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:

Anders W wrote:

Macx wrote:

Anders W wrote:

Macx wrote:

olliess wrote:

Macx wrote:

Stacking images should give you at least as good noise performance in the shadows as bracketing.

If you only stack (instead of bracketing), won't your shadows potentially take multiple "hits" from pattern noise, black clipping (if your camera does it), etc.?

Yes, that is a good point, and it's worth testing how much this adding up of "per-shot noise" will mean for the final image. There might very well be a break-even point. For the E-M5 the sensor noise is fairly low, so I would suppose that the perceived noise (signal to noise ratio) in the end image would still be lower, even with multiple exposures, because the read noise is fairly small compared to the shot noise caused by (lack of) exposure.

No, I think stacking only, without any exposure bracketing, is perfectly fine and it is sometimes the best you can do. It is merely less efficient than stacking with exposure bracketing when that's possible.

The problem with the stacking-only strategy, is that you need a whole lot of exposures to get anywhere. With stacking only, without exposure bracketing, DR grows in proportion to the square root of the number of exposures. So to get one EV more of DR you need four, not two shots. To get two EV more DR, you need 16 shots, to get three EV you need 64, and so on.

Are you sure about this? I would have thought that doubling the exposure would allow for one more EV, and it would be four exposures for two EV and so on? And while the many exposures is definitely the impractical bit, both during capture and when processing, but beside that I think I think it's fairer to look at the total sum of exposure time from the two methods.

Perhaps I should add to what I already said that it would indeed be correct to just go by the total exposure time if only a single sensor read-out would be required. But since you need to do multiple reads, one per exposure, the circumstances are less favorable. See this post for a somewhat related case:

http://forums.dpreview.com/forums/post/50399103

Thanks for setting me straight, and thanks for the links. I found a simple explanation at Wikipedia's article aboutSignal averaging, which I had in fact already read, but apparently forgotten again.

Let's look at some numbers though. Here are some data from sensorgen.info's info table on the E-M5: "ISO 400" gives us a read noise of 3.8 and a saturation level of 12661. Doesn't it then follow that two stacked captures at "ISO 400" would give us an averaged read noise of sqrt(2*3.8^2)=5.3 and a saturation level of 25322 (which translates into a DR=12.2). Compare that to the read noise of 6.5 and saturation level of 25041 for a single capture at "ISO 200" (DR=11.9). 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.

Or have I missed something again?

Another example: If we want to simulate an ISO 25 shot by stacking eight ISO 200, we'd end up with a read noise of sqrt(8*6.5^2)=18.4 and a saturation level of 200.328 which gives us a DR of 13.4.

No, I don't think you missed anything. As I originally pointed out, stacking multiple shots without varying the exposure between shots does increase DR, just less quickly so than one might hope. If what you want/need is just one EV extra, the four shots required are certainly within practical reach. And a burst of 16 shots (which the E-M5 manages in about two seconds before the buffer fills) for two EV extra is not out of the question either. But beyond that point, the going starts to get rough.

Stacking bracketed exposures is far more efficient. In this case, if you bracket in steps of one EV, you gain about one additional EV worth of DR for each shot you add (practically speaking; the way we can think of and measure DR in this case is a bit complicated). This means that the sky is the limit in terms of image quality. With four shots on the E-M5, you are already beyond what a D800 can do in a single shot at base ISO.

Both techniques (stacking with and without bracketing) are perfectly viable as long as the subject is static. It does not take a whole lot of extra work in the field and not in PP either. If you have a tripod or if the bracketing doesn't require slower shutter speeds that you can manage handheld, bracketing is better. But stacking without bracketing may be the best you can do in handheld shooting. Suppose, for example, that you want to shoot a dimly lit interior in some historic building where tripods are not allowed and that you need ISO 3200 to manage a single shot without camera shake. Shooting a burst of 16 will then move the IQ up to the level ISO 800 (actually marginally better on the E-M5).

Here's a modest example of what technical quality you can easily reach by means of stacking bracketed exposures and how it can vastly improve the situation even in base-ISO shooting (see the end of the post I link to):

http://www.dpreview.com/forums/post/51153320

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