ISOless sensors, read noise and photography - many questions!

Started Oct 3, 2013 | Discussions thread
(unknown member) Senior Member • Posts: 1,321
Think About Maximizing Signal During Exposure

" if the sensor is effectively ISOless then is base ISO always optimal (providing that you can see enough of the image on your camera to compose and focus in a sub-optimal exposure scenario), or are you as well off raising it in camera (providing highlights are preserved) rather than in RAW/pp?"

The answer is depends on the camera design. Some Canons perform better with ISO amplification because their analog-to-digital converter hardware works better with post-acquistion, in-camera amplification. The Leica M8 and M9s are essentially ISO-less until ISO 800 where the signal-to-noise ratio (and dynamic range) degrades significantly.

"- is it possible or likely that the camera's ISO implementation is better tuned to its sensor and RAW data than your RAW software? Aside from highlight preservation on exposure, are there other benefits to "doing ISO" in post?"

See above. Highlight preservation is not an issue if you maximize exposure (see below) at base ISO. The only benefit to using base ISO and increasing brightness in post has to do with maximizing the signal recorded by the sensor. Do not just think of noise. Instead think about the signal to noise ratio. The electronic noise of the sensor (read noise) is independent of ISO. When ISO is increased to one stop above base ISO, one stop less signal is recorded by the sensor when the shutter is open. At least this is what the light meter tells you to do. The SNR and dynamic range decreases by one stop. After the shutter closes the signal and the noise is amplified so the ADC operates at it's maximum performance level.The role of increasing ISO is to increase the light meter response so you can use a shorter shutter time or smaller aperture and still make the most of the analog to digital converter. Above base ISO less signal is recorded when the shutter is open because that's what the light meter tells you to do.

"- for a given sensor may it be more effective to raise ISO to some minimum/optimal level vis-a-vis its read noise?"

No. Some sensors will benefit (see above) and others will suffer. The only thing a photographer can do is maximize exposure of the sensor when the shutter is open.

How to maximize exposure

  • Set ISO to base ISO (unless your camera is one of the few that have better SNR above base ISO).
  • Choose the shutter time and aperture to maximize exposure while retaining any highlights important to the photograph you envision in your mind. This means you might intentionally overexpose unimportant highlights such as interior lights, car headlights, street lights etc., but you would not overexpose a blue sky. If all the highlights must be properly exposed to make the photograph, then the shadow SNR will suffer. In this case minimizing ISO is even more important because you need all the DR your sensor can deliver.
  • If the shutter speed is too long or the aperture is too narrow to record the photograph you envision, increase ISO by the smallest amount required to us appropriate exposure parameters.

Maximizing exposure at the base ISO, or the appropriate practical ISO above base ISO, insures you have the best shadow SNR and the best overall dynamic range.

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