TTL Flash Compensation is below.
First, your best flash lighting will always be an off-camera flash. To prevent the flat direct look, we need the gradient shadow tones this angle creates, which shows shape of the subject (not flat). This is not bounce yet, but a minimal try for better speedlight lighting would be a flash on a coiled hot shoe cord, handheld high and outstretched, to be high and wide on the subject (this was the purpose of those coiled cords). This can make a big difference, but watch out for dark shadows cast on near background walls. For less harsh shadows (on the subject), we could add a fill light very near camera (could be the internal flash if the cord allows it to open), the fill set to be maybe -1.3 EV down (at the subject). Umbrellas are better (soft light), but bare flash works too, if off-camera. Another light is slightly more preparation, and a stand becomes a fixed location, but if you have never tried this, you are missing out big time.
For example, professional studio portrait setups will have the main light (the strongest light) similarly “high and wide” on the subject, with another fill light (about -1 EV to -1.5 EV less bright at the subject) fully frontal as a fill light. The fill lightens the harsh shadows, but does not eliminate them. See that setup. But short of that, bounce from the ceiling or a near wall (with a small pull out bounce card on the flash for frontal fill) is a very similar effect.
But for a hot shoe speedlight, bounce flash is definitely the good stuff. Off-camera flash can be good, but bounce allows walk-around shooting, and is the best you can do with one hot shoe flash. My opinion is that if using camera speedight flash indoors, our first thought ought to be to first look up to determine if bounce flash is possible (a white ceiling not too high is ideal). Or, it can be bounced from a wall or the ceiling. Some uses have been to aim the flash head back over the shoulder at the top of the back wall. Definitely do try bounce flash so that you know about it, and how to do it. It is one of the best tools, another way to mimic off-camera lighting. If you have not tried it, you're missing out big time. Tilting the flash head up, to bounce the flash from the white ceiling, improves hot shoe flash photography in at least three ways:
How much more flash power? Ceilings vary, both height and color, but the ceiling reflection surface likely does not reflect much more than 50% of the light, and that loss is near one more stop of power needed (in addition to any path length increase). One rough old rule of thumb was that bounce needs about two stops more power (4x more power) than direct flash of the same subject... because, if we assume a 45 degree path up and back down, that is 1.4x farther distance than direct, which is another one stop more power due to inverse square law. A 60 degree path (from horizontal, 30 degrees from vertical, an equilateral triangle) is 2x distance, which is two stops longer. Straight up flash (seems more common) may more likely need at least three stops more power than direct (depends on path — the ceiling and flash and subject heights, and subject distance). We always want more power for bounce, from a decently powerful hot shoe flash. The old rule of thumb of "two stops more" notion worked better in the old days, when negative film had much more latitude than digital, letting exposure get away with more in the darkroom.
Today, TTL automation makes it much easier. The modern digital camera TTL light meter simply measures whatever preflash light actually comes back (from the same path), and sets TTL flash power level appropriately. But bounce does need significantly more power, and we often start near f/4 and ISO 400, and assume the TTL flash has sufficient power capacity to deal with it. Usually does, but Manual flash mode will be near full power. Higher ISO and/or wider aperture can substitute for power, but we definitely want a full size speedlight for bounce, not a dinky little minimal flash. And a good f/2.8 lens is a very nice thing for flash too, better at f/4. f/4 is wide open on a f/4 lens, but not on a f/2.8 lens. Also in the darker places where we use flash, focusing at the brighter f/2.8 seems a plus too.
P.S. I should mention, normally I don't want to use 45 degree or even 60 degree bounce (on the ceiling), unless very special cases of greater distance. Routinely, it's maybe 75 degrees, or often 90 degrees, because a lower angle runs strong risk of unexpected forward direct spill. We do use the bounce card to provide a bit of that forward spill, but in a more controlled way.
