Auto-ISO and TTL flash

Last night I picked up my D600 to get a few quick shots of some dinner party guests. They were all overexposed, even though the camera's exposure comp was set to 0 and the flash TTL setting was -1/3. I realized after the party that the camera was set to Auto-ISO, which I had been playing with earlier in the week, but rarely use otherwise.

Is Auto-ISO and a TTL speedlight something to avoid? Or did I miss some critical trick to make this work right?

Also, the pictures are pretty noisy, so I just checked the EXIF and the camera chose ASA 12,800! I know you can set a lower max ISO in the settings menu, but it bugs me that the camera defaulted to the maximum ISO setting for what could have been shot at 100. For now, no more Auto-ISO for me.

I avoid Auto ISO if using flash, because, well, we are using flash instead. But, it is just one opinion.

Which flash? It must have been the internal flash?

There are three generations of Nikon Auto ISO and flash, so it is hard to explain.
1. D300 and all older,
2. newer cameras, in between 1 and 3.
3. and most recent very few models, D800 on. I think the D600 is in this group too.


1. Never boosted Auto ISO if flash was present, being used. ISO stayed at Minimum with flash. It might boost ISO ONLY IF the TTL flash power was otherwise insufficient. This works best, IMO.

2. Always boosted Auto ISO fully according to the ambient, regardless if flash is used or not, so flash indoors was always highest ISO.

3 Nikon reconsidered this - Newest models only boosts Auto ISO two stops (to ISO 400) for external flash, but still always boosts ISO for internal flash, like 2.

Nikon does not specify this Auto ISO behavior with flash, but a simple test picture with indoor TTL flash will show the Auto ISO it obviously used.

Using Auto ISO to boost the ambient with flash makes the orange incandescent light become visible, adds ISO noise, and converts the TTL BL flash system to be fill flash instead of main flash. If an older flash with an actual TTL vs TTL BL menu, TTL mode will overexpose it..

Internal flash is tiny, and any distance more than a few feet, might need additional ISO. Shouldn't need maximum ISO.

Try flash without Auto ISO. Maybe use up to ISO 400 for the little internal flash if needed. 400 should do 10 or 12 feet, depending on aperture.

What was the camera mode, PASM? What was shutter speed and aperture? The little internal flash power needs a wider aperture, and the shutter speed affects ambient ISO.

Overexposure normally seems less likely, because there are a few factors suggesting a little underexposure is more often expected. Direct flash might often overexpose due to the dark background, but high ISO ought to rule that out.

Of course, it could just be normal metering issues. Subjects that are mostly black or dark color will overexpose.

I agree, avoid using Auto-ISO and flash. My problem was that I didn't know how it would behave. It won't happen again. I even tried to manually adjust the ISO to 400 with the button controls, but did not go into the menu to turn off Auto-ISO. The top screen showed me making the adjustment, though it also said "Auto-ISO." This was in Aperture Priority mode.

The flash was an external Mecablitz 52 speedlight with fresh batteries. The background was white walls, which would have suggested possible underexposure, but produced overexposure instead. As you say, ISO 400 would have been plenty, yet the camera defaulted to 12,800 with the Auto-ISO.

Here's another funny thing from the evening: when I got the camera out, they really didn't want to spend any time setting up a shot, posing, making sure I got it, etc. But later in the night I started showing pics of them from 12 years ago, and they were so happy about those... asking me to email them, etc. Yet they saw no current value in making sure we had a good record of last night's party.

I am puzzled, because the Metz 52 flash ought to be compatible with Nikon CLS communication, so camera should have recognized the flash was present on the hot shoe. For iTTL to work, it has to. And I thought D600 is a newest model which limited the Auto ISO increase with flash to only two stops (ISO 400).

Guessing, but probably the Metz 52 is a model that includes a specific menu for TTL vs TTL BL mode. (Metz does that much better than Nikon today. ) So with high ISO, the ambient is "normally exposed", and TTL BL will default to be fill flash and will back off, and will not overexpose subject in bright ambient.

However, TTL mode will overexpose in significant ambient, with the proper ambient exposure, and the proper TTL flash exposure, which sum is 2x proper exposure, which is 1 stop overexposed. My guess is that explains the overexposure, but does not explain why the Auto ISO was so high on a D600.

WayneF said:

I am puzzled...

Click to expand...

Yep, I was puzzled to. The expected behavior you described is also what I expected. The camera normally recognizes this flash.

ETA: EXIF shows "Flash: Fired, Strobe Return Light Detected, Compulsory Flash Firing, Flash Function Present, No red-eye reduction."