Is auto ISO fixed with fill flash?

Started 3 months ago | Questions thread
Kev The Doc Senior Member • Posts: 2,216
Re: Is auto ISO fixed with fill flash?

dbm61 wrote:

Jigal wrote:

Currently I'm mainly shooting pups in our living room using a bounce flash. I set the A6400 in P mode, the Godox TT685s to TTL and noticed that the photos were quite dark. Exposure compensation on both the flash and the camera didn't make any difference. The histograms showed that non of the graphs (white, red, green, blue) got further to the right than about two thirds of the way.

Suddenly I noticed that all images had an ISO value of 200. I had set the auto ISO range to 100-400. Tonight I experimented a bit and each time the ISO was set to half the maximum value of that range (for all photos). I didn't change the lower limit, so maybe it sets a value half way between maximum and minimum.

I couldn't find anything in a manual about this but I'm really surprised by this. I kind of expected the camera to select the lowest possible ISO setting in the range that the light provided by the flash would allow.

Is this the behaviour of the camera? Or is it influenced by this particular flash? (Should I look for the HVL-F60RM?)

Are there settings to make it automatically select a value inside the range?

Auto ISO and TTL are counter-intuitive. Auto ISO will try and boost the exposure before the flash even fires. Then TTL tries to sense the needed flash to fill in the rest. And bouncing a flash loses power. So you're using the worse possible scenario for the flash.

Try setting the ISO fixed to be either 100 or 640 (Sorry, I don't know where the dual ISO kicks in on the A6400 for sure, but it's probably 640). Then use TTL and see where the exposure ends up. If bouncing still underexposes, increase the flash compensation to get the exposure you're looking for.

I’m not sure how you’ve come to the conclusion that auto iso and TTL are counter intuitive? I’d say they work together to yield the correct exposure, but until you’re experienced and knowledgeable with flash shooting you might not always get the desired results. I had exactly the same problems as the OP when starting off, but now I realize I didn’t know what I was doing 😀.

I don’t disagree however that setting exposure manually (including iso) is one valid option, it gives the photographer more control on the outcome, the downside however being it’s a little slower if your in an ever changing lighting environment and needing to catch action as it happens. If you’re setting exposure manually and still getting underexposed shots then most likely your flash is putting out it’s max output but it’s still not enough (the flash tells you this on the screen for a few seconds right after the shot), so adjusting the flash comp won’t necessarily fix the problem, you may need to increase the ambient exposure on the camera.

 Kev The Doc's gear list:Kev The Doc's gear list
DxO One Sony a7R II Sony a6500 Zeiss Loxia 21mm F2.8 Sony 1.4x Teleconverter (2016) +4 more
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