Nikon's automatic wireless remote speedlight system makes it phenomenally easy and convenient to use a couple of remote flashes (like Main light and Fill light). Its Commander allows easy off-camera remote flash, and it provides an automatic point & shoot wireless remote TTL multiple flash system. Automatic setup is surely the largest factor for many people. Just set a flash or two out there and they can work. Immediate automatic TTL multiple flash setup, vs. tedious manual setup. However manual flash does offer absolute and full control (no automation). The commander system can also be used as a wireless triggering system for manual flash power levels (which is NOT to be confused with Manual flash mode - it is still very much Commander/Remote operation, and incompatible with much manual flash gear, such as optical slaves).
Basic camera models like D40, D3100, D5100, do not include a commander, but higher models like D90, D300, D600, D700, D7000 do (in the internal flash). If the camera does not have a built-in Commander, optional Commanders (SU-800 or SB-800, SB-700, SB-900 flashes) could be added to the hot shoe of all CLS iTTL models, giving most AWL features (all except one, no FV Lock in those cameras without Commander). Here is Nikon's Quick Guide for AWL setup instructions, with the menus of the various compatible gear.
Note that in this remote wireless mode, there are no flash exposure controls set on the flash body. The remotes are controlled from the Commander menu on the camera.
The Commander works well with one remote flash, but the system is at its best with two remote flashes (to be Main and Fill lights).
The idea is this: Using the Nikon Commander/Remote system, set the menu of the Master unit on-camera to Commander, and set each remote flash to Remote mode. Normally, you would set one remote flash to be in group A, and one flash to be in group B (so the commander can individually control them). Commander Group B refers to the remote flash(es) you set to be in Group B. You throw the two lights out there somewhere (a little more care and planing is certainly a good thing pictorially, but the system doesn't care what you do). Be sure to rotate the remote flash bodies so that their side sensor is aimed at the commander on camera (in a line of sight path).
The Commander MUST be on-camera (for communication), either the internal flash with Commander, or a speedlight with Commander on hot shoe, or connected to the hot shoe with a SC-28 type of hot shoe extension cord. Then the commander will individually preflash and meter each Remote flash group, and will set the individual group power levels so that they each meter the same intensity at the subject, regardless of their distances, or bounce, or modifiers. If using the internal flash or speedlight commander, this can include its flash too, if it is set to contribute in TTL mode (or it can be disabled with its Mode "- -" in Commander menu).
You can also set your lighting ratio in the commander menu, and the system will do that for you too - to make one flash be intentionally stronger than the other, for main light and fill light concept. The preflashes take a split second, activated by the shutter button, just before the shutter opens. This is immediate automatic wireless remote point & shoot TTL multiple flash setup, and it is an Awesome capability. But like all automation, not always extremely versatile. Certainly very convenient, but automation is never complete control - sometimes TTL can be a surprise - any camera metering is simply not always precise (reflective meters depend on the brightness of the colors of the subject and background), but we always have Flash Compensation to fix it. The main skill to be developed for TTL flash is always the Flash Compensation button - don't hesitate to use it, always consider using it.
A rose by any other name... FWIW, people mistakenly call this Commander wireless remote trigger feature Nikon CLS, but all features of current Nikon flashes are CLS. The Commander remote feature is technically named Advanced Wireless Lighting (AWL), but I even added CLS to the title headers here, because people do say CLS when they mean AWL. But CLS has a different meaning. CLS (Creative Lighting System) is the name of the current Nikon flash system, and specifically, CLS is Nikon's communication system between camera and flash. Basically, CLS provides all of the features like iTTL, AWL, FV Lock, Flash Color Information Communication, Auto FP High Speed Sync, and AF Assist illuminator (see page 5 of SB-600 or SB-800 manual, or page B3 of SB-900 manual, for this description). This article is about the Advanced Wireless Lighting option (AWL), which is the CLS wireless remote feature using Commander, including iTTL. I normally prefer to call it the Commander/Remote system, since the term AWL seems less well known. The term that indicates third party flash TTL compatibility with Nikon is "iTTL".
