Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
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Detail Man
Detail Man Forum Pro • Posts: 16,400
Re: Exposure basics, lesson two point one (& ISO)

Certainly. Remember that my entire post is written, as indicated at its top, under the assumption that exposure is kept constant.

For the same Scene Luminance, then the camera's F-Number would have to change to compare, right ? So, the implicit operating assumption is that a higher F-Number is better in the application ?

If exposure can be increased without negative side-effects, it is of course better to stay at ISO 200 and increase exposure to reach the ETTR criterion than to keep exposure unchanged and go to ISO 400. However, if exposure cannot be increased, it is better to go to ISO 400 and ETTR rather than to remain at ISO 200 and stay one EV short of ETTR and that was the point I was making here.

How would it be (assuming that the sensor is linear, and that the entire Full Well Capacity is effectively utilized) that losing 0.2 stops of DR (by switching from ISO=200 to ISO=400) is better. What matters is the ratio of the Full Well Capacity divided by the Read Noise. Make sense ?


Sorry, I missed that you are considering the possibility of "staying one EV short of ETTR" (at ISO=200). DR is still worse at ISO=400, and I presume that one would then only be motivated to use ISO=400 if: Scene Luminance is inadequate; and/or the desired F-Number must be high; and/or Shutter Speed must be kept at some value in order to avoid camera-shake. Those seem like separate issues to me that may dictate such a situation - but the DR and SNR are going to be worse.

I guess that what you are addressing is a situation where the (due to some or all of the factors listed above), ISO=200 cannot deliver "Full Well Capacity", and one is contemplating whether to post-push in RAW processing, or to push in-camera. Now I think that I understand your point.

Edit: I read your post published in the interim, and I now understand what you are talking about.

Question: How, then, can we know whether read noise is reduced or not by increasing ISO at a certain point on a particular camera?

Answer: By checking out the read-noise figures reported by Sensorgen (bobn2) here


Click on a specific camera model to see its read-noise values for different ISOs. Note that small differences may be due to measurement error and that the values become inreasingly unreliable at higher ISOs due to problems of measurement. In general, you should look for the point at which the read-noise values no longer tend to fall significantly when doubling the ISO.

If results for a certain camera are not (yet) available at Sensorgen but reported by DxO, you can find the essential information via the DxO DR curve. If the loss of DR is less than one EV when you double the ISO, read noise is still falling. When the loss reaches one EV when you double the ISO, it is no longer falling.

While Sensorgen and DxOMark both report the results for the main ISOs (100, 200, 400 and so on), they don't report them for the intermediate ISOs (except for the base ISO). As a rule, that doesn't matter much but exceptions exist. With the E-M5, for example, it is advisable to skip the intermediate ISOs between 200 and 400 since these don't bring the same benefit in terms of read-noise reduction as ISO 400. See here for details:


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