a Poll about Auto ISO action with hot shoe TTL flash

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OK, got this done, I think. I have my Auto ISO set for 100-8000, camera D500, lens Sigma 17-70 f2.8-4, SB-800.

Did the first test, about 12 feet away from some glass work in a dim corner of the family room.

1/60, f4.5 no flash, pegs at 8000 and is rather underexposed, no surprise there.
1/60, f4.5 flash on TTL, flash LCD shows ISO 400, VF Auto ISO shows ISO 400 as well as on the back LCD when checking Info.
Take a picture, however, and the ISO is 2000. I do have the flash at full power, with no EC, e4 is set for "Subject and background". This is 2000 because it is taking ambient into account, correct? If so, why show the 400 in the first place? When I change e4 for "Subject", the flash and camera show ISO 100, and the EXIF shows ISO 100.

Now, I wonder, what happens if the flash is set to TTL/BL? Same results, ISO 2200 this time with "Subject and background", 100 with "Subject".

I am not real "flash savvy", so I am not at all sure what should happen with TTL vs TTL/BL, I thought with the latter setting the flash was supposed to take into account ambient light, much like the e4 setting in the D500/D5, is that correct?

Not sure what this all means, I hope this is of some help. If I have done this wrong, wouldn't surprise me at all, or if there is something else you want me to try let me know.
 
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OK, got this done, I think. I have my Auto ISO set for 100-8000, camera D500, lens Sigma 17-70 f2.8-4, SB-800.

Did the first test, about 12 feet away from some glass work in a dim corner of the family room.

1/60, f4.5 no flash, pegs at 8000 and is rather underexposed, no surprise there.
1/60, f4.5 flash on TTL, flash LCD shows ISO 400, VF Auto ISO shows ISO 400 as well as on the back LCD when checking Info.
Take a picture, however, and the ISO is 2000. I do have the flash at full power, with no EC, e4 is set for "Subject and background". This is 2000 because it is taking ambient into account, correct? If so, why show the 400 in the first place? When I change e4 for "Subject", the flash and camera show ISO 100, and the EXIF shows ISO 100.

Now, I wonder, what happens if the flash is set to TTL/BL? Same results, ISO 2200 this time with "Subject and background", 100 with "Subject".

I am not real "flash savvy", so I am not at all sure what should happen with TTL vs TTL/BL, I thought with the latter setting the flash was supposed to take into account ambient light, much like the e4 setting in the D500/D5, is that correct?

Not sure what this all means, I hope this is of some help. If I have done this wrong, wouldn't surprise me at all, or if there is something else you want me to try let me know.


I wondered about your lens zoom? If 24mm or longer, then 12 feet f/4.5 should be no issue. But if using one of wide angle adapters, it starts becoming more iffy, 12 feet is marginal.

Many of the camera models only show the words "Auto ISO" in the viewfinder. No value shown. One has to look in the Exif (on rear LCD afterwards) to see the ISO actually used.

The D800 which I have, and the I'm pretty sure the D500 too, also show the actual ISO to be used, in real time. Which is great, it is good to know what ISO will be. However... if the flash power capability is deemed insufficient, Auto ISO will increase as needed to deal with the power that is available. This is apparently not known until preflash time, which is just milliseconds before the shutter takes the picture, so there is no time to display this new ISO. The new value still shows in the Exif on the Rear LCD, in red.

My only point is, this would seem very natural, if you are were in the insufficient power distance range. You could verify that by turning Auto ISO off, and setting the ISO 400, and taking the same flash picture, to see if you did have sufficient TTL power. The SB-800 LED would blink the maximum power warning then.

The strong fallacy of my logic is of course that you say the ISO 100 case did in fact work. That certainly sounds like sufficient power. I am puzzled too.

I am not qualified to speak on the "subject only / subject and background" topic. That's all new to me, my models don't have that. I will work on it.

