This page is about Nikon Auto ISO, because that's what I know. Auto ISO probably affects flash adversely indoors. White Balance for one. When the high ISO lifts dim ambient up to be significant again (meaning, to a meterable and usable level), then the TTL BL flash becomes fill flash for the ambient main light. And we discover incandescent lights are orange, and fluorescent may be green. The flash is like Daylight. This causes White Balance problems, which is when you need the filters on the flash, for example, an orange CTO filter to make the flash also be orange, so we can set Incandescent White Balance to correct the orange.
Or, much simpler, and likely better, we could figure out that since we are using flash instead, we don't need high ISO. Auto ISO can be turned off. The camera still works. :)
Nikon has had three types of TTL flash designs, all very different and incompatible, which are described here.
The SB-800 and SB-600 are unique flashes, able to do any TTL, TTL-D, or iTTL mode that the camera can do. All other Nikon flash modes have only one TTL mode, so some flash models are obsolete now for digital iTTL, but all can still do manual flash mode on all the models. See the Nikon Speedlight Compatibility chart. TTL on a modern Nikon DSLR will need an iTTL flash. Canon uses ETTL.
We are discussing the current iTTL camera models below.
Auto camera mode
In reference to Auto ISO, the DSLR camera will have metering modes A, S, P, M, and some models have Auto mode.
Use of Auto ISO
The Nikon Auto ISO menu has three settings.
In camera A or P modes, this is a Minimum in the sense that it is the threshold shutter speed when Auto ISO kicks in and starts increasing. But in camera M or S modes, Auto ISO instead increases when underexposure would result at the users shutter speed selection.
This Minimum Shutter Speed is Not an absolute minimum, shutter speed has to go lower as necessary when Maximum ISO is reached in dim light. In camera modes A and P, shutter speed naturally falls lower as the light becomes dimmer. But in Auto ISO, when it reaches this Minimum Shutter Speed, it holds there, it being the threshold when Auto ISO starts advancing instead. Therefore, this menu value will become The One Fixed Shutter Speed whenever Auto ISO is active... Auto ISO values between Minimum and Maximum ISO will always use this Minimum shutter speed (so give it some thought - Minimum or Maximum ISO are limits which will allow other shutter speeds, but if Auto ISO is active in between, this is the shutter speed you will be using). If this Minimum Shutter Speed is too high, you are probably always at high ISO. If too low, then the shutter speed may always be too slow. The more recent Nikon models add an "Auto" choice here (for Minimum Shutter Speed), which will use the handholding rule of thumb of the shutter speed being "1/focal length seconds" for the focal length in use. For cropped DX models, this will be 1/(focal length x 1.5) seconds.
Note however, if using flash, then there is also another Minimum Shutter Speed With Flash (E2 menu in some, or 1/60 second default in others). This is just because if we are using flash, then we probably don't want the slow shutter speed that the dim ambient may actually meter (Slow Sync and Rear Curtain Sync ignore it, and will use the metered slow shutter speed). With Auto ISO, some camera models ignore this flash Minimum (to use the Auto ISO Minimum Shutter Speed), but generally most models will use the fastest of these two minimums (with flash and Auto ISO).
But in any case, when Maximum ISO is reached, ISO can go no higher, so then shutter speed must be allowed to go even lower than this Minimum Shutter Speed (which will still be using that Maximum ISO). Because Auto ISO works as last resort, when the automated settings have no more range to go, and increasing ISO is the last resort.
In camera S mode, we set shutter speed, and as the light falls, the aperture opens wider. When the lens hits the wide open limit and can go no more, then Auto ISO starts increasing. That means if Auto ISO kicks in, the lens aperture will be wide open, possibly not the best general choice for every picture.
In camera A mode, we set aperture and as the light falls, shutter speed slows. The concept is when it hits maximum limit, then auto ISO starts increasing. But since the lowest 30 second value would be unacceptable, the Auto ISO menu adds an intermediate Minimum Shutter Speed setting, to be the threshold for increasing ISO. We can set any shutter speed as minimum, meaning this is the shutter speed active Auto ISO will always be using. So note that this Minimum Shutter Speed is sort of a replacement for S mode with Auto ISO, since it is the shutter speed that active ISO will always use (but we can still choose aperture). Shutter speed will of course go even slower if Maximum ISO is not sufficient.