With a hot shoe speedlight, we can often just tilt the flash head up, to bounce the flash onto the ceiling, or rotate it to bounce on a side or rear wall, flash aimed back over your shoulder maybe, up high. I certainly don't want to leave out walls, their level angles can be great lighting from the side, and the ceiling is not always possible. Just talking, but if nothing else, bouncing off of someones white shirt can work great too (a bit like an umbrella or reflector), the shirt is a lot larger than the tiny flash head, and it is an off-camera angle. Walls may be less likely to be white, and flash will be colored by wall color, but walls can offer better lighting angles, and can be closer than ceilings. However #3 is less applicable, the wall is on one side of the room. And of course, umbrellas are a similar (bounce) situation, but which can be positioned optimally, wherever we want them.
But speaking of ceilings and lighting the room, if the path goes additional distance up to the ceiling and back down, then the rest of the room simply lights up (if the room is not too large). The difference in distances (in the room) is simply a smaller percentage of the longer bounce light path (which also requires substantially more flash power). An alternative to more power is to reduce power requirements, with wider aperture, higher ISO, or closer subject distance. We said before that flash exposure could only be correct at one distance from the source, but the bounce light travels up to ceiling and back down, and most places in the room are in fact at about that same distance from the ceiling, measured that way... so the entire (small) room is nearly equally exposed, more or less. Not without limits, and bounce requires more flash power, but try it, it really works (if the room is not too large, or ceiling too high). Bounce flash has other great properties too (very soft light, Part 3 here), and is a wonderful effect at most opportunities with hot shoe flash. We can bounce on a rear or side white wall too, which creates soft light, and can be excellent lighting, but wall geometry does not have the same even illumination properties over the room distances that ceiling bounce has.
For a hot shoe flash, bounce ought to be the standard routine case whenever bounce is possible.
You should try this yourself. You won't understand until you do. Get up, set f/5 and ISO 400, and aim the flash head up at a white ceiling (pulling out the bounce card would be good too). Do everything else about the same as you always did. Your own results ought to strongly make the case.
One example of bounce: This snapshot is routine hot shoe bounce flash, sitting on the floor under a ten foot ceiling. ISO 400, f/5. The entire technique was merely aiming the TTL flash head up at ceiling, and using the small SB-800 pullout bounce card (adds catch light sparkle in eyes, necessary to add vitality, indicating something is inside there). TTL still meters bounce automatically and normally, but always watch any TTL for need of any necessary Flash Compensation. None was needed here, but it is never a surprise to need a bit (TTL BL mode is more likely to need a little, but it depends on the scene that you point the camera at). Just do what you see you need to do.
This is NOT flat frontal direct lighting. The point here is: Notice all of the shadows on the face? Around the eyes? Cheeks, on both sides of the face? In what crevices there are? Different Tonal gradients all over, all those different flesh tones which show shape and curves and roundness? Maybe easiest to see on the shirt, bright ridge on tops of folds, and the shadows inside? Shadows creating tonal variations... and showing shapes. Much more interesting and natural than flat frontal light, when not obliterated by intense flat frontal light (avoiding the “deer in the headlights” look is a good thing to do). This example may be slightly too much bounce card — not pulling the flashes’ pullout card fully out would be less direct fill, but less fill would add more contrast (darker shadows).
Basics: Direct flash is flat frontal light, creating no shadows for example on the subjects face. To make shadows offering tonal gradients requires an off camera light. The light is from a different direction than the lens sees, so it makes shadows that the lens sees, which sculpt and show the curves of shapes. Bounce from the overhead ceiling or from a side wall is essentially also off camera lighting. However, a frontal weaker light is then good as a fill light, frontal meaning near the lens to fill the same shadows the lens sees, to lighten dark shadows, but not strong enough to hide them. The weaker fill light intensity can add to control the darkness of the shadow gradients. The shadows (soft and filled, to be subtle) are slight tonal variations which cause the picture to approach a 3D look, much more than a flat 2D deer-in-the-headlights image. That's a very big thing. These are definite Goals, and very achievable. Again, there is more detailed on another page.