I misspoke before, this maximum power warning would not necessarily blink with Auto ISO increasing to 2000 or whatever. That could still have a little safety margin, it might not necessarily be maximum power. Nikon probably tries to avoid the warning.

I am not real "flash savvy", so I am not at all sure what should happen with TTL vs TTL/BL, I thought with the latter setting the flash was supposed to take into account ambient light, much like the e4 setting in the D500/D5, is that correct?

I am still unsure about the D500 E4 menu, but TTL BL, I can answer. Think of it this way:

The camera automation first meters ambient, and sets shutter speed and/or aperture for the ambient. Generally (outdoors), we rely on that to be a full proper exposure, which is the point.

The preflash from the TTL flash is metered, and the flash power level is set to be a full proper exposure by the flash (into the existing camera settings). That's how TTL works.

But on the near subject, this is TWO proper exposures, and 2x exposure, and one stop overexposed (on that near subject).

That flash mode is called TTL (as opposed to TTL BL balanced mode). The SB-800 has a menu for TTL or TTL BL. Most flashes don't, but we can set TTL mode (as opposed to TTL BL) by selecting Spot Metering (the flash Never does Spot metering, it just changes this flash mode, but ambient certainly can do Spot metering... so outdoors, things can change). Both modes are generically called TTL (Through The Lens, with whatever automation the metering system does), but TTL and TTL BL also have these different metering methods.

That TTL BL does (what balanced flash does) is to reduce this metered flash level to avoid overexposing the subject with that 2x exposure just mentioned.

So we generally have two flash situations, outdoors and indoors.

1. fill flash in bright sunlight.... Metering sets up for the ambient, so with it being a proper exposure, then more TTL flash mode will overexpose the subject. In bright sun, we would need to apply about -2 EV flash compensation prevent that, and to be a useful flash picture (flash can still add a lot). TTL BL mode knows this is its purpose, and does it automatically, typically about -2 EV if in bright sun. This number does not show anywhere, it just does it. If you also try to apply flash compensation, the BL result may ignore you, the system thinks it has its own job to do. But either way to apply about -2 EV flash compensation works pretty well in bright sun.

2. flash indoors, typically in low ambient (typically unmeterable, meaning that for the f/5 we set, maybe the shutter should be 1/8 second for the indoor ambient, however the camera mode A or P will set a default 1/60 second Minimum Shutter Speed With Flash (menu E2 default on those that have the menu). This means the ambient is underexposed 3 stops, since 1/60 is not 1/8, which I then call unmeterable... i.e., insignificant ambient level. Intentionally underexposed, because we are using flash instead. The TTL flash provides the illumination. And there is no significant ambient to "balance", so we hope the flash does come ahead on strong and illuminates the subject properly, as metered.

The Nikon manuals say it this way:

TTL BL - Automatic Balanced Fill-Flash:
The flash output level is automatically adjusted for a well-balanced exposure of the main subject and background.

TTL - Standard TTL flash:
The main subject is correctly exposed regardless of the background brightness. This is useful when you want to highlight the main subject.

In Nikon-speak, the "main subject" means the flash illumination, and the "background" means the ambient illumination.

It means:

TTL BL: Flash power is reduced so the subject is not overexposed. Fill flash.

TTL: Flash comes ahead on strong, as metered, as needed so to speak, it says regardless, but which will be overexposed outdoors (but NOT indoors in insignificant ambient). Flash is the main light.

Anyway, that is what TTL BL mode is.

Another issue is, with fill flash in bright sun, even if the flash gets compensated -2 EV (one way or the other, manual or TTL BL), the ambient was still considered a proper exposure, and we are still adding a bit more from the flash. The numbers are that -2 EV flash in a metered ambient is still 1/3 stop overexposed at the near subject. That is not much, but it is a little. By intentionally underexposing the ambient by 1/3 stop, now the subject comes out properly exposed, and the background is slightly darker, so the subject seems to stand out against the background better.
 