In camera M mode, we set both shutter speed and aperture, which cannot vary from your settings. Which then is likely usually not precisely correct for the metered light, but Auto ISO can be used to seek correct exposure. So then Auto ISO tries to be enough to make it be right, at that selected shutter and aperture. So in this way, Manual camera mode can become Automatic point&shoot, so to speak. Some users doing sports photography claim advantage of this (being able to specify the exact shutter speed and aperture they think they need, but allowing any ISO to become an automatic exposure anyway). Of course, they do try to stay in a good proper ballpark, but I suspect this is subjective. It is cute, and it does make Manual mode be another auto mode, but Auto ISO will have less possible range than if also allowing camera settings to vary as needed. We do always need to watch what the camera automation is doing.
I would suggest that setting aperture and minimum shutter speed for Auto ISO in A mode is essentially the same thing as setting same values in Manual mode with Auto ISO, but with greater possible range, allowing a faster shutter in the brighter light, and able to force a slower shutter when maximum ISO simply is not enough. The only alternatives (other than greater user attention) are a bad exposure.
Nikon Auto ISO with Flash
First, no Manual flash can deal with Auto ISO changing, so this question is about TTL flash. All camera models using Auto ISO always remain at Minimum ISO if a Manual flash is detected present (but remote flashes are not detected). But Auto ISO must be Off with Manual flash, else we see surprises of flash exposure changing. Likewise, all models of the Commanders always force Minimum ISO.
Flash introduces a couple more factors. Camera modes S and M always use the users selected shutter speed. But for camera A or P modes, the Auto ISO menu has a Minimum Shutter Speed that determines the threshold when falling shutter speed holds there, and Auto ISO starts increasing instead. This Auto ISO Minimum Shutter Speed is not an absolute minimum, shutter speed will have to go lower when Maximum ISO is reached in dim light. There is also another feature called Minimum Shutter Speed with Flash (1/60 second is default, it can be set slower in E2 menu of some Nikon models). This Minimum Shutter Speed with Flash will not go lower with flash (if A and P modes). Camera models do vary in how these two minimums interact, but generally, the fastest Minimum will apply and hold.
This Minimum Shutter Speed with Flash is not related to Auto ISO, and it prevents using the excessively slow shutter speed that the dim ambient would otherwise meter. An exception is if we instead want it, then we select Slow Sync, or Nikon's Rear Curtain Sync mode, when then both Minimums are ignored, so that the dim ambient actual metered shutter speed value is used (which is the meaning of Slow Sync, it allows the slow metered shutter speeds with flash). I think Canon users must select Slow with Rear, but Nikon Rear Curtain also selects Slow. Rear Curtain has no purpose if no slow shutter motion blur exists.
There have been at least three methods of how Nikon Auto ISO works with current digital iTTL flash, so camera models can act different, depending on age. There is no general answer to how Auto ISO will work with flash, it depends on the camera model. So TTL flash in dim places where flash is needed will see one of these three possible Nikon Auto ISO actions in your iTTL camera (this group A, B, C names are are my own made-up nomenclature to call them).
When a TTL flash is recognized as present on older Nikon iTTL DSLR models (D70 in 2004, until the D300S changed in 2009), Auto ISO normally remains at Minimum ISO (regardless of the ambient, which might be dim, but which is why we are using flash instead). Could say that Auto ISO turns itself off if it recognizes a TTL flash is present. However, Auto ISO still can increase, but only at the last instant when the TTL flash limits out at maximum power, and actually needs the help (hotshoe and internal flash, the commander has no communication to do this). You can see this action with a impossibly doomed ceiling bounce attempt at f/16.
Routinely, Auto ISO always only triggers at some extreme of whatever available limit exists. For examples of this available limit 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 Minimum Shutter Speed 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 limit is reached. In camera mode M with no flash, Auto ISO increases at any aperture and shutter speed not already giving correct exposure (actually, it can also decrease a little below Minimum). With TTL flash (on these older camera models), the ISO holds at Minimum ISO, but the limit changes to be the flash power, so the TTL 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 choose to use flash instead. You can turn Auto ISO off (except not if in camera Auto mode).