Learning: Beginners don't always recognize much of this yet in the lighting results, but the longer and harder we actually try to see, the more we learn to see (and then the more we cannot avoid noticing automatically). This is not difficult, it is all there to see, but it is NOT a skill learned overnight. Starting out, we do have to consciously think about “seeing”, and only then we start getting the idea about what we are trying to do, but had no clue about before. Creating and seeing the shadows is pretty much what lighting is all about. Try a shot like with this with no bounce card, with the card pulled halfway out, and another all the way out — and think about what you see. Note however, in contrast, if we want to hide such shadow differences, like a case of excessive wrinkles, you might prefer a stronger fill (closer to the main light level at the subject) from a much larger light (large umbrella), for softer more flat lighting. But lighting is actually about the shadows — those gradients of tonal shading caused by those shadows is the product of the picture. I don't necessarily mean any specific shadows, I just mean “pleasing” and “natural”, NOT abysmally flat or without detail. We must learn to look. It becomes obvious, when we learn what we’re looking for.
Seeing is not easy at first, we look but don’t yet see, don’t know what we’re looking for, and don’t yet realize that there is much more we don’t know to see yet. Maybe you’re faster than me, but I’ve been there and done that, learning things but spending too many years with no clue about lighting. But when we do ever realize there is more to it (when the light bulb comes on), then we can pay attention to what we see, and start learning. Equal lights on each side of subject for even lighting is NOT what you want, at least not for portraits. The purpose of your portrait lighting is to create these mild shadows. The trick is to start thinking about it. Finally actually being able to see it gives us the idea of what our lighting is trying to do. If you are a beginner, I am trying to point out that this idea of learning to actually SEE involves realizing and understanding what we look at. It is all right there in front of us, so looking is easy, obvious even, but seeing requires a little thought about what we see.
More about seeing — One point this linked page makes is this:
Lighting books are sometimes disappointing to novices, because they show pictures, and to get it, we have to look, and we have to see. So practice your seeing, which is much of the skill of lighting. They do say "photography is about light", however IMO, portrait lighting is more about the shadows (the gradient tonal variations). To recognize it or to control it, you have to be able to see it (meaning, an awareness to notice it and think about it). There is much known lighting information online, but seeing is an awareness which involves the thinking brain.
For this picture above, I merely aimed the flash head up, and pulled out the bounce card. A very casual snapshot. The overhead ceiling may not be the most optimum lighting angle, but it’s not bad either, way better than direct flash. It is a natural lighting, and there are certainly much worse things. Flat direct hot shoe flash is about the worst case.
See the depth of the lighting in the room behind? The distant background is illuminated, not dark like direct flash falloff would cause. The wall is more than twice the distance of the subject, it ought to be more than two stops down, but it isn't. We have sort of fooled the inverse square law — the overhead bounce is filling the room with light. The ceiling is sort of equal distant to all of it. So there are no dark "direct" shadows back there. The largest rooms will still be the same old problem (too large and far for flash), but normal rooms are pretty easy for bounce, mostly fully illuminated everywhere.
Or if the ceiling is not the best bounce chance (too high, or not close to white), try bouncing on a near wall beside you. Or behind you, flash aimed back over your shoulder (up high) at a rear wall is good too. This wall should be a light neutral color (very little color). Learn to correct white balance, and a white balance card will be very useful. Distance to this ceiling or wall can be an issue, and walls are often not white (can add a color cast, may need correction later). At extreme distances, more flash power, higher ISO, and/or wider aperture are big helps. If no suitable ceiling or wall, you're out of luck, unless you add an umbrella or another large reflector to bounce from. Bounce simply may not always be possible. Problem ceilings can include: Outdoors (none), gymnasiums (high), many restaurants (dark), etc. Look up and check. Some indoor problems can be easily solved by just moving slightly to a better ceiling area.
Importantly, the forward fill of the bounce card creates the catch lights in eyes. That’s a sparkle of vitality in there somewhere, a good thing.
Don't stand too close for portraits — For proper portrait perspective, stand back a bit, and instead zoom in if necessary. Stand back perhaps 7 or 8 feet from human subjects, and zoom in all you wish. For two reasons: a) to help prevent raccoon eye socket shadows from a steep overhead light bounce angle, and b) proper portrait perspective requires that much distance anyway (to avoid enlarging noses). A bounce card helps provide fill for this scene (and those eye sockets), just don't overdo it. A clear dome on a bounce flash is also intended to spill forward fill, but excessive IMO, can't reduce it... I like the card, it can be only partially extended. It does more than you might think.
