What I learned from Gollywop -- and what I wonder

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

Macx wrote:

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.

While stacking without bracketing does increase highlight detail while the bracketing approach does not, I can't think of single example where there is much to be gained from reduced highlight noise. From a noise point of view, this is the least problematic region of an image, the one where SNR is highest.

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.

This is a rather special case, though, that rests entirely on the fact that read noise is nearly cut in half by going from ISO 200 to ISO 400 on the E-M5, making the DR from a single shot at ISO 400 very nearly the same as that of an ISO 200 shot at double the exposure.

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.

Whether the image looks natural or not has little to do with whether you bracket or not. It has to do with the way the images are merged and tonemapped. There is nothing unnatural about the bracketed example I already linked you to in my previous reply here:

http://www.dpreview.com/forums/post/51173651

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.

Sorry, but why would that be a good yardstick? For non-bracketed stacking, the improvement in DR (as measured in EV) is ordinarily proportional to the square root of the total exposure time, or equivalently, the square root of the number of shots, so that four shots gives one EV more, 16, two EV more, 64 three EV more, and so on. For non-bracketed shots it gets a bit complicated because it depends partly on the number of shots, partly on the exposure difference between these shots. As a very rough rule of thumb, however, I would say that if the exposure difference between each shot is one EV, you gain one EV worth of extra DR per added shot.

Another way of thinking about it in the case of bracketing with a separation of one EV between shots is that the number of EVs with a quality just as good as the brightest highlights in a single capture is equal to the number of shots. So if, for example, you make 10 exposures and increase exposure by one EV at a time, you can render the ten topmost EVs of the scene with the same quality that the highlights have in an ordinary shot. Then, with the E-M5 at base ISO, you would have 11 EV worth of DR of ordinary quality below that point.

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