Nikon d750 AUTO ISO. My speculation

[FONT=&quot]Hi all,[/FONT]
[FONT=&quot]many people including me have commented how weird auto iso works with flash on nikon bodies (I have a d750).[/FONT]
[FONT=&quot]The ideal implementation would be the camera to get a similar to an ambient exposure and then use the flash (typically I have it dialed down) to only give a slight fill.[/FONT]
[FONT=&quot]My experience says though that with auto iso you are not sure what the camera would finally do. I have found many times that camera would use basic iso trying to illuminate the scene on flash and others use it only as a week fill light.[/FONT]
[FONT=&quot]
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[FONT=&quot]The reason I made this post is that I have noticed something that I would like to share with you, that looks to be the way to prebias the system on what to do.[/FONT]
[FONT=&quot]When my camera is set to AUTO ISO and below it gives the number of 100 then my shot would be shot at iso 100 and the flash would only try to light the whole scene.[/FONT]
[FONT=&quot]In cases though that I am at AUTO ISO and below it gives the number of 1600 (Typical iso for enough indoor ambient light to register) I would get a much more balanced exposure.

[/FONT]
[FONT=&quot]For what I am trying to say here I have not found enough time to extensively test it but to me it looks like that when you are at AUTO ISO with a flash unit the iso value below (the word AUTO) somehow gives a bias to the system what to do with the ambient exposure.

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[FONT=&quot]This is not how camera works in AUTO ISO when there is no flash.[/FONT]
[FONT=&quot]Have you found the same thing? Try it shoot a scene (indoor) at AUTO ISO 100 and set flash at +0 TTL. Then pick AUTO ISO 1600 with flash at +0 TTL.[/FONT]
[FONT=&quot]How these two look like to you?[/FONT]
[FONT=&quot]
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[FONT=&quot]I would like to thank you for your time[/FONT]
[FONT=&quot]Regards[/FONT]
[FONT=&quot]Alex[/FONT]

I think this was already covered in your recent thread at http://nikonites.com/d750/36412-how-your-auto-iso-flash-works.html#axzz4CQDzxdTC

Speaking of D750 (and several recent models):

If hot shoe flash, any Auto ISO increase with flash will be limited to 4x (2 stops, like from ISO 100 to ISO 400) above whatever Minimum ISO is set to.


This is easily seen and verified... Using camera Manual mode M (so that shutter speed and f/stop will not change), then simply reaching up and turning the flash on or off will see the Auto ISO changes just described above.


That is speaking of the recent camera models. Older models differ greatly, in two ways:

Initial iTTL models (D70 to D300 era) will never increase Auto ISO with any TTL flash. Makes sense, because we are using flash instead.

Intermediate age models (D300S until roughly just before the D800) will always increase Auto ISO for the ambient level for TTL flash. We might call this balanced flash, but which IMO makes absolutely no sense (see below about balanced flash).

Later models fix that and limit Auto ISO with hot shoe flash to 4x ISO increase, like to to ISO 400 is about right for bounce flash.


One ISO exception, for all models:

If the maximum flash power level is deemed insufficient for the TTL preflash measurement, Auto ISO will increase with any iTTL flash until the power level is deemed sufficient.


Balanced flash means that the flash level is adjusted down (less than metered flash exposure) so as to not overexpose the added combination with the metered ambient exposure.

The problem with indoor "balanced flash" is that indoor light is typically incandescent or fluorescent, which is typically orange or green, and this causes white balance problems mixing with white flash. To correct that situation, suitable color filters can be used on the flash to allow using incandescent of fluorescent white balance. But if not using the filters on the flash, balanced flash indoors may be about the last thing you'd want.

Unfortunately I think it is my english here. No we are not talking about my previous post. What I am describing here is rather different. Need to put my baby to sleep. Will comment tomorrow
Alex