ISO invariance understanding it - just checking something

Started 2 months ago | Discussions thread
Brian Kimball
Brian Kimball Regular Member • Posts: 460
Re: ISO invariance understanding it - just checking something

CESA wrote:

SigZero wrote:


Here are some rules, not all exactly matching your scenarios, but I hope it will clear some things.

With every increase of ISO (1 EV) You are loosing around 1 EV in highlights. Overexposing to te limits of the sensor (Zebra at 109% is Your friend here) and then pushing exposure down and shadows up (which means leaving them at real exposure level) gives you the best highlight protection and shadow noise performance.

In case od dual gain sensors the key is to overexpose not more than the limit of dual-gain change. So If the limit is at 640 ISO, then overexpose ISO 100 no more than 2,5EV and overexpose ISO 400 no more than 0,5EV - of course if having best shadow noise performance is the key.

In case you need to have better shutter speed and need to overexpose more then the limit, bump ISO to second gain settings (640 in Your case) and overexpose accordingly there - You will loose more highlights but get better shadow noise.

If the situation is the other way - if in order to keep highlights you need to make negative exposure compensation - if you need to cross dual-gain boundary it is better to underexpose more on lower ISO than underexposing on higher and crossing the dual gain level (is such case You will not saturate sensor to its full capacity).

Br, Pawel.

Thank you for your comment. I think I understood. However what if I want to shoot back-lit to get some flares from the sun? I need to underexpose to preserve the highlights. So in this case because I have to underexpose it is best to go to the first gain stage, use ISO400 say and underexpose by 1 stop for example or 2 stops.

Assuming your aperture and shutter speed are letting in the most possible light that your DOF & motion blur requirement allow, if you want to preserve highlights then you just need to drop the ISO until clipping stops, whatever ISO that is.  If you're still clipping at base ISO and need to preserve highlights, then stop down or increase shutter speed.

Let's say you end up with an exposure of f/4 at 1/60.  To get your midtones where you want them you need ISO 1600, but that clips highlights.  To protect highlights you find you need to drop your ISO by 2 stops, to 400.  So far so good?

Now, should you stop down or increase shutter speed so you can get back up to ISO 640 with no blown highlights, all for that half stop of read noise improvement?  No.  By letting less light in you are affecting shot noise and decreasing your incoming SNR proportional to your exposure change.  You would be trading read noise for shot noise.  Not worth it.

Now this means that when I recover the shadows I will be like stepping to the next gain stage? Equivalently in post? Do you know what I mean? When recovering?

Read noise is captured at the time you snap your picture.  Playing with image lightness in post processing does not change, either literally or figuratively, which conversion gain was used during capture.

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