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Basics of Flash Photography
Four Fundamentals we must know

Flash Indoors

Flash pictures are double exposures, but flash conditions indoors usually see only an insignificant amount of continuous ambient room light. The ambient light is dim (compared to daylight), which is why we need flash. Metering that low ambient room light is normally not possible with flash, because the camera normally enforces a minimum 1/60 second shutter speed when the flash is used (discussed above). The room where we need flash is probably closer to 1/2 to 1/10 second ambient. If 1/60 worked, we may not need the flash. So... this 1/60 is NOT about the flash. The 1/60 second commonly seen is just a minimum shutter speed limit with flash, since we do not need a slower shutter with flash.

Point is, this ambient room light is normally insignificant indoors, and we cannot reasonably attempt to balance it with fill flash, not like using fill flash in sunlight. The flash is the major light indoors, pretty nearly the only light indoors. We could use high ISO and raise the indoor ambient to be meterable, and then use the flash as fill flash. Or, we can use lower ISO for better quality, and generally ignore the insignificant indoor ambient - simply just use flash. Flash TTL mode is for when the flash is the major significant light source, and TTL BL flash mode for when the flash is to be balanced with significant ambient light (which is normally sunlight). Normally there is nothing to balance it with indoors.

Second point is that any automatically metered camera exposure settings are only about the room ambient light, which is normally insignificant indoors. Instead of trying to meter the insignificant ambient room light, we can instead choose an aperture more appropriate for our flash goals (like available flash power). The flash does not care what shutter speed is, but we can select a shutter speed to control (boost or reduce in our picture) the small amount of ambient room light (our choice, for example to be sure to keep out any orange incandescent light).

Camera exposure modes for flash INDOORS

No matter which P, S, A, or M camera mode you select, TTL flash is always still automatic flash exposure (even with camera M mode). TTL flash automatically chooses the appropriate flash power level for whatever aperture you set, regardless of how you selected it. Camera M or A modes are very popular for flash indoors, but let's discuss why.

Camera P mode (Program) is NOT a versatile choice for flash indoors. It might be best and great for fill flash outdoors in sunlight, it knows enough to be very convenient then. But indoors with a hot shoe flash, it works of course, but P mode will use 1/60 second at f/4, every time (Slow Sync or Rear Curtain Sync will bypass the 1/60 second lower limit. Higher ISO will stop down aperture). This includes the "scene modes" on those cameras with them. In dim light where flash is needed, it will be maximum aperture, but it may not be the maximum of the lens. I am saying f/4 as one generic value here at lowest ISO with external flash, some models are f/5, and internal flash will be slightly wider. P mode enforces a Maximum aperture, which can vary depending on which camera model (see list at right), and which is made worse with high ISO. Minimum shutter speed with flash can vary with the E2 menu Minimum, the Slow sync setting override, and with Auto ISO shutter setting. Nevertheless, the common P mode default will be about f/5 and 1/60 second, for any and every indoor flash picture. We can argue that 1/60 second f/5 for every picture is more dumb than smart.

In minimal lighting (indoors), this f/5 maximum result depends on ISO. Higher ISO setting will worsen and limit this maximum aperture more. Here is from the Nikon D90 and D300 manuals (typical):

I would imagine this maximum aperture would be a problem for the later Nikon camera models that use high ISO indoors with flash if Auto ISO. We can turn Auto ISO off. And there are better modes for flash indoors than P mode.

This f/5 is a reasonable guess for average flash situations, conservative and probably always works, which is good, but which is not exhibiting great intelligence about the scenes requirements. Every flash picture you ever took, or ever will take, in P mode will be 1/60 f/5 (P mode, indoors with flash, low ISO). If you bought the expensive f/1.4 lens, P mode will never use that wide aperture for flash indoors. If you specify ISO 800 for conditions needing more light, camera P mode probably will only use f/7 then, maybe when you most need the wider aperture (P mode seems counterproductive for indoor flash). We really do not have to accept f/5 1/60 second for every flash picture we take indoors (see A or M mode). But P mode works very well with fill flash outside in daylight however, and knows about maximum sync speed.

Camera S mode (Shutter) -- ??? Wrong choice indoors. It will work of course, the TTL flash is automatic for any aperture, but what is the aperture in S mode? (rhetoric, it surely is always wide open in S mode when indoors where we need flash). Instantaneous flash could not care less about shutter speed, so why are we giving preference to that? Perhaps for action outdoors in daylight, but we cannot exceed maximum sync speed anyway, and aperture is critical for flash power. Mode S has no advantage for flash indoors. Flash power level is about aperture. Indoors with no significant ambient to interfere, the short flash duration is what stops action. The flash duration is faster than any possible shutter speed. Instead think camera modes A or M for indoor flash.

