What I learned from Gollywop -- and what I wonder

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

olliess wrote:

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.

No, I agree it isn't perfectly efficient, but I think there are a couple of places where it could shine.

Firstly, if we want to increase highlight details, I don't see any way around it. Admittedly, I can't think of many times where lack of highlight detail is a problem, but even so.

Secondly, it seems that for the E-M5, stacking for example two ISO 400 captures gives less noise than one ISO 200 shot. The thing to note about this, is that this is with the same total exposure time. Here we're not talking about taking the ideal exposure and then another to bracket or stack. Here we're replacing the ideal exposure with two shorter exposures that yields a better result when stacked than the ideal one.

Thirdly, and this is a subjective thing, but I wonder if it's easier to get a "realistic" look from this way of doing HDR than the normal bracketing practice that sometimes produce a bit surreal images.

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.

It's probably more practical in most situations. I think my personal yardstick is going to be the summed total of the length of the exposures when considering the advantage to multiple captures over a single capture.

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