Auto ISO

eastside

Senior Member
I made 3 photos; manual mode, same subject, same focus. (1) 1/100, F8, Auto ISO2800; (2) 1/200, F8, Auto ISO9000, (3)1/400. F8, Auto ISO12800. I use single-servo AF mode (AF-S),EV Compensation for each photo was O, GrP Group Area (5 point). Why did the auto ISO change so much: 2800 to 9000 to 12800? After all the only change I made from photo to photo was doubling the shutter speed one stop. So I expected the ISO to double when going from (1) to (2) and also from (2) to (3). I was not in Liveview. Active D-Lighting was set to N. Center weighted 12mm. Not using a tripod.

As someone who has done film for over 50 years, this ISO change is confusing.

Two possibilities: I don't know what I am doing OR a slight defect in my 750.

eastside
 

Bikerbrent

Senior Member
Keep in mind that your camera is much more reliable than your eyes at detecting light levels. The human eye and brain tend to smooth out smaller changes in light level. What was your lighting environment. Was it outdoors or indoors with light from a window. If so, were there clouds in the sky that could partially block the sun at times. If indoors under artificial light, what kind of light source. Florescent lights and LED bulbs sometime flicker in sync with the power cycling so depending on when the camera reads the exposure might have some effect on the ISO setting. As long as the exposures are correct, I would not worry about it.
 
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mikew_RIP

Senior Member
First where all 3 correctly exposed, next slight movement from you could cause a different exposure to be needed due to a change in metering point, i use auto iso in manual 95% of the time with a D500 and think i may have had discrepancy's in the expected iso but not sure,as i get correct exposure i never gave it any more thought.
 

TieuNgao

Senior Member
I agree with both Bikerbrent and mikew; it's not a perfectly controlled environment regarding the light source and framing (I did similar experiment in the past). If your 3 photos show the same level of exposure then the Auto ISO works.
 

eastside

Senior Member
To Bikerbrent. Mikew, and TieuNgao,

Thank you for your responses. The experiment was done indoors looking outside through a closed window at a a stone wall bathed in sunlight. A 1/2 stop is now understandable to me.
 
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