Camera A mode (Aperture) commonly will use 1/60 second indoors (Minimum shutter speed with flash, menu E2 on many Nikon models) at whatever aperture you set for most cases of flash indoors. This is NOT a shutter speed for flash, the flash could not care less about shutter speed. It's a Minimum shutter speed with flash. Since you are using flash, slower than 1/60 second is normally pointless. (For flash in sunshine outdoors, the shutter speed cannot exceed maximum shutter sync speed, so we may see warnings of HI exposure in A mode unless we know to set f/16 and maximum sync speed for the sun. Use P mode with flash outdoors to prevent that). Slow sync or Rear Curtain sync modes will bypass this minimum shutter speed limit, and of course camera mode M has no limit.

You can choose a wide aperture to minimize flash power (like for bounce which needs a lot of power), and reduce recycle time. Or you can choose a narrow aperture for closeup subjects, for depth of field, or to not overpower the subject. Again, f/5 is good average and reasonable choice in many cases of bounce or distance. For bounce from high ceilings, or from a rear or side wall, you probably may want to use ISO 400 too, and/or f/4 (for less risk, and for faster recycle). But ISO 200 and f/5.6 may work on low ceilings (watch your flash Ready LED for indication of power used). We normally always get 1/60 second shutter indoors with flash, which doesn't affect the flash, but it does affect the continuous ambient room light. There are often menus and options to allow other shutter speeds than 1/60 second (Flash Shutter Speed menu, Slow Sync, Rear Curtain Sync), but if you want other shutter speeds, then you should be using camera M mode (flash indoors), because it is so much simpler to do anything you wish there.

Camera M mode (Manual) - Here is a big deal to know: The TTL flash mode is still automatic. It is only the camera settings that are manual. Any automatic camera meter reading was determined from the ambient light, which is not about the flash. That automatic metering (like A or P mode) is helpful to match ambient outdoors in daylight, but the light is insignificant indoors now, which is why we need flash. So indoors, camera Manual mode lets us set things up for the flash picture double exposure, any way we want it. We could simply set M mode to f/5 1/60 and duplicate what P mode will do (flash indoors, ISO 200). Or, we can change things a little to suite better purposes.

Aperture controls the flash power needed (camera M mode or any other mode), and therefore the flash recycle time. We set aperture for the flash power level (judged by recycle time). Trying to be very clear: Aperture and ISO certainly affect the flash exposure, so we do see the exposure of manual flash mode at a fixed power level change with aperture or ISO. We have to manually set that manual flash power level to be correct for the aperture or ISO. But automatic TTL flash mode will simply automatically react to aperture or ISO with same automatic "correct" TTL exposure, by using a different flash power level (automatically). So we may not see it change, may not realize it changed, but flash power level does change big time with aperture or ISO. But flash power is relatively limited, not infinite, so we must pay attention. Watch your Ready LED.

In camera M mode, we can set any shutter speed up to the cameras maximum shutter sync speed, or we can set it as slow as we want it to capture more of the dim room ambient light. The flash exposure does not care what shutter speed is, and will not change. The flash duration is fast, faster than maximum shutter sync speed, so therefore, actual shutter speed controls only the ambient room light, fast shutter to keep it out, or slow shutter to allow it in - our choice. Studio portraits always want maximum shutter sync speed to keep the ambient out (for example, the modeling lights). Family snapshots may want shutter slower, for the "warming" look of the incandescent room lights. Again, it is our choice. We do not have to accept 1/60 second. We can think about it instead. But as first beginning step in using camera M mode for indoor flash, we can always use 1/60 second and f/5 as a starting point, a home base. This is the same values P mode would have used, and it will do the same thing in camera Manual mode, and it usually should give a decent picture (remember, the TTL flash is always automatic flash exposure). But with camera Manual mode, there are other choices too. We can more optimally change aperture or shutter speed, for the specific purposes mentioned. A little experimentation will be very helpful.

The automatic flash in TTL mode gives automatic flash exposure in EVERY camera mode (including camera Manual mode). This is true for direct flash, bounce flash, flash in umbrellas, etc. Not to say it is always perfect, it's not. We often have to add a bit of flash compensation, and then it is perfect (How light meters work may be of interest). In automatic TTL flash mode ("Through The Lens" metering for flash exposure), the first thing that happens after the shutter button is a preflash. Some of that preflash reflects back into the camera lens, where is it metered (measured), and the flash power level is set to give the correct exposure for the lens aperture in effect at the moment. What varies in automatic TTL flash operation is the flash power level (the camera exposure settings remain constant, whatever we set, or whatever the ambient light meters). We can choose the lens aperture ourselves to determine the ballpark flash power level that will be required, but we always get correct TTL flash exposure, regardless of which aperture we choose (within the range of what the flash power can reasonably deliver). This means however, that A (Aperture) and M (Manual) modes are very important and useful for flash indoors, since the aperture setting controls the flash power (and recycle time).

