The Sub-Menu for Part 4 (a few pages):
Any flash picture is effectively a double exposure, once from the flash, and once from any ambient continuous light that might be present. These two effects work differently, with different rules. The ambient may be an insignificant level indoors, or it may be overwhelmingly significant when using fill flash in bright sunlight. We might be able to ignore the ambient indoors, but generally must match it in daylight. There are always these two effects to be considered in any flash picture. Note again, they have different rules.
The camera system measures ambient and TTL flash separately and independently. The camera meter, and settings it may choose, indicate only the continuous ambient light it sees (it is never about the flash), and camera automation sets the camera settings appropriately. Then later, the TTL flash system meters the preflash to set flash power level for the aperture setting it discovers in effect at the time. But we don't see this TTL meter - all we see is the picture result on the camera rear LCD. The flash of course affects the picture exposure, but it does not affect the exposure of the separate ambient light.
The TTL flash metering doesn't ever change automatic camera settings, however the flash presence does have a few effects. If the camera is aware that the flash is present, then:
The exceptions we see in camera A or P modes are if Slow Sync, or Rear Curtain Sync on Nikons - which ignore this Minimum Shutter Speed menu. The purpose of Slow Sync is to "Ignore any Minimum Shutter Speed menu, and use whatever slow shutter the dim ambient metering actually meters", no matter how slow. On Nikons, Rear Sync does too, because it only has meaning for a slow shutter that will cause motion blur, then it makes the blur trail follow the action, instead of lead it (Part 5). In contrast, camera M mode will allow any shutter speed at any time, but will not exceed Maximum shutter sync speed if the camera is aware the flash is present.
OK, so knowing that instantaneous flash is not affected by shutter speed (see Part 2), and knowing any continuous ambient light is so affected, we can use that fact. For example, indoors, using incandescent modeling lights with studio lights, we can use maximum shutter sync speed (without affecting the flash exposure at all) to reduce that ambient continuous so that the unwanted light does not affect our picture. One purpose could be to prevent orange incandescent light in our flash picture. Or to prevent normal room lights from affecting our carefully lighted studio setup. Studio flash surely always uses camera M mode and maximum shutter sync speed, for this reason. But in a normal room snapshot setting at low ISO, we might intentionally use a slow shutter speed to show and maximize the contribution of that continuous room light, for a warmer "ambient" look. Caution though, at high ISO, without incandescent white balance and a CTO filter on the flash, you surely get excessive orange.
Normally it is our choice, to control the continuous ambient light with shutter speed, to block it out or allow it in, independent of the flash exposure. We only need to know that any flash picture is a double exposure of the two situations. With flash, we always have two decisions to make - proper aperture so flash power level is possible, and indoors, and shutter speed about if we want the ambient included or not (outdoors in daylight, we are probably going to have to account for the sunlight).
The flash exposure will not care about whatever shutter speed we use, but we have choices, and our choice ought to be intentional. When we use the camera's A or P exposure mode indoors, when we turn the flash on, we see the automatic shutter speed increase from "slow" (perhaps 1/4 second metered in the dim room?) back up to 1/60 second. Not for any reason other than this is the lower limit set by the camera when using flash - we do not need a slower shutter speed if we are using flash. Many DSLR models have a menu for this called Flash Shutter Speed (often Nikon menu E2, D90 E1, default 1/60 second). There are ways around this limit - camera Manual mode allows any permissible shutter speed, or Slow Sync or Rear Curtain Sync option ignores this limit and uses whatever slow shutter speed the camera actually metered for ambient. This minimum lower shutter speed limit with flash (often a 1/60 second default for minimum shutter speed with flash) is simply because we might be able to hand hold 1/60 second in case there is some degree of ambient light. But you may prefer to keep out the continuous ambient, because incandescent is orangeish in color. Or if you are using the very fast flash for the purpose to stop motion, a 1/60 second shutter may allow dim ambient to blur what the fast flash stopped. If you noticed the metered light reading BEFORE you turned on the flash, it was surely slower than 1/60 (if dim indoors - but in sunlight, you get the actual that it meters). So indoors (with insignificant ambient), it seems much better to always use camera Manual M mode with flash, which allows us to set the shutter speed as we wish, either slow to allow the continuous ambient in, or fast at maximum sync speed to keep it out. It is our choice, but the automation will choose 1/60 second, possibly not for reasons useful to us. The Speedlight flash power used in TTL mode will depend only on the aperture and ISO values, and the TTL flash exposure is still fully automatic even in camera M mode.