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Thanks Jim. The problem is sometimes about conflicting reports, but those are all pretty recent models, and reports say those do 4x Minimum ISO with hot shoe flash indoors, and their very recent date seems good confirming logic. Naturally, I hope you say something if you see it different.

My main uncertainly is the few models at the borderlines of change, like D5000, D3100, D7000, D5100.

I also have a D5100 which hasn't been used in a while, so I'll try your test with it later today.
 
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Wayne, I was at 70mm on the lens, and that also showed on the Flash LCD as I would expect. I do have my D500's set to actually show the ISO in the VF when in Auto ISO. This is why I found it interesting that when set to TTL with "Subject and Background" it would show 400 when in fact it actually used 2000. My guess is that much like the "number of shots on a CF card", this is the "best guess" as to "best case", and once pre-flash and all the other hoop-te-do is done to balance, the actuality is 2000.

The other thing I found interesting was that the e4 setting and the TTL/TTL-BL don't seem to have a conflict. It all seems to sort itself out.

The bottom line for me is simple, I'm glad I don't use flash much so I don't have to fully understand all of this. It also tells me that I should do a few test shots any time I am wanting to use flash. I have enough trouble now with moving my focus points around :D
 
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D5100 + 18-55 VR II set at 35mm. In auto-iso with minimum 100 and maximum 3200 at f/5.6 and 1/80s the camera chose iso3200, but the image appears to be correctly exposed.

Same setup with SB-600 mounted in TTL mode at 1/80s it chose iso 400, and the image is slightly overexposed.

Is that what you needed?
 
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D5100 + 18-55 VR II set at 35mm. In auto-iso with minimum 100 and maximum 3200 at f/5.6 and 1/80s the camera chose iso3200, but the image appears to be correctly exposed.

Same setup with SB-600 mounted in TTL mode at 1/80s it chose iso 400, and the image is slightly overexposed.

Is that what you needed?


Yes, thank you. That confirms my notion the D5100 was my group C (4x above Minimum ISO with hot shoe flash), same as the more recent models. My hesitation was because the D5100 came out a year before the D800, and while the D800 immediately raised comments about it, the D5100 did not, AFAIK.
 
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Does any Nikon camera resolve ISO at the time of flash, if not it appears to be a dangerous thing to do, use auto ISO and flash

They all do when and if it is realized that the flash power capability is insufficient for the requests of the situation (requests for distance, stopped down aperture, low ISO, etc - assuming Auto ISO and assuming iTTL flash).

Otherwise no, it is the TTL power level that the preflash metering adjusts for whatever conditions that the ambient metered already determined and changed the settings. Whatever settings are in place by ambient metering, or by the users manual input is what the flash wakes up and discovers it must use. The flash power level is adjusted accordingly, as per the TTL preflash metering (calculated using those settings). This works well, if the flash has sufficient power capability as needed.

If not Auto ISO, OR if not TTL flash, the ISO always stays at the manual ISO setting or at the Minimum Auto ISO setting. Only TTL flash power level can react to ISO changing.

There are ifs and buts, when presence of the flash can make some changes.

The power capability just mentioned.
The Auto ISO 4x Minimum limit with hot shoe flash previous discussed, on the later model cameras.
Camera modes A and P and Auto enforce a Minimum Shutter Speed with Flash (typically default 1/60 second minimum, or as per a E2 menu that may be present).
And of course, all recognized present flashes enforce the Maximum Shutter Sync speed, typically 1/200 or 1/25o second with flash.
Mostly, these are Not about exposure at all, just about other concerns or preferences.

Auto ISO is not bad for flash due to exposure concerns. The system knows how to do it.