Note: Flash compensation changes TTL flash power level, and it only applies to TTL flash, either on hot shoe or TTL remotes. Manual PC cord flash is affected by the flashes power level setting, but the camera cannot affect that. The camera has an Exposure Compensation which affects both the metered ambient and the TTL flash, and it has Flash Compensation with only affects the TTL flash. These two compensations add together for the TTL flash.

Auto ISO and Flash

Auto ISO probably affects flash indoors. There have been at least three methods of how Nikon Auto ISO works with flash, so camera models can be different. The following is speaking of Nikon Auto ISO, in dim places where flash is needed:

In any Nikon DSLR camera model, Auto ISO always remains at Minimum ISO (Auto ISO is not used) WHEN the hot shoe flash (recognized present) is in Manual flash mode, or when using Commander/Remote mode. Otherwise ...

1. When a TTL flash is recognized as present on older Nikon DSLR models (D700, D90, and D300 and older), Auto ISO usually remains at Minimum ISO (regardless of the ambient, which might be dim, but which is why we are using flash instead). Auto ISO is increased only when the TTL flash limits out on power, and needs the help. Because, routinely, Auto ISO always only triggers at some extreme of whatever available limit exists.

   For examples of this philosophy, camera mode S has to hit the limit at the lens widest aperture before Auto ISO triggers. Camera mode A has to limit at the shutter speed in the Auto ISO settings (this intermediate menu limit prevents it having to first reach the actual 30 second lower shutter limit. The shutter speed will still go lower if necessary, after maximum ISO is reached). In camera mode M with no flash, Auto ISO triggers at any aperture and shutter speed not giving correct exposure. With TTL flash, this limit is the flash power, and on these previous camera models, if the TTL flash is turned on, the flash has to approach its maximum power limit before Auto ISO is increased. My own notion is that this is as it should be, we don't want a high ISO when we are using flash instead.

   In this older system, a confusing quirk for Nikon flash is that the Auto ISO value shown in the viewfinder never shows this ISO increase for flash. With TTL flash on, the viewfinder always shows the Minimum ISO setting, probably always ISO 200 or 100, with no hint if Auto ISO will trigger. Probably because this ISO requirement is not known until after the TTL preflash, and it could only show a few milliseconds then. But if the TTL flash power would be insufficient, then ISO is increased as needed, and the final Exif data shown on the rear LCD result will show the higher ISO value (in red text) when it has triggered. To see this one time, you can force it by using bounce flash with an impossible f/16.

2. But the newer models (D300S onward, D7000, D5100, etc.) begin in dim ambient by first increasing Auto ISO based on ambient, regardless if flash or not (intending to balance flash to the orange incandescent ambient?) So in dim places where flash is needed, the flash is always working into highest ISO. If you don't want high ISO, of course turn Auto ISO off with flash indoors. High ISO means that mixing flash with incandescent lights needs a CTO color filter (allowing Incandescent White Balance to be used), and the newer flashes provide a filter holder.

   To determine how your camera model responds to Auto ISO with flash: If with Auto ISO, and if TTL flash is turned on in dim ambient, you see high or Maximum ISO value in the viewfinder, you have this newer version. If you always see Minimum ISO, you have the first version above.

3. The D800 (and surely the D600?) has changed the method yet again. Auto ISO still uses Maximum ISO with the internal TTL flash (the previous new way 2. above if in these dim places). But if external TTL flash (on hot shoe), Auto ISO is only increased to be two stops above Minimum (if in this dim ambient. Two stops is like ISO 100 to ISO 400, and 400 is very reasonable for bounce). Logic and reason finally won. :)

Repeated again, in any Nikon DSLR camera model, Auto ISO always remains at Minimum ISO (Auto ISO is not used) WHEN the hot shoe flash (recognized present) is in Manual flash mode, or when using Commander/Remote mode.

With studio flash (or any remote manual flash), the camera does not even know the flash is present (not recognized by the Nikon system). It is absolutely necessary to turn Auto ISO OFF! It cannot be used with manual flash. We set the power level of manual lights as appropriate, which cannot tolerate ISO changing on its own.

So when indoor flash always discovers that it is dealing with high ISO, we can of course simply turn Auto ISO off, and set what we want. The camera will still work if Auto ISO is Off. :) I admit that I am not a fan of Auto ISO, especially not with flash, but I understand many people choose to always leave it on.

Continued