only on distance, and which is not affected by the reflection of subject colors (see more). It is not actually underexposed, the same image at 100% crop at right shows it is just black. A correct exposure should show black to be black, and white to be white.
Reflective meters (in cameras) ARE affected by the reflectance of subject colors (see How Light Meters Work). So Left: TTL mode does dramatically overexpose black as expected, because the reflective meter is seeking a middle gray result. We might favor this brighter view sometimes, it could be desirable, but since the black paper background is actually very black, and this is not, it is greatly overexposed (at least 2.5 stops).
Right above: TTL BL mode sees this same black metering situation, but this TTL BL direct flash is held back by the D lens distance watch guard, which is unusually accurate for this lens, which says 1.5m, 4.92 feet (measured distance is 5 feet). The system says: "Whoa man, this metered value is simply excessive flash for the Guide Number at five feet" (like the left picture), so the system limits the TTL BL flash power (about same as GN flash mode then). Instead of being like TTL, it is the same as the GN picture above it (except slight distance value variations). This is the idea about the D lens, and this overexposure is not uncommon for direct flash in general, when metering often sees the dark background behind direct flash. So this is a good result, and when the D lens is accurate, this is very reasonable. However beware, the reported D lens distances are often quite wrong, especially for zoom lenses (see more). If a lens reports distance as less than it is, then the problem is that direct TTL BL flash is held back incorrectly, pictures are too dark.
This stuff happens. Both results are as expected. But both can be compensated. NOTE: I am NOT saying TTL will overexpose. I am saying reflective meters will overexpose Black.
In contrast, White scenes are routinely underexposed by reflective meters, but the D lens distance only tries to protect against overexposure. The meter has no clue what the subject is, or how it should be, but the TTL BL system can compare the direct flash metered value to the D-lens Guide Number expectation.
TTL Mode and TTL BL mode same as above, but both with FV Lock. FV Lock prevents D-lens distance from helping guard TTL BL direct flash against overexposure... when it is correct.
The only difference in procedure was that the FV Lock button was pressed before the shutter button, so that the early occurrence preflash exposure is locked and used. Camera models with Commander have the FV Lock feature.
Both are overexposed, which is expected due to the reflective camera meter always trying to bring black up to gray. White scenes are routinely underexposed, same reason, automation seeking a gray result. Many typical scenes do average out closer to middle tones, and do meter more correctly (which is how reflective meters work), but this scene is all black, and far from typical.
However, both are brighter with FV Lock (see TTL metering area, in your camera manual, FV Lock section). TTL mode only slightly different with FV Lock, but TTL BL is greatly brighter now, and same as TTL (which does no BL, no D lens correction). Remember, this is a special all black scene (to be difficult). It seems FV Lock obviously ignores protection by the D lens distance - no doubt because some of the FV Lock function is to allow you to aim the camera elsewhere after metering. So ignoring D lens distance is not unreasonable at all, and this can be good or bad in typical scenes for other purposes too, depending if D lens distance is wrong or correct. Not the only factor, but when TTL BL is underexposing, try FV Lock (especially if flash is on a hot shoe extension cord, not at same distance as the camera. That case ought to be TTL Mode anyway, which Spot metering can select).
All this black is NOT a typical subject, intentionally difficult, and the overexposure is expected (unless we compensate it). One point is that the D lens distance can protect TTL BL direct flash from this (when the D lens distance is actually correct). But a second point about FV Lock being rather different may not be expected.
So there are several ifs and buts possible regarding TTL exposure. Normal reflective metering issues always strongly apply. Always watch results and use Flash Compensation when it can help to correct flash TTL exposure. Flash Compensation is the magic key for any TTL flash.
If the flash has a TTL vs TTL BL menu, select TTL indoors. Disables balancing and D lens checks, which can correct problems. Flash models without the menu do TTL BL mode by default, and can be wonderful out in bright sun, but may tend to underexpose indoors. Flash compensation works, but two possible work-arounds for that case (TTL BL flash indoors) are:
The black shadow is my subject.
Biggies: iTTL is definitely a metered TTL system which meters a preflash. However, D lens distance info is also provided to protect TTL BL direct flash from overexposure, but not always correctly, especially not zoom lenses (see more). D lens distance only affects TTL BL direct flash, not bounce and not TTL. However, FV Lock bypasses that D lens property, and can significantly brighten both hot shoe TTL or TTL BL for bounce too, which is something we need to realize happens. Also, if using fill flash in bright sun, if FV Lock is used, then there is no visible evidence of TTL BL mode being "balanced". It becomes as bright as uncompensated TTL mode. FV Lock metering is NOT necessarily the same as would be metered without FV Lock.