Some top models show ISO in the viewfinder, and in this older system, a confusing quirk for Nikon flash is that the Auto ISO value shown in the viewfinder never shows this ISO emergency increase for flash. With TTL flash on, the viewfinder always shows the Minimum ISO setting, with no hint if Auto ISO will trigger (due to insufficient flash power). 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.
Then about 2009, Nikon got the idea to allow Auto ISO to go high with TTL flash, first on the D300S. Actually, the camera meters the ambient to set whatever high ISO the ambient needs. I think this change may be related to the concept of balanced flash, TTL BL, which is the Nikon default, with the flash to become fill flash then. Of course, in bright sun, Auto ISO should stay at minimum. However, the system cannot use ISO 3200 with flash, which would be really excessive flash exposure. At least minimum 1/128 flash power at 24 mm might be ONLY Guide Number 48 at ISO 3200, which could be usable in some situations, not less than f/4.8 at 10 feet. This leaves no TTL range to deal with things. But ISO is allowed to go pretty high (group B). So at the instant when TTL preflash is metered, ISO is lowered from the ambient value to be a usable flash value. But lowering ISO from the 3200 that the ambient metered and set the camera exposure for, to whatever lower ISO value the flash can deal with... underexposes the ambient. Which is probably good, because incandescent ambient that shows up is orange. And the flash does add illumination. Nikon started adding filter holders to the flashes then though, so we could correct it with a CTO color filter. Forcing fill flash seems to be the plan. Or of course, we could always turn Auto ISO off with flash if we knew to. The camera will still work. :) Except Auto ISO is always On in camera Auto mode.
Then Nikon changed the method yet again (fixed it again). Except the internal flash is still the previous group B way, Auto ISO increasing for the dim ambient. But with hot shoe TTL flash, this is corrected, and returns almost back to the "Older" way group A above. Except Auto ISO will increase slightly with external TTL flash, but only to 4x Minimum ISO (two stops), like to ISO 400, which is very reasonable for bounce. Logic and reason won one. :)
The vast seven stop range of the TTL flash mostly only sees ISO 400 then. But if the distance is very short with TTL flash (2 or 3 feet) or a wide aperture, even ISO 400 may be too much, and when true, Auto ISO will be reduced below 400, as necessary. And if the opposite, when and if the flash power is insufficient (for the mode A, P & S flash limits of shutter speed or aperture), the Auto ISO will be raised above 400 as needed. It does seem prudent to turn Auto ISO OFF with flash, and to actually plan for the situation.
Internal flash Auto ISO action in recent Group C cameras
The list at right is true for most models, but I'm unsure of a few models at the group division lines when it changed. I would be very appreciative to receive any camera model list Corrections, thank you (see How, below).
These group C camera models (my own nomenclature attempt) will limit the Auto ISO increase with TTL hot shoe flash to two stops maximum ISO increase, or to 4x Minimum ISO, presumably usually to ISO 400.
A fully powered speedlight (Nikon SB-800 or Yongnuo YN565 class) has 24mm guide numbers of about:
Full power GN 98, x2 for ISO 400 is GN 196 (feet)
1/128 power GN 8.5, x2 for ISO 400 is GN 17
So these TTL flash can operate over a seven stop range of power level, which at 24mm in this ISO 400 situation is from GN 17 to GN 196. Which provides for very many common situations, and TTL at these extremes might range from f/16 at 1 foot (at 1/128 power to stop motion in water drop splashes), to f/2.8 at 70 feet (a ball game?). There are charts in the Nikon flash manuals describing these ranges (regarding manual ISO, not Auto ISO). GN for longer zooms could approach up to 2x this 24mm GN, which both the water drops and the ball game would surely use.
But exceptions exist, due to limitations of maximum and minimum flash power level capability.
Suppose we try f/4 at 24mm at ISO 400 (and I did try it).
If at 6 feet, this is GN 24, and it uses the ISO 400, at very low power.
If at 3 feet, this is GN 12, which the flash at 24mm cannot do, even at 1/128 power. So the camera used ISO 200, out of necessity.
Suppose we try f/22 at 22 feet. This is GN 484, but the flash at 24 mm can only do GN 196 at ISO 400. So it used ISO 1400, out of necessity. Technically of course, the camera meters the TTL preflash instead of using guide numbers, but the guide numbers give us humans a clue about the capabilities and possibilities. And there are differences. The TTL power level adapts to switching to ISO 1400, but it changes the ambient metered at ISO 400. However, the flash did still illuminate the scene, but differently. How noticeable depends on the scene distances from the flash.
