What I learned from Gollywop -- and what I wonder

Started Mar 26, 2013 | Discussions thread
olliess
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Re: What I learned from Gollywop -- and what I wonder
In reply to Macx, Mar 28, 2013

Macx wrote:

Even if we presume the same amount of exposure, the lower averaged noise would still give an advantage to multiple captures than the single one.

Well maybe here is a way to think of it:

Your goal is to capture an image without clipping, but with a certain shadow area "out of the noise."

The "ideal" exposure would simply be a single exposure which gives enough exposure to the shadow area that you want to capture. For our purposes, let's assume we can't do this because the sensor has an insufficient DR.

We could stack N unclipped (ETTR) images, which is certainly advantageous compared to a single unclipped exposure, but we also accumulate read noise as sqrt(N), so it is not perfectly efficient compared to the ideal exposure, where the read noise counts only once and we are only collecting photon noise with increasing exposure.

Now suppose the scene can be spanned by exactly two bracketed exposures, which we combine. The first exposure is the same ETTR as above, and then we expose a second time with the "ideal" exposure which clips the highlights but captures the shadow area with an adequate SNR. The first exposure incurs "1" fixed time cost, while the second exposure improves as the "ideal" exposure, and therefore this method may be more efficient compared to stacked exposures.

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