This picture here was TOO CLOSE, only about 4 feet, and the nose is enlarged too much. Stepping back a bit can only help it, always at least six feet, and a couple more is better. She's too young to care, but ladies don't like that. You can SEE these results, so much of the skill is to watch your results to judge the desirable degree of this frontal fill. Every bit helps, so do think about it. Not so much flat fill as to wipe out all traces of your soft bounce gradient shadows. The only trick is to be able to actually "see" your results. You have control — you don't have to pull the bounce card all the way up (but it is often about right). Experiment some, see what is possible (seeing more situations is called experience).
105 mm was a popular "portrait lens" for full frame 35 mm film, simply because it forces standing back sufficiently to have a sufficiently sized head and shoulders shot. 70 mm DX (1.5x cropped APS sensor) is the same equivalent view of 105 mm Full Frame, and you don't want to be much closer than this. Zoom in all you want, but stand back just a bit (6 to 10 feet), not too close for bounce or for portraits. Note that portrait perspective is determined by where the camera stands... Perspective is NOT about the focal length, any lens can only see the same thing if standing in the same place. Focal length may determine where the camera stands, but perspective is determined only by where the camera stands.
Walls: Not about lighting, but while we're at it, bounce is typically indoors, a wide view likely shows some vertical lines of edges of walls and windows and doors and furniture. If your camera is aimed a little up or down, these vertical lines will lean, looking bad. You can see this in your viewfinder if you look for it first, so think to look first. The solution to keep everything straight is to keep the camera level, not aimed up or down. For kids on the floor, get down on the floor with them. The flat back of the camera should be kept vertical if possible, parallel with the wall (level, to keep the wall straight).
For the camera in A or M mode and a hot shoe flash in TTL mode — starting at f/4 at ISO 400 is relatively fail-safe power-wise (maybe ISO 800 for a smaller flash or a higher ceiling). For a subject at 6 to 8 feet, tilt the flash head almost straight up at the ceiling routinely for 8 or 10 foot white ceilings, even 12 feet at maximum power. Higher ceilings can work, but likely needs more ISO, but higher ISO means it is harder to keep the orange room lights out with shutter speed. Watch for the Ready LED blink warning about insufficient TTL flash power. For a more distant subject (more power yet), maybe the flash head is tilted slightly forward to aim it at a point on ceiling almost halfway to subject (but be aware of direct forward spill from a low angle).
Camera M mode is very popular for indoor flash (where the ambient is dim and insignificant). The TTL flash is still automatic flash exposure in any camera mode, even M manual. If we set the same f/5 aperture in camera M or A mode, the TTL flash automation will do the same thing in either, f/5 is f/5, and there is no difference to the flash or its automatic TTL exposure. All camera A mode does is set a shutter speed too, probably always an automatic 1/60 second Minimum when indoors with flash. This 1/60 second is a limit, not a correct exposure for the indoor ambient, which normally needs it to be much slower. Reach up and turn the flash off, and you'll see shutter drop to 1/15 or maybe 1/2 second (indoors). But since we are using flash anyway, this 1/60 is just a more reasonable Minimum shutter speed with flash than much slower (BTW, camera Slow Sync flash mode would override to let camera A mode use the slow shutter, actually metered for the ambient). However, if outdoors in daylight, the shutter is much faster (as metered in bright light, but limited with flash by Maximum sync speed), and yes, we need to match that ambient then (sunlight intensity is overwhelmingly important). But indoors, ambient is typically dim and insignificant and easily ignored, which is why we're using flash.