"But my popup flash still flashes" is a common first question. Used as a Commander, it must still flash, in its way. The popup flash is also the commander, which must flash commands to the remotes (before the shutter opens). The internal flash can be another contributing group in the lighting, but we normally disable it. This is done in the Commander menu by setting the Internal Group Mode to be "- -", which turns it off to disable its contribution to the picture lighting (after the shutter opens). The commanders (except the SU-800, which is commander-only) include two functions, Commander and Flash. The pop-up camera flash for example, can be another regular flash unit (group) during the shutter exposure, and as Commander, it also flashes command signals to the remotes - which occur before the shutter opens. These commanders include a menu to disable the internal flash (with mode "- -") , which is usually done, to prevent the on-camera flash from contributing to the exposure. And it does prevent it, however, we humans will always still see the flash continuing to flash command signals but which occur slightly before shutter actually opens. To better understand this, we can experiment with Rear Curtain sync, and a long one second shutter speed once, to see when the flashing occurs with respect to shutter being open. There can still be issues with this flashing causing the subject to blink, solved with FV Lock (next page). Note that Remote mode has no Standby mode, it is always listening and ready. If you see Standby in Remote mode, it means your batteries are dead.
The images below are quickly done samples, to show automatic point & shoot TTL operation. No deep point here, just showing what the system does if we just throw a couple of lights out there. We put one light in Group A and one in Group B, so the Commander can control them individually, and the TTL system takes care of it. All results below use Commander TTL (except the pictures showing setup). The point I make is that the system does a lot of automation. These lights were intentionally sloppily thrown out there, intentionally at different distances here, just to show it works. Better planning is surely always a good thing, there is more to lighting, but the TTL system can in fact tend to many details. TTL exposure correction might sometimes be necessary, so simply just watch, and use whatever Flash Compensation that seems necessary to make it perfect (none used here this time, which was a little surprise).
This exercise uses the D300 internal flash as Commander, and two remote SB-800 flashes in two 45 inch Smith Victor reflected white umbrellas. The horse is metal, and this background is a reflective gold paper. One point of choosing the horse is to cast a clear background shadow, to judge flash intensity and softness. The dark harsh shadows were intentionally included to aid showing that the automation sets the flash exposures to be equal at the subject.
Your picture need not be a tabletop scene like this... it might be a portrait, or a group, or a macro, or any picture where you would use flash.
The point shown here is that the Commander system TTL automatically sets the two groups of flashes to be equal at the subject, for effortless automatic point & shoot wireless remote multiple TTL flash. The umbrellas are not required, but this tries to also show how easily the umbrellas convert the harsh dark shadows to be very soft light, with only vague hints of shadows. All you have to do to achieve this is to open the umbrella and set them close. Everyone can do this, and it really matters (when the subject is stationary, and not running around the room). Umbrellas are much of the point of remote flash.
Other things you should notice in the pictures: (we do have to look at this stuff... to see what is happening):
First row - left picture, shadows, far legs, rear of neck, front of neck, and of course the background shadow - as compared to the umbrella next to it. See how the large umbrella wraps soft light around the subject? The neck, and the far legs, are in shadow, or not... You have learned to see these shadows when you look, right? Umbrellas are magic (and are trivially easy).
Second row - greater distance from light is a darker sharper shadow. Up close is important for umbrellas, to make the light appear larger, to better allow it to wrap around the subject more.
But the main point here is that the commander system automatically meters and sets the left and right lights (in different groups) to be equal at the subject, even though the light's distance ratio was more than a one f-stop difference.
Direct - Only Left light turned on - at 64 inches
Umbrella at 60 inches - Only Left light turned on
Direct - Both lights above. Note that this is NOT actual Main/Fill lighting, but is instead simply two lights, either side of camera. (Proper "fill" is normally located very near the lens axis, to light all the shadows the lens sees, without adding a second set of shadows.)
Umbrellas - Both lights above.
Two lights are often good, but two sets of shadows are never good. Frankly, the single close umbrella seems better than two direct flash.
The point here is that the shadows above show the left and right TTL flashes are set to equal meter equally at the subject, regardless of their distance (or size or modifier, or whatever). This is what the Commander does.
The lights here are intentionally set at different distances, and wide spaced to show two shadows, to show that the Commander TTL system sets each group to be equal intensity at the subject (assuming the lights are in different groups to allow individual control). The purpose of including the bad shadows here is to show that the two lights were automatically adjusted to equal intensity at the subject. The two distances are more than one f-stop different, so the TTL system meters the far one, and in this case, turns its power up more than 2x brighter, to be equal at the subject... automatically, not requiring our attention.
One umbrella's sheer size tends to fill its own shadows and make the light soft (in large degree, if umbrella is close). Note the fundamental basic principle is obviously true, that the close umbrella is softer, creating vague indistinct shadows (the closer, then the larger, then the better). The distant umbrella is less soft, causing darker sharper shadows. And two lights let one fill the other ones shadow.
