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Old 21st February 2019
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Re: My latest foray into FF mirrorless - and why I'm sticking with m43

Originally Posted by pdk42 View Post
OK, maybe a bit of explanation is needed...

1) Stacking of images with the same exposure to reduce noise

When you stack multiple images of the same exposure, you blend them such that each contributes its own proportion to the final image. So, with 4 exposures, each gives 25%. This is performed automatically using "median" blending in Photoshop. You can do the same with pretty much any software that can handle layers with varying opacity, such as GIMP etc. The resulting image will have lower noise, because the random effect of noise means that when stacked it will cancel out. The signal on the other hand will not cancel out.

The end result is that the image noise will be as if the stacked images were taken at a lower ISO. The final effective ISO will be halved for each image stacked - so 8 stacked images will reduce the ISO by 1/2*8 = 1/16. Hence, ISO 6400 will appear like ISO 400.

Another way of looking at this is to imagine that instead of capturing 8 shorter images that you capture 1 longer one. So, 8 images at 1/800 sec each is the same as 1 image at 1/100 sec. It's as if you've increased the exposure and hence reduced the ISO.

So why do it? Well, if the subject is static and you're shooting handheld you will be able to take 8 short shots and keep the subject sharp, but a shot 8 times longer would be blurred due to camera shake.

Doing it is fairly easy and quick on the E-M1ii:

- Use 60-fps electronic shutter

- Set the frame limiter to the desired number of frames

- Press the shutter - you'll capture 8 shots (or whatever) in no time

- Import, align and stack in Photoshop as per Dave's instructions above


You can do it on any camera of course, but having a very high FPS and a frame limiter as the E-M1ii does makes it much easier.

Stacking of images with bracketed exposure for HDR

This is more often used of course. In the case, the stacking is more complex since the images need analysing to find the bright and dark areas and the stacking then needs to vary the opacity by these areas. However, it's a technique that works well if done sensitively.

Stacking of images with same exposure to increase resolution

The technique above for reducing noise can be tweaked to increase resolution if there is a little "dithering" between shots (i.e. slight movement of the subject in the image). In this case, you do exactly the same as above, but you upscale the images before stacking. This will increase the apparent resolution since movement of the subject will be across the pixel boundaries resulting in slightly improved resolution when the image is de-mosaiced.

You'll also get noise reduction for the reasons discussed above.

This is what's happening with hand held hi-res. The camera is taking up to 16 shots and arranging for the IBIS to be off between shots to introduce some dither. The upscaling, aligning and stacking is then done in camera to produce a single file. Very clever!

You can do the same using Photoshop, but in my experience of trying it, the resolution increase is not as good as I observed from my playing around with the E-M1x (see my other post on this in E-M1x section).

Hope that helps!
It certainly does help. Thanks, Paul......
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