Manual camera mode can help with TTL flash indoors. The mode A normal 1/60 second can allow in some of the orange incandescent light. Flash is not affected by shutter speed (part 2), but the ambient room light is affected, so camera M mode simply allows us greater control to set shutter speed to any speed we choose, to control the ambient light (to allow more or less of the ambient into the picture, Part 4). The orange incandescent room light is likely insignificant at 1/200 second (which then is a virtually black picture if without flash, which can be a big White Balance plus when with flash). 1/200 second makes orange incandescent become 1.67 EV less significant than at the slower 1/60 second. Camera M mode lets us choose shutter speed with TTL flash. And the ambient becomes even more significant at high ISO (because high ISO turns the flash power down). If using high ISO to balance the incandescent room light too, then ambient becomes the main light, and the flash is fill (fill is maybe best if not bounce then), and then you will need a CTO filter on the flash, to be able to set Incandescent White Balance. Otherwise, to avoid highest ISO, you surely want to turn Auto ISO off for indoor TTL flash (Manual flash mode cannot work with Auto ISO).
For studio flash, where the lighting is carefully prepared, certainly Auto ISO must be off, and to avoid interference from the room lighting, good practice is to use low ISO and maximum shutter sync speed and maybe f/8 (to help keep out the continuous room light). That’s a good plan for flash bounce flash too, except speedlight bounce power likely needs f/4.
In general, a fast shutter speed (maximum sync speed) solves so much for indoor flash, and camera M mode can provide that. However, some people like a somewhat slower shutter speed, maybe 1/60 second, to intentionally pick up a little bit of the warming orange incandescent room light. Sometimes we tend to like the warming effect, if not overdone. Camera M mode allows control of ambient with shutter speed.
This picture above was ISO 400, f/5 1/160 second, 70 mm on 24-70 mm lens on a 1.5x crop sensor Nikon D300 in M mode (SB-800 flash TTL mode) I was sitting on the floor under a ten foot ceiling, so it was close to full flash power. The 1/160 second is arbitrary in some degree — the flash exposure isn’t affected by shutter speed (Part 2), but shutter speed affects any ambient light you want to let in or keep out. A fast shutter with lower ISO keeps it out (Part 4). A reasonably lowest ISO for bounce flash is likely ISO 400, and choices are always possible, but for that reason, for indoor TTL flash, I love camera M mode with maximum shutter sync speed to shut out any orange incandescent room light.
Note there are two different concepts called a bounce card. One type is a fairly large scoop to block the ceiling, and redirect most or all of the flash forward. Macho guys may like the big ones, but which is pretty much just direct flat frontal flash in lieu of ceiling bounce — larger than the flash head, but it is no umbrella. The bounce card discussed here is instead a small card allowing the light to continue up unimpeded to the ceiling, which still does the bounce on the ceiling from an off-camera angle as the major light source. More finesse involved here. The small card adds a little direct frontal fill (filling whatever shadows it sees, shadows a bit lighter, less dark and harsh), and the card adds catch light sparkle in the eyes, but the bounce still does the lighting and makes the shadows.
Use a small white bounce card, for example, a pullout bounce card. If none, there is no need to buy anything. It is trivial to make one from thin white cardboard or paper and a rubber band. That will work as well as anything. I would suggest starting at about 2x3 inches (maybe a business card), with about half of it above the flash head. A bounce card has two purposes, to add a slight forward direct flat fill, and it also adds wonderful catch light reflections in the eyes (part 3c). Adding the sparkle of catch lights in the eyes is one of the major purposes of the bounce card. The vertical angle shown here is typically about right.
The purpose of diffusion domes is to do this forward spill too, for fill and add the catch lights too. But usually a flood of forward spill that's too much, and IMO, more finesse is a good thing. The dome is too tiny to have any effect diffusing the light for softening — it just spills light in every direction away from the subject, and reduces intensity of the bounce. We like to imagine the dome reflects back from all the room walls in all directions, but the inverse square law distances usually make that be just wishful thinking (is the ceiling closer than the walls?) Look at your actual shadow results carefully. Can you identify any side shadows from the walls all around? This is the time to believe what we can actually see happens (what you can see is what counts in pictures). Direct forward light is flat (even). It does not make shadows ON the subject, it makes them behind the subject. The shadows we can see on the subject are from the bounce, and NOT from the forward spill... that angle is same as the lens, and frontal fill is flat and shadowless on the subject. It fills (lightens) the bounce shadows, but most of which should remain, hopefully.