Note that you can still control the lighting. If the lights are in different groups (A or B or C), then the Commander TTL system attempts to make both lights equal at the subject, regardless of distance or modifiers. So this is an even lighting, and a bit flat. But there is some control possible, next. Camera Flash Compensation applies to every flash, but you can specify individual flash group compensation, to be more or less light, individually, in the Commander menu.
Umbrellas - Both lights on. Right fill light compensated -1 EV in Commander menu, to be slightly less flat (just this one picture).
This is simply two lights either side (not technically a main/fill relationship), and the point is not that this is considered the best lighting principles. A fill light is normally placed close to the lens axis, specifically so it will not make a second shadow. So this is just a quick example of two lights set up hastily, hoping to show that the system is very easy.
Commander menu: Note that you can set the mode for the built-in flash group to be "- -", which disables the internal flash. It is still the commander, and it will still flash commands before the shutter opens, and its flashing won't look much different to you, but then it will stop when the shutter opens, and will not contribute lighting to the picture (if disabled with mode "- -")
This is the -1 EV compensation of the right fill light (for Group B). You control the lights from this camera Commander menu. You can adjust your lighting ratio here (as shown), so the Commander will do that too. Group B of course refers to whichever light you placed in Group B. I set the right light to be Group B here.
Hot shoe flash. The previous off camera flash is a less flat, more interesting light, and just about as easy as hot shoe. Either way, direct flash is always sharp dark shadows, but bounce or umbrellas are soft vague shadows.
Umbrella setup - always located reasonably close to subject. These are intentionally at haphazard different distances just to show it works. Unimportant to be same distance with Commander, but close allows umbrellas to appear larger, to work better (to be more soft), and also needs less flash power.
Either way, direct or umbrellas, Commander provides automatic point & shoot wireless remote multiple TTL flash.
Here is something slightly more serious... just as easy, but at least I actually arranged the subject and the lights a little bit, took only a few seconds. The orchid is rotated for best view, and the umbrellas are pretty much in front and close. You can control the lighting ratio in the commander menu. Otherwise, it is point & shoot TTL. Really, this is incredibly easy.
Technically, this is just two lights either side of camera. It is NOT Main and Fill light. A Main light is normally high and wide, and a Fill light is normally very close to lens axis (to fill without making a second set of shadows).
We must keep light off of black backgrounds, but velvet is different and magic. I had background separation here, and this is cheaper velveteen cloth, which is OK here. High quality black dress velvet is best (from fabric store), it is jet black even with zero separation from subject. If you will set the object on the velvet, then definitely get a yard of the good stuff (dress velvet). I use masking tape around my fingers to pick up the dust specs which might show in the light.
These below are Commander, and two SB-800 in two Smith Victor 45 inch umbrellas. D300, f/11 1/200 second, ISO 200
Left umbrella only
The goal was not about this being great lighting. These lights are merely in front of the subject, each side of camera, without much thought.
Right umbrella only
The goal is to see the differences contributed by the individual lights, and to see that carried over into the "both lights" lighting.
Both lights, with right compensated -1 EV for ratio
This is easy, just playing around, try it. Basically we just set a couple of lights out there (one light to a group for individual control), and the TTL system meters them, and sets their power levels to give equal results. You can also add compensation of one in Commander menu to set your lighting ratio (maybe fill at -1 EV). And there is nothing to keep you from thinking about your lighting a bit too, choosing your setup and watching the results. The camera's Flash Compensation button is how you control your TTL flash results, how you adjust what the automation does. All Commander examples on this page used TTL metering of two flashes in two groups, and all here used +1 EV Flash Compensation.
I get enthused and excited, because the Commander system is so impressive in its way, within its limitations (summary below). Remote flash allows using umbrellas, which in fixed studio situations, just provide so much more. So let me try to show you one more.
OK, far out now. This next is a silly setup - a couple of similar little white statues quickly set up so that each statue is nearly 6 feet from their respective umbrella fabric apex (which is not "close"). But far apart, statues are about 8 and 10 feet from the other umbrella, one side more than the other, to create an unequal lighting situation (intentionally clumsy). Both umbrellas are aimed more in the middle. The light at right is more in front of its statue, the other less so, more from the side. This was about the only attention given, and then only so I could tell you.
Make no mistake, what is shown here is not goals, this is Not great lighting. This is just a very quick intentionally careless setup, to show specific points. The point was, with respect to each statue, this lighting can be said to be unplanned and random (could say haphazard, some lights just thrown out there), and the two statues are two very different lighting situations, both silly, the bright light close to one, the dimmer light (fill ratio) closer to the other.
