NOTES
1. The following is relevant only for raw shooters; the considerations are different when creating JPEG in-camera.
2. The usage of high ISOs (6400, 12800) is not analyzed here.
Opinions, questions posted on several forums convey the impression, that some users of the Canon 7D do not fully understand the way the camera is working in conjunction with the ISO selection. I created a small explanation with demonstration of the issues regarding
a. the 1/3 EV ISO steps,
b. ISO 100 and 125.
http://www.cryptobola.com/PhotoBola/Canon7D_ISO.htm
contains the explanation; however, that may be far too detailed for many photographers.
Following is a summary of the findings:
- the camera's hardware does not support the fractional ISOs, they are created by underexposing the previous lower respectively overexposing the following higher full-stop ISO,
- the +1/3 EV ISO steps, like 125, 250 reduce the dynamic range by 1/3 EV,
- the -1/3 EV ISO steps, like 160, 320 cause an "overmetering" by 1/3 EV, which may cause clipping,
- the ISO steps 100 and 125 cause about 1/4 EV higher exposure than it should be: ISO 100 is rather 119, thus the metering for ISO 100 may cause overexposure.
Note, that DPP and LR/ACR compensate for the over- and underexposure; however, they can not reverse the noise effect caused by the underexposure of the +1/3 EV ISO steps, nor the possible clipping caused by the overexposure of the -1/3 EV ISO steps.
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