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The Auto ISO does not change - always 2500 when set to 5000


rtcary
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I do a lot of event photography where the lighting is quite low, so my solution is to use bounce flash with the flash set to TTL and

Mode: Manual; SS: 1/125; f\stop: 5.6; ISO: Auto (Max = 5000). The results are amazingly good, yet the ISO is always 2500. Doing events for the museum has just started due to a relaxation of COVID regulations and my A7RM3 is new for me.

I do need to try fixed ISO, but this behavior I find interesting.

Any suggestions as to why it picks 1/2 of the Max setting?

Todd

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I never thought about this before, so I did a simple test.  I use an a850, but I bet your camera works the same way.  I set the exposure mode to P and the ISO to AUTO.

I took two shots -- one in dim light and one in bright light with the flash ON for both.

In dim light, the camera sets the ISO to 1600.

In bright light, the camera sets the ISO to 200.

So in AUTO ISO mode, the camera sets the ISO to the ambient light. 

There may be some way to override that on your camera -- while still using AUTO ISO.  I don't see a way to do it on mine.

Is there a problem choosing a specific ISO?

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I ran some tests with my normal settings:

Mode: M; SS: 1/125; f/5.6; ISO: Auto (Max = 5000); Flash: TTL.

In the dark part of the room, ISO = 2500; with light streaming in through the window, ISO = 2500.

Conclusion: as XKAES suggested, why not a fixed ISO setting - the Auto ISO + flash TTL seems to confuse the A7RIII.

Without the flash turned on, the ISO does change according to the light conditions.

Anyone else able to duplicate my results with exactly the same settings?

Todd

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A long time ago ( during the era of film cameras such as Minolta Maxxum/Alpha/Dynax 7 and 9 where auto flash was specified between ISO 25-1000 in the body owner's manuals), there was a limit to the ISO number where auto flash (TTL) could be used since there was few films that could be available with such high ISO and the fact that the electronics were probably incapable of cutting down the flash fast enough for these high ISO settings providing unreliable exposure.

My guess is there is still a limit to the ISO number where a flash can be used in TTL but it is not published anymore and the liberal use of Auto ISO is probably showing these limitations.

Did you try fixed ISO at 5000 and see if your results are still good ? ( or maybe the body will revert down to 2500 or not allow for fixed 5000 with a flash mounted and powered on the body) ?

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6 hours ago, rtcary said:

I ran some tests with my normal settings:

Mode: M; SS: 1/125; f/5.6; ISO: Auto (Max = 5000); Flash: TTL.

In the dark part of the room, ISO = 2500; with light streaming in through the window, ISO = 2500.

Conclusion: as XKAES suggested, why not a fixed ISO setting - the Auto ISO + flash TTL seems to confuse the A7RIII.

Without the flash turned on, the ISO does change according to the light conditions.

Anyone else able to duplicate my results with exactly the same settings?

Todd

I used P mode in my test, and you used M mode.  That could make the difference.  The ISO changed automatically in P mode.  Maybe it doesn't in M mode.  I'll give it a try and see.

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I think I've found the answer -- although it is just a footnote in my owner's manual -- and it may not apply to your camera, but I think it does.

The AUTO ISO setting is only available in the P, A, and S exposure modes.  In M exposure mode, the ISO is set to the camera's default (for my camera it is ISO 200), unless you manually set it to something else.

So in M mode you choose the ISO, but in the other modes the AUTO ISO will vary depending on the ambient light -- and the AUTO ISO range of your camera.

But this only makes sense.  You use M mode when you want to be in charge of the exposure.  That means you can't use TTL, ADI, BOUNCE or any CAMERA-CONTROLLED auto-exposure, since you have set the speed and the f-stop.

So back to your original question.  You can only use TTL, ADI, or BOUNCE CAMERA-CONTROLLED flash in auto-exposure modes -- P, S, and A.  You can choose either AUTO ISO where the ISO will vary based on the ambient light, or you can set the ISO to whatever you want -- you can change it for any particular exposure, of course.

Edited by XKAES
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You might wonder, "Why does the camera allow you to set the ISO in M exposure mode?" -- where you set the shutter speed and f-stop to whatever you want.

The camera only displays the set shutter speed and f-stop, but if you dial in the ISO that you are using, the camera's exposure scale will display how far off your set shutter speed and f-stop are from what the camera's TTL meter THINKS is a correct exposure. 

That's the exposure for the AMBIENT light -- and not considering any electronic FLASH light.

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