No, this really does not always have to be done the worst way, and I certainly am not advocating crummy lighting, and yes, honest, a little study of lighting is a very good and useful thing. But here I am only showing what the Commander system can do to deal with multiple point & shoot flash. It seems a mighty good thing, and real easy to do.
Just showing the setup. D300, ISO 200, 1/200 second f/5.6 (near maximum flash power at this distance), and using internal flash commander. Internal flash is disabled with mode "- -" (shown above). Two SB-800 flashes zoomed 24mm, in 45 inch white umbrellas, not very close, not like if actually planning to photograph a tabletop statue. Camera Flash Compensation set to +1 EV (commander TTL often needs it). Light at camera left (group A) set to -2 EV compensation in commander menu, to be weaker fill (allowing some shadow tones, so the subject is not so flat and boring). Light at camera right is group B, 0 EV. Different groups were used of course, for the individual light control. All settings are the same the same in both pictures below, everything except where the camera is pointed (where the TTL is metering). The two sheets of red paper are not quite the same identical color in real life, but close.
Note I used center metering, and the TTL is adjusting both lights for the spot at A now - this picture was not concerned with either statue - Speaking of THIS picture of the setup. It certainly is concerned with the statues in the pictures below, when we aim the camera at them. But, this is a picture too.
In this picture, note the left side is not as bright, and the shadow B on left side of statue from right light (both statues) is much darker than any shadow on right side of statue, because light at left is compensated -2 EV. Two stops is a lot of ratio (one stop is more common), but we are not actually achieving two stops of actual ratio here (in the pictures below). You can see the right light is more bright, but it spills over to add on the left side too (see the shadow at B?), because the lights are back so far, flooding the room with light, spilling and mixing everywhere. Ratio would have more effect if the lights were closer (combining less). So for visually better results (below), I cranked the ratio up to two stops, hoping for one.
Note that camera tripod C in foreground is dark, because the internal flash group was disabled (mode "- -"). There is still a lot of spill light bouncing around the room though. That is only the lens on it, the camera body was removed to take this picture.
Note the light D at camera left has its body rotated 180 degrees backwards, to aim its sensor (by battery door) towards the commander at camera. Sometimes the light's sensor is hidden behind the umbrella, which is not good for line of sight to the commander. Certainly there can be problems, but umbrellas are normally pretty close, and often work fine anyway - if at close range like here. Regular main and fill lights, for close portrait or tabletop, normally seem little problem when close, but hiding the background light behind the subject is doomed. Often for tabletop work, the umbrellas are even slightly behind the camera (at the side of camera, slightly farther back than camera), and this close position always works - reflects back from subject and background I suppose. The commander signals are weak, and my notion is that close reflections can be strong enough, but distant reflections are hopeless. Keep an eye on it though, not all things work. It really should be line of sight, and distance hurts.
Next, from tripod position, a 70-200mm lens is zoomed tight at 200mm, and individually aimed left and right at each of statues (from 10 feet - extreme, but to provide access to both ends of the table). All settings always the same though. The camera aim was just rotated and the shutter button pressed again. Nothing else changed, except of course, now the TTL system is individually metering and setting both lights for each smaller area which it sees when aimed at it. Saying, regardless of position or distance, the light at left is always a couple of stops weaker than the light at right (simply because I said so for ratio - and the gradient shading tones this adds seems like magic).
Are you aware that you are seeing the shadow gradient tones that define the shape of the subject? This is caused by the unequal main and fill light concept. Can you imagine the result if this had been direct on-camera flash? It would have been excessively flat, no gradient shadow tones on subject. So the one wise thing I did was to compensate one light to be weaker than the other (in the commander menu), which causes the subtle gradient toning results. Believe me, some ratio is much better than if both lights were equal. This result is not flat and featureless (like direct hot shoe flash would be, or like two equal lights would be). Instead, a major point here is that this ratio achieved some tonal range (mild and controlled shadows), gradients which show shape, more natural, more interesting light, which clearly makes all the difference. Look at the shadows on the water jar for example. The gradient shadows sculpt the curves to show the shape. Not just a bright white blob. Note: One could substitute a close white reflector board for one light, as a way to provide fill to soften gradients.