What is bad about high ISO with flash indoors is that the indoor lighting is typically incandescent or fluorescent, which is typically orange or green in color, and their white balance simply does not match the white flash. High ISO boosts the ambient, and makes that wrong color be easily seen instead of minimized. There is really no good solution for mixed white balance from mixed lighting. But this is why the color filters are furnished with many of the flash models, as "a" solution, if not good. The orange or green filters can modify the flash color to match the ambient color, than then the ambient white balance can be used. A more common solution (considered better by many) is to use low ISO with flash, to leave the indoor ambient underexposed so its color does not show up... since we are in fact using flash instead.

This color issue is why older camera models stayed at Minimum ISO with flash, and it is why the newer cameras limit ISO to only 4x Minimum ISO with flash, because while high ISO adds range capability, it also adds serious color issues indoors.

There were a few intermediate age camera models (D300S, and I think D3000 and D3100 era) that switched thinking, and always allowed Auto ISO to go high for the ambient, regardless of the flash. My notion was that Nikon was promoting their "balanced flash" notions, but this did not last long before they corrected it. We cannot balance flash with different color ambient. However, the internal flash on the latest models still do that (high ISO with flash indoors), but hot shoe flash does not again.
 
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It's hard for me to get my mind around all this stuff. And even if I did, I use flash so seldom I would soon forget the details. I will stick to my simple method of using fixed iso with the flash in manual mode. Requires a bit of trial and error, but at least I know what I'm doing.
 
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In Nikon-speak, "subject only" means "flash" and "subject and background" means "ambient". Why they cannot say ambient is mystery to me. :)
I think it goes a bit like BL on speedlights. I did try BL just a bit, but saw no change. I suspect I just did not exercise enough of a range to get it to. BL seems to mean "ambient" also, in much the same sense (I once heard it was Background Light, then later Balanced Light... but it still seems to mean Ambient Light is given preference).

I wish they would publish some kind of matrix, at least in concept, of how all these inter-relate.
Re: f/18. I don't know your subject distance, flash to subject, 9r aperture, or Guide Number, but there is an existing algorithm, in all iTTL models (since the first D70), that if the TTL flash has insufficient power requested for the situation (f/18 could be a situation), then Auto ISO in all models will boost ISO so that the existing flash power can be sufficient. For example, f/18 bounced on the ceiling would be expected to fail, but Auto ISO about 2000 can come though to save the day. Since your first test at ISO 400 did not do that, I presume f/18 was a situation needing that. The hot shoe flash (most Nikon models) should have been blinking the maximum power level warning then.
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I was purposely aiming for extremes, the first test was with very low light and lowest base ISO setting. When I dialed up to F18 I wanted ambient just out of the equation to see how it adjusted the flash power. The flash was basically providing all illumination -- even at max (set) ISO, at F18 I got a black frame. At that setting, with subject+background, it ran the ISO all the way to max (allowed). With subject only it stopped at 1800, which I judge to be maximum flash output.

The part that is inconsistent to me is how it limits itself is very different if it's near the base ISO. If it has adequate flash power for subject-only to be at base ISO, it will allow up to two stops more in subject+background to get some ambient light in.

But... if the flash power is such that it needs to bust through the base (or maybe 2-stops over base) to get subejct exposure with full flash power, then it opens itself up to go any number of stops to get ambient, not 2 stops more. I really expected that subject+background to be ISO 7200 (2 stops over 1800), not 102400 (the max set).

My GUESS Is if I had the base ISO set at 1600 or so, then second exposure (subject + background) at F18 would be at 6400 (2 stops up). It's not dark enough right now to reproduce, but if I remember will try later.

Then if I'm really wanting to be confused I'll flip on BL on the flash and see if anything else changes.
 
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It's hard for me to get my mind around all this stuff. And even if I did, I use flash so seldom I would soon forget the details. I will stick to my simple method of using fixed iso with the flash in manual mode. Requires a bit of trial and error, but at least I know what I'm doing.

Yes! With my D800 I often leave the pop-up flash set in commander mode all the time, because in commander mode it doesn't use auto-ISO at all, even if it is turned on.