Don't overdo the size of the bounce card. It is adjustable, and you can always use less of it. You watch your shadow results to judge this. Big is not better here. As a rule, some is good, but less is more. Don't overwhelm your nice soft bounce lighting by overpowering with flat forward fill. Look at your results, think about what you "see" in your results. Identify your shadows, and the light source of those shadows. You should be able to see evidence of your bounce shadows, that is why you did it. The exception about big: If you have no bounce possibility, and only have direct flash, then big is the whole idea, like an umbrella. But with bounce, bounce is the whole idea (the ceiling is big itself). The picture of girl above used the SB-800 pullout bounce card, perhaps a bit too much fill there, but overall it is a fine size (its effective maximum area above flash head is about 1.75 x 1.5 inches, WxH). Note that we do not have to pull it all the way out (I normally do, unless a close subject). It is more than sufficient, probably too much up close. Experiment a little, to learn to see what it does. You should know what it does, so you can control what you want it to do.
How large a card? I can't answer exactly, but the built in card works really well. Bounce card size is related to the direct flash distance, and to the distance to ceiling and back down (and reflection loss there). Close direct distance increases effect of the card, and far ceiling height reduces the bounce illumination. We are trying to balance these, with the card being half or less of the ceiling illumination. Extreme direct distance possibly may need a little more card, but less than you might imagine, because the bounce path is also longer and weaker there too. Even then, even for groups, the pullout card seems a great size. Never swamp the bounce with fill. Pay attention, watch your results, do what you see you need to do, practice makes perfect, yada yada.
Rotating camera up on end for portrait orientation: If direct flash, this makes terrible shadows visible behind and at the side of the subject. If bounce, the bounce card cannot be aimed forward when rotated to the side of the camera. Event photographers use "flash brackets" to cure this, allowing rotating the flash back around to still remain directly ABOVE the lens in all cases (a good example). Direct shadows are largely hidden behind the subject then. For bounce flash, this also aims the pullout card forward normally. Or, the bounce can still work without the flash bracket, using a small business card attached with rubber band on the side of the flash then, so that the card can still aim forward from the side. The only real card problem is when aiming the flash backwards to a rear wall (no card then, no eye catchlights). A rubber band works fine, but the black bands shown here are very inexpensive kids wrist bands (perhaps three for a dollar?), a little like a heavy rubber band, just the right size and stretchy enough, and the professional black version is shown.
I admit to having done stunts like this in the past, maybe everyone has. This is cut from a white Fun Foam sheet, held with a hard rubber kids toy bracelet. The pullout card is also shown extended here only for a size reference (not otherwise used here). The concept is called a "Better Bounce Card" and its popular on the internet. You can even buy them, or variations.
"Better" is arguable however. At least for bounce, it puts out way too much forward spill, and one opinion is that it negates benefit of the bounce. It can improve a direct flash shot, since we can argue it is a little larger than the direct flash head, but not much. But it generally negates bounce, with direct flash overpowering the bounce contribution. Novices tend to like anything cute that makes light, and it does, but hopefully they will soon advance to the point of actually looking and seeing and judging the gradient tones they see created in the image (which is better than the flat frontal light). The gradient tonal shading is all important.
Anyway, bottom line, if you overdo the forward spill (fill), you simply overpower the bounce with flat frontal light, and you are back to flat direct flash. If you cannot see your bounce shadows, you're probably wasting it. Why bother to spend power to bounce then? Pay attention to (study) the shadows you are trying to create, and trying to partially fill. I don't necessarily mean we intend to create specific shadows some certain way. I just mean we want some shading, and we want to control their fill and softness. We want their effect. Just saying, Learn to look and see if your goals were accomplished. Realizing we can see is the first step. We all started blind, but once we wake up and look, and think, and see, then we are on our way.
Bounce is a grand start. Maybe my snapshot above is not a Rembrandt, but I've seen worse, and bounce offers a lot of reward for simply pointing the flash head up. If you don't bounce yet, you are missing out on a really big thing. Bounce does need more flash power, but it is large soft lighting (creating mild shadows, not dark and sharp edged. See Part 3 about soft).