Just to be clear ... a ratio is not often appropriate for a wide area, like the first setup picture, or for a group portrait, etc. Uneven lighting is bad news then. A ratio is normally appropriate (and desirable and interesting) for one object, one face, one statue, etc. But still, notice that the camera-left side of both subjects is the dark side, with gradient shadows, simply because the left light had -2EV compensation (this is not as much contrast as -2 EV lighting ratio, but our lights here are not main and fill, they are just on either side of subject).
Clumsy as this setup was, both statue pictures could be a lot worse. Not fancy, but it seems not bad for point&shoot flash. I did use umbrellas, and I did use ratio, but I did no more than was stated above - basically I just threw a couple of lights out there, not even very cleverly.
You might notice the histogram data graph is not very close to the right edge, even on these white objects. This white barely makes 200 on a 255 scale. Looks better, in this case. Don't overdo exposure, the histogram is not a light meter. Notice the small shadows on table caused by the statue base. In both statues, the left side shadow is darker because left light is weaker fill, set to be -2 EV. Right side shadow is weaker, hard to see even, because right light is stronger, filling the other ones shadow. Neither statue has a proper main/fill relationship. The left statue just has two side lights, and right statue has a backwards ratio (frontal "fill" stronger than side main). Normally the main light is at 45 degrees, and weaker fill is the frontal light, but which is backwards here. But not bad though, it still has a ratio to show shading.
Anyone can do this, and of course with a little thought, it only gets better. But again - This presentation is in no way pretending to be about proper lighting of tabletop photography. Farthest thing from it, more intentionally poor. Any way you can use an umbrella is better than without, but this is instead about point & shoot flash. Of course, it could be more, certainly all of the potential is there, and nothing prevents us from thinking more about the lighting, to plan it better. But the point about the commander system is that each of these pictures meters each light individually, and controls each lights power level individually, and the specified fill ratio was observed no matter where the lights, no matter where we pointed the camera. In the left picture, the power level of both lights were set appropriately for that subject (seen by lens). In the right picture, both lights were set appropriately for that subject. The overall exposure of both lights was right, no matter the distance of the lights (within reason of course, and +1 EV Flash Compensation was used, more routine than not for Commander). TTL meters what the camera sees, and AWL meters both groups A and B individually, and sets their power levels to be equal at the subject, and then compensates them as specified, as desired. Pretty danged good stuff, automatic point & shoot wireless remote TTL multiple flash. We could of course instead do everything manually, which does offer more control, but then we have to work at it a bit, do it all ourself, and be aware of more things. But this way, with Commander TTL, we just point & shoot, and if we don't like the result we see, we simply tweak it a bit (Flash Compensation typically).
#2 Watch the Ready LED.
As always, be mindful of the Ready light blinking, indicating insufficient power capacity for the job at hand - when our settings are too optimistic, causing the TTL metering to request more flash power than the system can deliver. The warning in the flash LCD (upper right corner of LCD on most flash models) can even tell you how much more TTL power was requested than was available (hot shoe or remote TTL). The remote flash should be set to beep this warning. Read and understand SB-600 page 29, SB-700 page D-29, SB-800 page 33, SB-900 page D-4, or SB-910 page C-4 about the insufficient power warning. Wider aperture, higher ISO, and closer distance reduces demand for power, to give you more effective flash power to solve this problem. The camera gets the Ready signal from a hot shoe flash properly, and the viewfinder symbol can blink. But the communication path is only from Commander to Remote, so the camera cannot know when the remote flashes recycle. The Ready icon in the camera viewfinder is just a hopeful rough approximation (a guess, with safety factor), which never blinks to show remote power was insufficient. So watch the flashes own Ready lights, or better, set Remotes to beep the warning.
#1 The TTL flash is fully automatic flash exposure. No matter how you may set ISO or aperture, the TTL system will still try its best to give you a proper exposure, if it has sufficient power to comply. Flash Compensation is the way we adjust automatic TTL power level and results. This is a Big Deal, important.
The camera's TTL metering "controls" the automatic TTL flash exposure, and our own Flash Compensation "adjusts" or tweaks the TTL flash exposure, to do it our way. As always, sometimes flash compensation is necessary (often is necessary - all commander pictures above also used +1 EV Flash Compensation). Just simply do what you see needs to be done. This is the key, simple as that. The only difficult part is making novices realize this is necessary. Cameras simply cannot always get everything right. So simply just watch and fix it. If it needs a bit more light, add a bit of +EV Flash Compensation (or -EV if too bright). Flash compensation is how we fix TTL. Note that Compensation is Not reset when the camera is turned off.