I think they are making this much more complicated than it really needs to be, maybe through lack of documentation.
 
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It's hard for me to get my mind around all this stuff. And even if I did, I use flash so seldom I would soon forget the details. I will stick to my simple method of using fixed iso with the flash in manual mode. Requires a bit of trial and error, but at least I know what I'm doing.

Knowing is the same as having control, which is a good thing. :) The dumb automation often doesn't know. :)

I agree with the fixed ISO for flash. It seems fundamental, and it is required for manual flash of course. I use TTL or manual flash about equally, they both have advantages. For me, TTL is fast and easy for the quickies, but manual is for the more studied and lengthy or critical sessions. But I do think ISO ought to be low with flash. Sunlight and flash are about the same color, so no big deal there, but matching indoor lighting that varies so much in color is far from trivial.



The part that is inconsistent to me is how it limits itself is very different if it's near the base ISO. If it has adequate flash power for subject-only to be at base ISO, it will allow up to two stops more in subject+background to get some ambient light in.

I see it simpler, that the reason is that 4x is still low ISO, the fundamental goal. But if we use bounce flash, ISO 400 is typically needed anyway, at least helpful for ceilings 1o to 12 foot high. Bounce is an extremely popular method, and ISO 400 allows automation to include it to a large degree.

The internal flash in TTL mode will still use the very high ISO indoors, but right, the commander mode never allows Auto ISO to increase. I assume TTL remotes could, the commander still has to meter and then set power levels, so it could be computed. And Commander sets manual levels too, but I suppose if we declare a remote manual flash should use 1/4 power, it ought to use 1/4 power.
 
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I see it simpler, that the reason is that 4x is still low ISO, the fundamental goal. But if we use bounce flash, ISO 400 is typically needed anyway, at least helpful for ceilings 1o to 12 foot high. Bounce is an extremely popular method, and ISO 400 allows automation to include it to a large degree.

The internal flash in TTL mode will still use the very high ISO indoors, but right, the commander mode never allows Auto ISO to increase. I assume TTL remotes could, the commander still has to meter and then set power levels, so it could be computed. And Commander sets manual levels too, but I suppose if we declare a remote manual flash should use 1/4 power, it ought to use 1/4 power.

I do not quite follow that. First, obviously no internal flash, and bounce flash wasn't involved (with the speedlight it knows that). But all that aside, here is what confuses me.

Nikon apparently thinks that it's worth two stops higher ISO to get more ambient light when you turn on Subject + background when there is adequate flash power to shoot at the base ISO set.

So when there is not adequate flash to shoot at the base ISO, who does it think it is ok to go up 6 stops? Remember this was a flat scene, so there was by definition adequate flash power at 1800 ISO to light the whole scene. When I switched to subject + background though, it added more than 6 stops to try to get ambient light.

Why this decision to go up so high? Why is the magic number 2 stops when near the base ISO, but no limit when (well) above it? That's a radical change in the amount of noise present (which presumably is why it uses only 2 stops as a limit normally).
 
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I do not quite follow that. First, obviously no internal flash, and bounce flash wasn't involved (with the speedlight it knows that). But all that aside, here is what confuses me.

Nikon apparently thinks that it's worth two stops higher ISO to get more ambient light when you turn on Subject + background when there is adequate flash power to shoot at the base ISO set.

So when there is not adequate flash to shoot at the base ISO, who does it think it is ok to go up 6 stops? Remember this was a flat scene, so there was by definition adequate flash power at 1800 ISO to light the whole scene. When I switched to subject + background though, it added more than 6 stops to try to get ambient light.

Why this decision to go up so high? Why is the magic number 2 stops when near the base ISO, but no limit when (well) above it? That's a radical change in the amount of noise present (which presumably is why it uses only 2 stops as a limit normally).


