Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
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texinwien OP Senior Member • Posts: 3,320
Re: Exposure basics, lesson two point one (& ISO)

texinwien wrote: Could you clarify a couple of points for me?

Jack Hogan wrote: It appears that if one wanted to capture the very same tonal range from a scene with the EM5 and the G3, same exposure and all, one would have to set the EM5 at in-camera ISO 3200 while the G3 at in-camera ISO 1600, with roughly similar resulting SNR (EM5 slightly better as per DxO).

I'm following your example, so far.

On the other hand, if one set both at 3200 maintaining the same exposure they would no longer be directly comparable, but the G3 would appear noisier to a naive observer.

Would it appear noisier, and would the observer be naive to believe what he saw?

He wouldn't be naive to believe that he saw more noise (he would).

I do not think that this is a given. Exposure and ISO were kept the same in both photos for the E-M5. Exposure was kept the same in both photos for the G3, but ISO was doubled in the second photo.

I think there are at least two scenarios in which the observer would not see more noise in the second G3 photo than he saw in the first.

Scenario #1: We achieved perfect ETTR on the G3 sensor with our choice of ISO and exposure parameters in the first photo, and our entire scene was a uniformly-colored and uniformly-lit, textureless gray card. The second G3 photo will be completely blown out, since we doubled camera ISO, practically halving the saturation point but did not concomitantly half the exposure. All (or at least most) pixels will be blown out, hiding any increase in noise. Note: We could have also taken a picture of something other than a gray card and only blown out some large portion (that I haven't bothered to calculate) of pixels in the second photo - enough to cancel out any noise increase due to bumping up ISO. Additionally, if we were able to salvage some pixels and wanted to use the image, the steps in Scenario #2 would apply to those pixels.

Scenario #2: We achieved something less than perfect ETTR on our first photo, and our scene was of some arbitrary subject (doesn't have to be a gray card). Assume we are shooting RAW and want a uniform brightness in the 4 output images we generate. We are using RAWTherapee to convert RAWs and always start out by setting exposure settings to neutral (backing out any automatic tone or gain adjustments applied by RAWTherapee upon opening our RAWs). Assume that the group of 3 similar photos (the first G3 photo and both E-M5 photos) have the 'correct' brightness. Our 2nd G3 photo will be one full stop too bright, since it received the same exposure at twice the ISO. We will have to adjust the gain of this image downwards one stop in order to equalize its brightness with the other 3 images. My understanding (and feel free to correct me if I'm wrong) is that the noise inherent in an image becomes more apparent as we brighten that image, and becomes less apparent as we darken the image. I think (but have not done the math yet) that darkening our 2nd G3 image by one stop in order to match the brightness of our other images will almost exactly cancel out (at least in terms of appearance or what one can see) the amount of visible noise in this photo.

I believe you have made an error in logic here, and that is that you relied on the DxOMark graphs as predictors of camera behaviour in a test you have set up that, unfortunately for you, does not follow DxO's testing protocols. That is, DxO adjusts exposure along with measured ISO as they test, and their various graphs are based on that. In your test, you have adjusted measured ISO, but have not adjusted the exposure, in sync.

He would be naive in thinking that to capture the same tonal range (shadows and highlights) of the same scene with the same lens and Exposure (ss and f/n) as his friend's EM5 he would have to use the same ISO in the G3. His naivete would cost him a ton of IQ: one stop of highlight headroom and 1 stop of SNR, and he possibly would lose his National Geographic desk

As well he probably should. Any photographer naive enough to think it was safe to just copy the settings from his friend's camera over onto his own camera (of a different sensor generation and made by a different manufacturer) probably shouldn't have a National Geographic desk

The same could be said of a film photographer who copied a friend's camera settings onto his camera, not caring that he (our photographer) was using a different brand of film than his friend, and that he (our photographer) had no knowledge of the characteristics of the film his friend was using. Naive, indeed.

On the other hand an intermediate photographer who understands this subject, would know that to capture the specified tonal range at the given ss and f/n with a G3 he would need to set his camera on ISO 1600 independently of what others need to do to capture the exact same scene and tonal range subject to the exact same artistic constraints. Given the same objectives, his friend with the EM5 and the same lens would need ISO 3200 at the same exposure.

That's correct - perhaps he'll get lucky and be asked to take over the National Geographic desk that was vacated by our hapless naive photographer

What effect does bobn2's statement that "the bigger exposure will win" have in scenario #2, or does it have any effect at all?

It doesn't apply, because exposure stays the same in every scenario here, only ISO changes.

The fact that the exposure stays the same while the camera ISO (and DxO "measured ISO) changes is the very fact that invalidates the predictions you've made regarding your hypothetical experiment.

I look forward to your reply. Perhaps I've overlooked something - it's certainly possible. I'm fallible, after all


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