Frankly, I am normally very likely to be at +1 EV Flash Compensation with Commander Remote TTL and two flashes. My wild guess is that this was intentional design, since the TTL group preflashes are metered individually, one at a time, so the system has no clue if you are overlapping them to cover the same area, or not. Two overlapped flashes do add to 2x the exposure, one stop overexposed, which needs to be backed off a stop. The system is good about trying to prevent overexposure. But the flashes don't add until fired together, so it can't really know. I routinely start at +1 EV flash compensation as my baseline with Commander and two flashes. For one hot shoe flash, not as much compensation is needed, since one flash cannot overlap, then 0 EV is my starting baseline for one hot shoe flash (for TTL, but TTL BL may need a little more). It is pointless to get upset because your TTL BL unit often needs + 2/3 EV, and my TTL unit may only need 1/3 EV. The numbers are not important, what is important that we both have to be alert about it. But all this definitely also varies with each situation (with each scene in front of the camera), so we always have to watch it. You will quickly start recognizing the situations to expect certain results, but always just do what you see you need to do.
Note: Flash Compensation is definitely NOT a "set one time and forget it" value. This is NOT compensating the flash unit, it is compensating the scene in front of the camera, and the next scene is likely different. Once set for bounce, it might still be appropriate for walking around in the same room, in the similar situation, but it may be rather different next time in a different situation. It entirely depends on what you aim the camera at (see What Light Meters Do), so pay attention to results, at the time, all the time. Simply just do what is necessary, as you see it is necessary. Learning to do what is necessary (flash compensation) is the main key to successful flash pictures. Seriously. This is the difference between Wow! and Ugh!, allowing you to be the hero instead of the goat. You are the photographer, this is your job, so simply just watch and fix it. This may require an attitude adjustment, that is to say, compensation and retry works much better than does just complaining that the camera does not always get it right. :)
Many lighting things are possible, but there are sort of two major lighting situations for two lights: 1) Equal lights on each side of camera, illuminating an area evenly, like a group portrait. Or 2) a portrait (one person) is a Main/Fill situation, with intentional shadow gradients (from Main, softened by Fill), which make the face greatly more interesting than flat (even) featureless light. But subtle soft shadows, not harsh dark shadows. A large close Main light at an angle to make soft shadows, but with frontal Fill to reduce degree of them. The concept is the same as one Main light and a reflector for fill, but a second light is more versatile than a reflector. One idea is that the Main umbrella is large and close (to be soft), and perhaps as much as 45 degrees high and wide (to cause the shadows - maybe a bit less than 45 degrees), with the fill umbrella near the camera lens axis (frontally fills what the lens sees), which is necessarily back near the lens to be able to see around it. For a portrait of one person, we typically want the Fill light to be about -1 EV down from Main (a good general purpose value), which is a lighting ratio, not really compensation. Sometimes more ratio is good, like -2EV. See more about fill and ratio.
For ratio, the fill light power level is simply compensated to be less than the main light. To create a -1 EV fill ratio, consider that there are two ways.
1) Just set Main 0 EV and Fill -1 EV in the Commander menu (shown above on the horse). If you also need +1 EV Flash Compensation, set that too, it adds to both groups.
Or alternatively, since we probably need +1 EV FC, and since the Flash Compensation button only goes to + 1 EV, then
2) Same thing could instead be Commander Main +1 EV and Fill 0 EV, and Flash Compensation 0 EV, which is same one stop difference, and same one stop flash compensation - but it leaves the Flash Compensation button free for any other need.
Flash compensation is a major key to great flash pictures. Just watch, and simply do what you see you need to do. If you don't like the result, just fix it to be how you want it. No one else will do it for you. Once we realize this is our job, it becomes pretty easy.
In situations like fill flash in bright sun, TTL BL flash mode will reduce the flash automatically, automatic compensation of the flash metering relative to ambient metering. We never see the EV numbers then, but we see the effect of reducing the flash exposure. Applies to Commander or hot shoe TTL flash (the Commander does TTL BL). We can still choose to tweak it with additional values of flash compensation, for a different result.
However, even if Flash Compensation seems like magic, still remember that the flash only has so much power capability. If TTL is underexposed because it is already limited out at its maximum power, telling it to try harder by adding Flash Compensation can't help (that then needs wider aperture, higher ISO, or closer distance. Or a bigger flash - maybe gang two flashes in the same umbrella - double power is one stop.)
Continued - Part 2: Summary of Advantages, and FV Lock (subject Blinking), and unwanted Trigger signal.