To say the obvious: There is no camera intelligence that can treat one area of the picture different than any other area. It may choose one area to meter with preference, but the camera settings can only use ONE aperture setting and ONE shutter speed setting. The flash can only use ONE power level. These single settings affect everything in the picture (there is no magic). ISO, aperture and shutter speed affect the continuous ambient light exposure. ISO and aperture and flash power level affects the flash exposure. TTL automation tries to determine some of those settings, but the one big thing important to realize is that our manual Flash Compensation also affects the flash (power level and exposure), as we see fit. If TTL automation does not give the result you want, then you can moan, or you can simply fix it. :) Flash compensation is a main trick for getting great flash exposures. Flash compensation should be considered (if not used) for every TTL flash picture, and certainly in any new situation.
Maybe this is overly simplified, but regardless how we might reach the final choices, remember the exposure goal is in fact simple: No matter how mixed and complex the scene, our photo can only set one aperture, one shutter speed, one ISO value, and one flash power level (in each flash unit anyway). Maybe we specify some ISO and aperture, like f/5. If we want to expose the ambient properly, we set shutter speed to do it. If we want to expose the flash properly, we set the flash power to do it. There are choices of course, but these two exposures (ambient and flash) is all there is to do. Sometimes these values are a compromise, sometimes a tool, or maybe we ignore a very underexposed ambient, but each parameter has just one value. Look at your results, and when the one value seems wrong for your purpose, simply just fix it. That is your job. Just do what you see you need to do. Flash Compensation is the tool we use to adjust what the automatic TTL flash is doing. I am suggesting we remember to approach it as a simple problem, instead of as incomprehensible magic. Learn a few basics, and it becomes very understandable (honest, it really does).
Flash Compensation only affects relative TTL flash power levels. Exposure Compensation (on Nikons) affects Both ambient exposure and TTL flash exposure. And these methods all add for a final TTL flash value.
Understanding these few fundamentals will greatly simply and explain much that you see happening with flash. See a homework page about trying this concept.
Then, first a quick section on "Modes". TTL BL vs TTL modes towards page end below.
Organization is a big issue here, sorry. Some material (like this flash mode section) are basics, you might skim past pretty fast. But IMO, other sections below contain substantial meat, so I hope skimming catches it all.
There are many options, and various modes to use flash, several modes on the flash itself, and also modes on the camera. Some top flash models have every mode, but some low-end may only have one mode, Manual or TTL, or maybe only a couple of modes.
Menus: The camera flash menu that selects TTL, Manual, or maybe Commander, only applies to the internal flash when its door is open. This menu has no effect when the popup door is shut. External flashes have their own menu (SB-300 and SB-400 are exceptions, sharing the camera internal menu). If a hot shoe flash is connected and is turned on, it will flash with the shutter (possibly not if it is in some slave modes).
A quick summary of these modes on the flash menu itself are these: (Nikon nomenclature, SB-800 and SB-910 have all of this).
The difference in A and AA mode: In A mode, we must enter the cameras f/stop and ISO info into the flash menu, so it will know how to compute a proper exposure. Whereas, AA mode is on the hot shoe, and can automatically obtain this f/stop and ISO info directly from the camera hot shoe. AA mode is Auto Aperture, just meaning it gets f/stop and ISO info itself, from the camera hot shoe - so AA mode must be on the hot shoe. But A mode can be on or off camera, and we must enter f/stop and ISO directly into the camera menu. So actually, current Nikon flashes only do A mode if off camera, and automatically substitute AA mode if on camera, since AA mode can get the necessary f/stop and ISO values automatically (assuming the communication CLS system). Otherwise, the A and AA modes are the same, except... two other things. Today, some commander versions can specify AA mode, and program and compensate and trigger it remotely. And AA mode does do a preflash, and A mode does not. The purpose and use of this AA preflash is unknown AFAIK... We can imagine it might balance with other flashes (but the commander really does not). Especially puzzling since the AA flash also meters and controls its own flash. On Nikons, flash compensation on camera body, or on flash body, works with hot shoe AA mode.
Note the camera light meter scale that we see is only about ambient light... the TTL system has its own invisible light meter system that it uses for the flash. TTL flash is automatic, point&shoot flash, metering and responding automatically to whatever aperture and ISO that either you or the ambient metering has set (TTL simply responds to it). However we can control TTL flash using Flash Compensation, to tweak it in.
