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Author Topic: Diffraction at small apertures ...  (Read 7738 times)

JeffKohn

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Diffraction at small apertures ...
« Reply #20 on: September 25, 2009, 02:13:25 pm »

Focusing-stacking software/technology is still immature. I guess it works OK for some types of images; but it has problems in many cases. Not only do you have alignment issues due to lens breathing, but even in properly aligned images you will often get artifacts such as halos. It seems the biggest problems are when you have overlapping planes of focus, such as a foreground element that sticks up in the frame and overlaps with the background.

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Guillermo Luijk

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Diffraction at small apertures ...
« Reply #21 on: September 25, 2009, 03:04:51 pm »

Quote from: bjanes
Does anyone have any experience or suggestions with this technique?

I have tried TuFuse, which is a particular coding by Max Lyons of this Exposure Fusion algorithm. This algorithm classifies pixels according to 3 variables: correct exposure, microcontrast (in the near surrounding area) and saturation. By weighting more some of those variables, this algorithm can be easily used for totally different purposes: e.g. HDR fusion (tone mapping) by weighting more the correct exposure, or focus stacking by weighting more the contrast variable.

I did a try and was not bad, but displayed halos in some areas (see the word 'Formas' in the calendar).
However I think I simply was too demanding with the algorithm. IMO these algorithms work well with a quite in-focus set of images, not images with such a strong defocusing as the ones in my test. We also need lenses that minimise changes in the entrance pupil position (perspective), distortion and exact FOV when changing the focusing distance.

Original set of 3 images: shallow DOF with different focus distances:




Result from TuFuse contrast fusion:




What I find interesting is the discussion about 'when do focus stacking'. At first I simply thought focus stacking could just be an unnecesary improvement in DOF that could be achieved anyway by properly stopping down the lens.

Thanks to Max Lyons forum, I understood the real reason for justifying focus stacking: it is very recommended or it can even become a must, everytime the scene is to be captured with a very high resolution (i.e. devoting many pixels to every area of the scene, typically when stitching). In that case the maximum allowed blurring is so small that the tradeoff between diffraction + DOF simply becomes impossible; any combination of both effects will blurr image details spreading over several pixels because of the high pixel density with respect to image detail.

- For example, if you stitch several shots done at 60mm, it will be very difficult to achieve in every single shot non-blurred pixels at all distances. If we want sharpness in 100% crops the tele lens will need several shots at different focus distances because it needs to be stopped down more to achieve high DOF.
- This would not happen if a single shot with a wide angle is done over the same FOV. In that case the final lower resolution will limit the captured detail before diffraction + DOF does.

Of course the stitched image will allow larger prints than the single shot, both to be observed at the same distance and providing the same degree of detail. After all this is the goal of stitching.

Regards
« Last Edit: September 25, 2009, 03:58:35 pm by GLuijk »
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