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Auto ISO and Flash

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 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 my own made-up nomenclature to call them).

Nikon DSLR
Auto ISO action with TTL hot shoe flash indoors

Group A - ISO stays at Minimum ISO

D2H Jul 2003
D70 Feb 2004
D70S May 2005
D200 Nov 2005
D80 Aug 2006
D40 Nov 2006
D300 Jul 2007
D3 Jul 2007
D60 Jan 2008
D700 Jul 2008
D90 Aug 2008

Group B - is at high ISO for dim ambient

D5000 Apr 2009 ?
D300S Jul 2009
D3000 Jul 2009
D3S Oct 2009
D3100 Aug 2010 ?

Group C - is at most 2 stops (4x) above Minimum ISO

D7000 Sep 2010
D5100 Apr 2011 ?
D4 Jan 2012
D800 Mar 2012
D3200 Apr 2012
D600 Sep 2012
D5200 Nov 2012
D7100 Feb 2013
D5300 Oct 2013
D610 Oct 2013
D3300 Jan 2014
D810 Jun 2014
D750 Sep 2014
D5500 Jan 2015
D810A Feb 2015
D7200 Apr 2015

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).

Thoughts on Auto ISO action with the recent Nikon Group C camera models

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 24 mm 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 24 mm 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 24 mm 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 24 mm 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 24 mm 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 24 mm 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.

A summary of ifs and buts about other cases.

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.


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