These camera modes affect how the ambient light is metered. Then the flash modes above affect how the flash power level is adjusted. Camera A mode is very popular for flash, but I would suggest camera P mode for fill flash in bright sunshine, and camera M mode indoors.
In dim light, P Mode is normally near widest aperture too. Or it does if no flash, but P mode also enforces a Maximum aperture with flash. At lowest ISO 100, a hot shoe flash limits to typically f/4 or f/5 maximum aperture, and internal flash is typically f/2.8 or f/3.5 maximum aperture. Which gets far worse at higher ISO. In P mode with flash, this maximum aperture limit is stopped down 1/2 stop per each stop of higher ISO. I realize that's the relation of Guide Number to ISO, but don't know why it's needed here? A and S modes don't need this, and the one stop more restrictive limit for a hot shoe flash would be a pretty small hot shoe flash. But your f/1.4 or f/2.8 lenses won't be much help with flash in P mode. This is counterproductive with flash, because if you really need high ISO, the lens is stopped down more, unable to use much of it. Camera P mode and Auto ISO seem an especially poor combination for indoor flash (but camera P mode is often a great choice for fill flash in bright sun).
Many Nikon camera model manuals have two charts of this, internal and external flash (shown here). This is the D7100, representative of the others.
We specify camera mode appropriately for the property important to our current picture, to ensure it is what it needs to be. Maybe we need wide aperture for more effective flash power capability or a blurred background, or stopped down for more depth of field. Or a fast shutter speed to stop motion in ambient light, or a slow shutter to blur a waterfall. Whatever we think it needs, we have choices.
The Nikon iTTL flash system (controlled by the camera metering) includes TTL and TTL BL metering modes. Some iTTL flashes (SB-600, SB-800, SB-900/SB-910) have a menu to select which TTL or TTL BL mode for the camera metering to use. The other iTTL flash models (SB-700, SB-400, camera internal flash, and Commanders) have a menu that says TTL, but these are TTL BL by default. Most non-Nikon-brand iTTL flash models for Nikon cameras also default to TTL BL mode, because Nikon is a TTL BL system. The camera controls the metering, and the flash just flashes at the programmed level.
The camera light meter that we see only meters the ambient (the flash meter is invisible to us). The camera meters the ambient, and sets aperture and shutter speed and Auto ISO for the ambient level. The flash presence can impose minimum and maximum shutter speed limits (shutter speed does not affect flash exposure). These settings are not about the flash (we can manually choose f/stop and ISO to favor the flash however). In many camera models, Auto ISO is certain to use very high ISO in dim places, and the flash has to work into whatever ISO and settings it discovers to be in place (exceptions are the older DSLR, and a few most recent new cameras). So turning Auto ISO OFF is an extremely fine idea when using flash.
TTL BL is automatic balanced flash (if in bright ambient). Balanced with ambient just means the metered flash power level is reduced, to serve as fill level, but specifically reduced because the sum of a proper ambient exposure and a proper flash exposure adds to be two proper exposures (of the near flash subject), which is 2x proper exposure, which is one stop overexposed, by definition. So, TTL BL mode backs off on the flash power, which becomes a reasonable fill level. Typically TTL BL flash in bright sun is reduced nearly two stops less than the ambient... automatically, pretty much perfect for fill flash in bright sun. That is what TTL BL is.
Using camera P mode for the bright ambient, and using TTL BL flash mode (default on SB-700, SB-400, and the camera internal flash), normally gives very good point&shoot fill flash results on human portraits in bright sun. Camera A or M modes will do the same if you set the same aperture that P mode selects, which is just saying that you must know to set up near f/11 (bright sun, Sunny 16, ISO 100) to keep shutter speed within maximum sync speed - otherwise you just see a flashing Error - wider aperture cannot give correct exposure since the shutter speed cannot go faster than sync speed. But P mode can adjust aperture, so it knows how to do this.
TTL mode can be selected on any of the flash models by setting camera Spot metering mode. Camera Spot metering changes the flash metering system from TTL BL to instead be TTL mode. Don't misunderstand, the flash never does Spot metering, the flash metering always reads a center area - but the ambient can do Spot. If Spot metering is set, the camera just changes from TTL BL to be TTL mode metering. It is the camera that meters and controls this. The Exif reports which flash mode was used. This metering mode change is necessary, because Spot metering is only about the ambient spot, with no concept of the background or total frame exposure, so meaningful balancing of flash is not possible. Which is what TTL mode is, just regular flash. no balancing.
