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ISO adjustment for Aperture

Canon 5D

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#1 ncheel044


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Posted 03 December 2017 - 02:39 PM

​Looking for a bit of advice for how much of an ISO change would be recommended for a full stop change of F/4 to F/5. I will be shooting in natural light. The attached picture is an example of what I would like to work on for the next shot, just want to go into the next dive somewhat prepared as the opportunities are not that often. I am mostly happy with the result after some work with Lightroom, however you can see there is a bit of softness on the lower left fin of the turtle that I am hoping a move to F/5 could do away with. Or if I should be trying something else all together. For simplicity let's just assume that the lighting conditions are the same or similar for the next dive. Here's the info from the example photo. Also a side note, I cannot adjust the shutter to anything less than 1/200 for the types of shots I'm looking for. Thanks!


Shot on Canon 5D Mark IV - 24mm 2.8 EF IS lens - spherical dome port - natural light

F/4 - ISO 500 - Shutter 1/200 - Manual white balance


45 feet deep, mix of sun and clouds, sun at back

Attached Images

  • 4P0A6362.jpg

Edited by ncheel044, 03 December 2017 - 03:31 PM.

#2 Tom_Kline


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Posted 03 December 2017 - 03:19 PM

f/4 to f/5.6 is a full stop not f/5. Closing down by one full stop is halving the amount of light passing through the lens so requires a doubling of the ISO to compensate if you keep the shutter speed the same.  BTW I use auto-ISO in combination with aperture priority auto-exposure quite a bit for my non-flash underwater photography. This is because of the need to keep the lens stopped down to work OK with a dome port.

Edited by Tom_Kline, 03 December 2017 - 03:21 PM.

Thomas C. Kline, Jr., Ph. D.
Oceanography & Limnology
Currently used housed digital cameras: Canon EOS-1Ds MkIII, EOS-1D MkIV, and EOS-1DX; and Nikon D3X. More or less retired: Canon EOS-1Ds MkII; and Nikon D1X, D2X, and D2H.

Lens focal lengths ranging from 8 to 200mm for UW use. Seacam housings and remote control gear. Seacam 60D, 150D, and 250D, Sea&Sea YS250, and Inon Z220 strobes.