This necessary ISO shift is good stuff, and both the Nikon and the Yongnuo flashes see this same action (deviating from ISO 400 in extreme cases). The camera controls TTL.
Top camera models do show the current Auto ISO value in the viewfinder right beside where it says Auto ISO. But in this situation (when ISO may be modified), it always continues to say ISO 400, even if conditions do switch it to ISO 200 or ISO 1400. Group A cameras (TTL flash at Minimum ISO) also do this too, viewfinder remains at Minimum ISO in group A, except ISO can heroically increase when the TTL flash power is deemed insufficient. These cannot meter and switch until the TTL preflash, so there is no time to affect the viewfinder. But the rear LCD result does show Auto ISO in red.
And Group B (TTL flash at high ISO) also must do it. Obviously Group B cannot do much flash at ISO 3200. Because ISO 3200 GN is 5.67 times IS0 100 GN, or maximum 24mm GN 98 x 5.67 = GN 555 at ISO 3200, therefore 10 feet would require f/55.5. This does not happen. However, minimum 1/128 flash power at 24 mm might be ONLY Guide Number 48 at ISO 3200, which could be usable in some situations, not less than f/4.8 at 10 feet. So ISO will be pretty high (if ambient ISO is). I assume the same algorithm is used in all models (limiting ISO as necessary). So ISO 3200 is likely reduced some for TTL flash at last instant. Which then ISO also obviously affects the ambient exposure metered at ISO 3200, underexposing it now. However, it is also lighted by flash, so we might not notice much. So, there are unknown things happening. My guess is ISO 400 sees much less degree of change, already with a more reasonable and usable seven stop TTL range.
How does "Auto ISO with flash" work in your model?
This property is not in the specs, but we should know what to expect. It's very simple to determine how your camera model responds to Auto ISO with TTL flash. The LCD Exif data will show the ISO value used, or a few top models will show current ISO in the viewfinder.
Results for hot shoe flashes
So if the flash picture remains at unchanged Minimum ISO, you have the first version A above.
If you still see the high ISO for the flash picture, you have the middle version B.
If you see only two stops increase above Minimum (4x, like ISO 100 to ISO 400), you have the most recent version C.
Being up too close is a problem for this test. If you get ISO results lower than expected, please repeat same thing again, but at 12 feet, or at f/11, or even both, just to be clear.
My own notion is that groups A or C are much better for TTL flash, however we can always turn Auto ISO off (but Not if in camera Auto mode).
Note that the single gray histogram shown on this data screen is the B&W luminance histogram, not real data, which will not show clipping, so it's totally useless in that regard. To see the real actual data, we should look ONLY at the three RGB histograms if concerning clipping (these are shown on a different data screen here).
Repeated again, in any Nikon DSLR camera model, Auto ISO always remains at Minimum ISO (Auto ISO is not used) WHEN the internal or hot shoe flash is in Manual flash mode, and is recognized present, or when using Commander/Remote mode. Manual flash simply cannot automatically respond to ISO changes. 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 - which cannot respond to ISO changing. We set the power level of manual lights as appropriate, which cannot tolerate ISO changing on its own.
The internal TTL flash is same Auto ISO action for groups A and B, but internal flash Auto ISO stays high in group C. Its lower power probably often needs it.
Manual flash can never increase ISO (assuming the camera can recognize it is present). Manual flash cannot react to Auto ISO changes, so Auto ISO must be turned off (and the camera does that if a manual flash is recognized present).
Likewise, use of Commander in any model never increases ISO, TTL or Manual.
An off camera flash, or a flash brand that is not recognized present by the camera, will not prevent Auto ISO increases. Auto ISO must be turned off to use manual flash.
So when indoor flash 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. :) There are times it is best turned off.
Otherwise, if Group B Auto ISO is on indoors, the camera exposure automation will fully meter for the (probably orange incandescent) indoor lighting. So then we have to use Incandescent white balance to correct for it. But then the white fill flash will appear very blue, so then we need an orange CTO filter on the flash head. Or instead, we can simply turn Auto ISO off, and use Flash white balance normally. The camera really will still work fine with Auto ISO turned off. :) Bounce flash at ISO 400 with Flash white balance is likely the very best choice anyway.