Flash Compensation is the extremely important tool we use to adjust TTL flash exposure results. Manual flash power mode is easy for bounce too, just adjust the manual flash power level as seen needed, but for TTL flash mode, Flash Compensation is the control for that same flash tweaking (as seen needed). For direct flash and for bounce flash, and for on-camera flash and for off-camera flash. TTL flash is automated metering, but might not always be precisely correct (because different scenes meter differently). Flash Compensation is easy, and the correct result is a BIG deal. Always keep compensation in mind, and use it when necessary. Often TTL is pretty close, but sometimes it is not. If there were only ONE tip about using TTL flash, it would be Flash Compensation! This is quite easy to do, simply watch results, and do what you see you need to do. Simply make it look like you want it to look. Moaning and complaining never helps. Instead, simply just fix it! You are the photographer, this is your job, and this is the difference between Wow and Ugh. If your result is Ugh, then simply change flash compensation to make it be a Wow. Flash too bright? Use -EV Flash Compensation. Flash too dim? Use +EV Flash Compensation. If it requires a little extra to look good, why not do it? This really is the whole point, and it is the photographer's choice, and your action is the only way it will happen. It will be about like the same situation last time, soon you will likely already know about what it will need. You will become real good at this real fast, you only need the realization that it is you who actually has to do this. So do it. Do it. Do it. Hoping to get your attention. :)
Manual flash mode: Adjust the manual flash power as seen needed. If result is too dark, give it a bit more power level, and vice versa. Auto ISO must be OFF with Manual flash, because Manual flash power cannot react to ISO changes (the ambient exposure does react). To help this, Auto ISO should stay at Minimum ISO if the camera recognizes a manual flash is present. And Manual flash cannot react to Flash Compensation and/or Exposure Compensation controls. The Manual flash power is what you set it to be, as you judge it, and is adjusted manually. Or you could meter the flash with a hand-held light meter, and set the camera settings accordingly for it (or change the flash power until it meters as desired). Except for HSS mode, shutter speed does Not affect flash exposure if not exceeding Maximum Sync Speed.
TTL flash mode: The control named Flash Compensation is the way to affect the TTL flash exposure, to be more or less flash exposure as seen needed. TTL flash works into whatever Auto camera settings were set by the ambient metering, and then TTL flash power is metered separately for that current camera setting situation. Therefore changing f/stop or ISO or shutter speed can affect the contribution from ambient light (if any), but it will NOT affect the exposure of the TTL flash (TTL will simply adjust power for the new situation, assuming it has sufficient power to comply).
There are two camera controls, Exposure Compensation and Flash Compensation. Exposure Compensation affects the ambient metering (which controls the Auto settings), and Flash Compensation affects the separate TTL flash metering. However, most Nikon models add the Exposure Compensation value to the Flash Compensation value, so that both affect flash (the current advanced Nikon models do have a menu E3 to optionally separate them). I think Canon keeps these two separate, and there are pros and cons either way. Adding them allows making only one exposure adjustment for the TTL BL balanced situation, maintaining the balance of flash vs. ambient. Controlling them individually requires separation.
Speaking of Nikon (which I use and know), if you want to affect flash only, use Flash Compensation. If there is no significant ambient level, then it matters much less which (and Exposure Compensation has more range than Flash Compensation). If you see this icon in the viewfinder or Info display , then you do have some flash compensation turned on somewhere. It could be in Flash Compensation, Exposure Compensation, or some hot shoe TTL flashes themselves have a flash compensation control. Flash Compensation and this icon adds them all numerically. It seems less confusing to standardize on only one place to set it.
At least one time, do experiment in tests (camera fixed motionless on a tripod for same scene and metering), and apply different levels of Flash Compensation, a few intentional different levels around the right level, a few third stop increments, both more flash and less flash than the "right" result, to be sure you see and understand and realize the control and range that you have. Do this both indoors, and as fill flash in bright sun (of a person turned away from the sun). As the sole light source, we need to get it about right, but as a fill light, we have choices, preferences (called lighting ratio). Seriously, this experimenting practice is how we learn, doing and seeing and understanding.