I think actually, that the ISO 400 limit just matches what Canon does. With flash, Canon has a hard Auto ISO limit of ISO 400. Which seems very reasonable to me, but there are always complaints wanting Auto ISO to go higher with flash. Nikon is not a hard limit, recent models are just 4x Minimum now, but Minimum can of course be any value. But 400 is a natural for much bounce flash. Except the Nikon internal flash still will not prevent very high Auto ISO values for ambient. It sounds like the D500 may have menus to change that now.

Or of course, Auto ISO can always be turned off, and we can set ISO as desired for the situation. The camera can still work. :)

I am puzzled by the D500 ISO options too, but I don't plan to buy one just to play with to figure this out. :) But obviously, there are changes again now.

The D500 has a new additional Auto ISO menu line, of Maximum ISO with Flash. So its ISO limit does not have to be the same Maximum as without flash. IMO, it only makes sense for it to be much lower for flash, but it is optional now. It could be set to Maximum ISO 400, which sounds right to me in any reasonable event.

And like any of the models, if the computed flash level exceeds (reaches?) maximum power capability, then Auto ISO will increase as necessary so the flash power capability can work (limited by Maximum ISO of course). We don't see that new final ISO value however, until in the Exif on the rear LCD after the picture is taken. And setting Maximum to 400 might prevent this action?

Then menu E4 is new too, now says Auto ISO can affect both subject and background (ambient), or just subject only (flash). I'm guessing, but the reports here suggest the choice of Both is like my group B, i.e., ISO will go high (probably Maximum) for the indoor ambient, trying to illuminate the whole scene. And it is the default menu, which seems a step backward again (IMO... but I suppose some may want that). Subject Only would suggest it's only about the flash, only the ISO needed by the flash is considered.

I didn't know about these new D500 menus for Auto ISO with flash (and do realize that I have never seen a D500), but it seems clear when trying to figure out what Auto ISO does with flash on the D500, these new menus must be considered and specified what was used, or else we don't really know nuthin' about the situation. If we knew which menus are chosen, it surely explains a lot. Maybe more complication now, but it does look like the D500 could be configured to do about any action desired. I don't see any other way to summarize it.

And of course, Auto ISO can always be turned off. It is absolutely necessary to turn it off for any manual flash, because manual flash cannot react to ISO changes. It's just a tool, which we should use however it gives the maximum benefit to us.
 
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I think actually, that the ISO 400 limit just matches what Canon does. With flash, Canon has a hard Auto ISO limit of ISO 400.

Sorry, just to make sure we are on the same page, the 400 was 2 stops over the base I set at 100. Or so I thought.

I did another experiment similar to the last, with the base ISO set to 500.

At F4.5, base ISO 500, shooting against a wall:

Subject = 500 (500 displayed in viewfinder also)
Subject + background = 10,000 (a bit over 4 stops, 2000 was displayed in the viewfinder)

I shifted to F16 so that the flash would not have quite enough power at ISO 500, so it had to move up:

Subject = 1400 (viewfinder showed 500, presumably inadequate power at 500, so it shifted up of necessity)
Subject + background = 102,400 (about 7.5 stops)

All of these were taken with in a fairly dark room where I could hardly see the wall about 12' in front of me (impressively it focused fine).

I experimented a bit and when I came down to F13 it reduced the ISO to 57,600 (when in subject + background), and then came down, but never below 7200 (this lens was F4.5).

I just don't see the pattern for the subject+background. I turned on a room light which provided enough light at F4.5 that ambient needed 40,000 ISO. Shot with the flash on subject+background it used 7200 with base set at 500, or about 3.5 stops, but not nearly enough for ambient to do much. That's actually less than the 10,000 it did the first time, but still more than the 2 stops I was expecting.

But... generally "Subject" does exactly what I would like -- bumps the ISO only when the flash is not strong enough. I think I'll just set it and forget it.

Incidentally in none of these did I get an indication of inadequate flash power, and all shots looked approximately the same exposure when viewed.
 
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