In contrast to balanced TTL BL, TTL mode comes ahead on, using full metered power, regardless if any significant ambient is present or not. Not reduced or balanced. For fill flash in bright sun, then you have to know to set about -2 EV flash compensation yourself, otherwise TTL will give an overexposed subject (if the two full exposures add). Setting manual TTL flash compensation for fill works great (and offers control), you just have to know it is required. Whereas, TTL BL does that automatically, point&shoot fill flash, called Balanced. And TTL BL is often the default mode, unless you set Spot metering mode.
TTL mode is very good for indoor flash, when we want full metered flash level. And since the ambient is normally low level indoors (dim, where we need flash), usually low enough to ignore , then we can set Spot metering to get TTL mode with flashes that otherwise default to TTL BL. A little tricky to explain, but Spot metering implies different things with flash indoors. It sets TTL mode of course, but also, if the ambient indoors is too low to be significant (unless we have Auto ISO ON), then Spot metering means nothing for an ambient level too low to care about. Camera A or P mode will set a Minimum shutter speed for flash anyway (often 1/60 second, overriding slower Spot choices in dim light). Or camera mode M works great indoors with flash, and Spot metering can not change settings if camera M mode (the TTL flash is still automatic flash in camera M mode). So there may be no Spot metering with flash indoors, but Spot will change the flash metering to be TTL mode - giving full flash level as metered, no balanced adjustments. Again, flash does not do Spot metering - it always meters its own center area regardless (not exactly same as Center Weighted metering, but reasonably similar).
In other situations in bright sun outdoors, some users instead reduce the bright ambient exposure about two stops, and then do full TTL flash level. This makes the subject stand out against the darker background, and it helps stop subject motion in fast sports. You would not call it fill flash however.
Cameras use reflective metering (metering is affected by subject colors, so sometimes not precise). Therefore to achieve better results, we may need to use Flash Compensation to tweak what the automation does with flash power level, in either TTL or TTL BL mode. Simply watch the flash exposure result in either mode, and then do what you see you need to do for a perfect flash result. This is key for great flash pictures. Flash is never fully point&shoot, we have to watch and help a little. It is quite easy.
Hint: Scene situations vary, but indoors in normal dim ambient, you may like routinely starting TTL BL mode near +⅔ EV or +1 EV Flash Compensation in the general case. Or you can instead use Spot metering mode to use TTL mode indoors. You will quickly learn to NOT leave it in Spot metering mode when you go back outdoors into bright ambient. :)
Again, Flash Compensation only affects relative TTL flash power levels. Exposure Compensation (on Nikons) affects Both ambient exposure and TTL flash exposure. And these methods all add for a final TTL flash value.
Is it required to turn Auto ISO OFF for flash?
The camera internal flash, the SB-700, and the SB-400, are TTL BL mode by default. Others have a selection menu on them.
Anytime I say -1.67 stops, of course I mean -1 2/3 EV on the camera dial ( -1.67 is easier to type. -2 EV is pretty close to same.)
Camera mode A lets you set the aperture - a concern for flash power requirement. But really, if insignificant ambient, camera M mode obviously offers the most choice. Camera mode A and M can set the same aperture. TTL flash is always still fully automatic flash exposure in any camera mode, including camera mode M. If you set camera M mode, and also set 1/60 second shutter speed, then this is exactly the same as camera A mode, which would have set 1/60 second too (that is all the camera A mode does with flash in dim light, it sets the 1/60 second Minimum Shutter Speed with flash - Rear Curtain sync and Slow sync are exceptions.) We have just seen that the flash does not care about shutter speed. You can still set the same aperture for both A or M mode, both are same thing then, maybe f/5 for bounce flash. The TTL flash is automatic flash exposure either way.
But camera M mode allows setting any faster shutter too, up to maximum sync speed, for control of the ambient indoors (to shut it out, or to show it stronger). There are two flash modes, TTL and TTL BL balanced flash. TTL flash mode is arguably best indoors, or TTL BL if it is the only choice, it works too - sometimes BL just needs a little more compensation, but either may need compensation, so it may be a difference without a distinction (speaking of flash indoors).
Continued - Fill Flash in Sunlight