Some users of flash pick up this compensation thing quickly and take off running with it. It is not at all difficult, if you want a brighter flash result, just add a bit of +EV flash compensation as seen useful (or vice versa, with -EV). Yet, some others never seem to get it, or can't be bothered to get it right. I think the difference is our attitude, the willingness to step in and take charge ourselves, to affect our outcome. We are the only ones who can do this, and certainly for basic hot shoe TTL flash, that dividing line is at those willing to consider using Flash Compensation as necessary (if using manual flash, we just adjust the manual power level as needed, same thing — TTL is just a closer starting point, often very close). The key is willingness to simply look at the result we are getting, and then to stop and fix it (in real time, then and there). It seems it ought to be the easiest obvious thing, to simply do what we see we need to do, but some seem not to grasp this, or can’t be bothered. We do have to try just a little, but then it becomes very easy, and rewarding. It can be as good as you want it.
Actually in fact, I find very little need to use Flash Compensation now, because I shoot Raw, and it’s so easy to just slightly tweak exposure in the Raw processing, when and if necessary.
Flash Compensation is NOT a constant, NOT set one time and forgotten. We are NOT calibrating the camera or flash unit. We are compensating the specific scene in front of the camera now, that scene which TTL meters. To say the obvious bluntly, only novices naively assume the camera metering ought to always be right. But alas, real life is certainly not like that, reflected meters actually have little clue (See How Light Meters Work, stuff we must know). Different scenes meter differently, however some scenes are similar, often once compensation is checked, we can walk about in the same room, bouncing without much concern, but every different scene is a new situation. Not all situations require Flash Compensation, but Flash Compensation should be reconsidered in every new situation. Just notice your results! With a bit of practice, soon you just pretty much know what to expect before the first shot.
The TTL flash automation will meter the scene and set the flash power required, for bounce too. ISO 400 f/4 is a fairly fail-safe TTL bounce starting point, usually adequate power is available in most situations. If under lower ceilings, it may be possible for bounce to use ISO 200 and/or f/5.6 (both together may barely even work on a ten foot ceiling at maximum power level, if standing). How to judge this? Always watch the flash Ready LED, how long it takes to be ready again. A quick or immediate recycle time means low flash power was used, so there was more power available for you to use. But a relatively long recycle time (1 or 2 seconds, depending on flash and batteries) means you may be at risk, since you are working near maximum available power. You can watch the Ready LED once at full Manual power to see how long it takes.
On Nikons, a flashing Ready LED (if Nikon blinks three times immediately after exposure) means the flash was at maximum power level, which likely was insufficient power for the situation, and underexposure) for TTL. Greater Flash Compensation cannot produce more flash power if at maximum, the only solution is wider aperture or higher ISO (or perhaps closer distance, or a larger flash). You do want some margin on power, and we do like faster recycles, but bounce is often working near the high power end. Read and understand Nikon SB-600 manual page 29, SB-700 page C-5, SB-800 page 33, SB-900 page D-4, or SB-910 page C-4 about the insufficient power warning. ISO 400 and f/4 are reasonably fail safe beginning values for bounce in most (reasonable) situations, a good starting point. But if recycle is very fast, it means you are using low power, and might be able to use more power, like f/5.6 and/or ISO 200. Judge the TTL power used by the recycle speed — low power is quick recycle, and high power takes a couple of seconds. If you get the blinking Ready indication about insufficient TTL power, then you were at maximum power, which might have been near sufficient, but the situation likely needs wider aperture or higher ISO, or closer distance.
Flash is easy, but it works a little different than you may be used to. There are three aspects, the physical "lighting", and the exposure, and the white balance. The way you arrange the lights controls the lighting (the shadows). You can aim the flash to create different lighting situations, direct or bounce or umbrellas, etc, your choice. It is not magic, you control it (at least you should). Flash is just a light which helps illuminate your subject. Then you simply have to get the exposure right. TTL and then added Flash Compensation make this easy. Then the proper White Balance just makes the picture snap in as "right". But it is just a light, and an exposure. Treat it that way, and it's easy. Just watch your result, think about it, and make changes to improve that result. Flash Compensation is how you control TTL exposure. Only novices assume the camera always can meter it accurately. More experienced users treat Flash Compensation as an Essential Basic Tool, not always required, but always considered.