Bottom line: Certainly the camera is NOT always right (for various reasons). Whatever the result, whatever the reason for it, regardless if we know the "why" or not, the best advice and only good procedure for flash is to watch your results, and stand ready to use Flash Compensation as seen needed. For manual flash, we just adjust the flash power level. For TTL flash, we use Flash Compensation on the camera. Watch, and just do what you see is needed. It is not difficult at all, and you have complete control.
There are two TTL modes (TTL and TTL BL, previous page), which meter differently.
Or instead, simple flash compensation works too. I routinely start near +1 EV indoors with TTL BL flash. Or, compensation is possibly not always needed if using Spot with camera M mode as above.
Did I mention that Flash Compensation is how we control TTL flash? Watch the results, and just do what you see you need to do, to get the result you want.
No matter which P, S, A, or M camera mode you select, TTL flash is always still automatic flash exposure (even with camera M mode). TTL flash automatically chooses the appropriate flash power level for whatever aperture you set, regardless of how you selected it. Camera M or A modes are very popular for flash indoors, but let's discuss why.
Aperture controls the flash power needed (camera M mode or any other mode), and therefore affects the flash recycle time. We set aperture for the flash power level (judged by recycle time). Trying to be very clear: Aperture and ISO certainly affect the flash exposure, so we do see the exposure of manual flash mode at a fixed power level change with aperture or ISO. We have to manually set that manual flash power level to be correct for the aperture or ISO. But automatic TTL flash mode will simply automatically react to aperture or ISO with same automatic "correct" TTL exposure, by using a different flash power level (automatically). So we may not see it change, may not realize it changed, but flash power level does change big time with aperture or ISO. But flash power is relatively limited, not infinite, so we must pay attention. Watch your Ready LED.
In camera M mode, we can set any shutter speed up to the cameras maximum shutter sync speed, or we can set it as slow as we want it to capture more of the dim room ambient light. The flash exposure does not care what shutter speed is, and will not change. The flash duration is fast, faster than maximum shutter sync speed, so therefore, actual shutter speed controls only the ambient room light, fast shutter to keep it out, or slow shutter to allow it in - our choice. Studio portraits always want maximum shutter sync speed to keep the ambient out (for example, the modeling lights). Family snapshots may want shutter slower, for the "warming" look of the incandescent room lights. Again, it is our choice. We do not have to accept 1/60 second. We can think about it instead. But as first beginning step in using camera M mode for indoor flash, we can always use 1/60 second and f/5 as a starting point, a home base. This is the same values P mode would have used, and it will do the same thing in camera Manual mode, and it usually should give a decent picture (remember, the TTL flash is always automatic flash exposure). But with camera Manual mode, there are other choices too. We can more optimally change aperture or shutter speed, for the specific purposes mentioned. A little experimentation will be very helpful.
The automatic flash in TTL mode gives automatic flash exposure in EVERY camera mode (including camera Manual mode). This is true for direct flash, bounce flash, flash in umbrellas, etc. Not to say it is always perfect, it's not. We often have to add a bit of flash compensation, and then it is perfect (How light meters work may be of interest). In automatic TTL flash mode ("Through The Lens" metering for flash exposure), the first thing that happens after the shutter button is a preflash. Some of that preflash reflects back into the camera lens, where is it metered (measured), and the flash power level is set to give the correct exposure for the lens aperture in effect at the moment. What varies in automatic TTL flash operation is the flash power level (the camera exposure settings remain constant, whatever we set, or whatever the ambient light meters). We can choose the lens aperture ourselves to determine the ballpark flash power level that will be required, but we always get correct TTL flash exposure, regardless of which aperture we choose (within the range of what the flash power can reasonably deliver). This means however, that A (Aperture) and M (Manual) modes are very important and useful for flash indoors, since the aperture setting controls the flash power (and recycle time).
Note: Flash compensation changes TTL flash power level, and it only applies to TTL flash, either on hot shoe or TTL remotes. Manual PC cord flash is affected by the flashes power level setting, but the camera cannot affect that. The camera has an Exposure Compensation which affects both the metered ambient and the TTL flash, and it has Flash Compensation with only affects the TTL flash. These two compensations add together for the TTL flash.