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Instead of using multiple images focused at different points to extend the depth of field, can I also reduce it?

Does a technique exist to reduce focal length by stacking multiple images that are exactly the same except for focus? An example of a situation one could think would be mimicking extreme shallow depth of field as in a medium format close portrait capture. Indeed, my intended focus stacking method would also not be simple, but theoretically possible.

Here is an example image of Katja Schuurman.

  • Wow. Veeeeery interesting question! I will make some experiments. There is a Photoshop filter called Lens blur The point is to find out how to prepare a deepth mask with the multiple shoots. This is a common technique in 3D rendering, but I need to test it on real life shoots. – Rafael May 31 '16 at 20:52
  • :D See below for @Athnas' spot on pointer to some people who have done this. – Kelley van Evert May 31 '16 at 23:01
  • Could this effect be created via a tilt-shift lens? – Wayne Werner May 31 '16 at 23:52
  • @Kelley, sure, it can be done, but it not says how they achive the deepth mask with something like Photoshop ;) – Rafael Jun 1 '16 at 8:06
  • There is a chance that HeliconFocus generates a deepth map. If that is a normal grayscale image it can be used with the "lens blur" of photoshop. I will test that later. – Rafael Jun 1 '16 at 8:09
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Yes, it's possible With the focus stack you could estimate a depth map of the scene. Then this map is used to selectively blur the image to emulate the effect of shallower depth of field.

See for example: https://graphics.stanford.edu/papers/focalstack/

You could of course use other methods to generate the depth map, such as moving the camera (as the default Android camera app does), but this does not use a focus stack, and is thus not what you are asking.

  • So cool. This is exactly what I was looking for. I knew it had to be somewhat possible, thanks for pointing me to it! :D – Kelley van Evert May 31 '16 at 22:58
  • @KelleyvanEvert For the best result you should eliminate the noise by taking many pictures for each focus setting and expose to the right (maximum exposure just shot of overexposure), align the images and take the average. The lower the noise, the better the focus stack method will work as local contrast is used and presence of noise makes it harder to determine this accurately. – Count Iblis May 31 '16 at 23:50
  • A large number of pictures is of course preferred because of noise, but I think it would be better to have the focus change slightly between each picture, so the additional pictures also help increase resolution in the depth map. This is just my gut feeling and not based on an experiment. – Atnas Jun 1 '16 at 5:46
  • Is there an implementation of this available to try? – Chris H Jun 1 '16 at 11:46
  • I don't know of any implementation of the menioned paper, but with this tool you should be able to generate a depth map. github.com/jean80it/DephtInition – Atnas Jun 1 '16 at 18:56
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Yes, although it is actually a bit different than what you described; it is most typically referred to as the Brenizer Method. See this question for much more info:

What is "bokeh panorama" (also called the "Brenizer method")?

  • No, not really what I mean, though thanks for reminding me of this method. I really am just interesting if a technique exists to reduce focal length by stacking multiple images that are exactly the same except for focus. An example of a situation one could think would be mimicking extreme shallow depth of field as in a medium format close portrait capture. Here, the Brenizer method wouldn't be of much use. (Indeed, my intended focus stacking method would also not be simple, but theoretically possible.) – Kelley van Evert May 31 '16 at 17:17
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    @KelleyvanEvert - What exactly do you mean then? Part of the issue is that you didn't describe any actual real world problem you are facing in the question. Why did you ask this question? What exactly are you trying to do? – dpollitt May 31 '16 at 17:17
  • (See my revised comment above. I pressed "enter" too quickly. Currently looking for an extremely shallow depth of field portrait to illustrate possible use; I recently saw a good one but can't seem to find such a good example now..) – Kelley van Evert May 31 '16 at 17:26
  • @KelleyvanEvert It's better to edit your question with any additional information/clarifications, than to put them into comments. Comments are meant to be more like footnotes and relatively temporary/deletable. – inkista May 31 '16 at 17:58
  • Here we go. This photo of Katja Schuurman: !Katja Schuurman – Kelley van Evert May 31 '16 at 18:09
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If opening the aperture all the way (possibly using a very low ISO and/or an ND filter) still doesn't get you low enough depth of field, the chances are you need a faster lens (one with a wider aperture).

Pushing or pulling the focus and stacking is unlikely to help more than a tiny bit as the sharpness falls off fastest close to the focus. Fake bokeh tutorials are plentiful (my link is for a GIMP tutorial chosen at random). You've probably seen them.

What you might be able to do is use the layer mask approach (common in such effects) to combine an image with the subject in focus and an image focussed closer than the subject. This has similarities to focus stacking but without the automated tools and with fewer images. Whether the results are any better than you'd get using a fake bokeh approach is up to you.

  • Thanks for the reply! It is not really the case that I really long to make these photos (I'm quite content with my Zuiko 50mm 1.4 :D ), it was more of a theoretical question I had. If really needed, I'd guess I'd actually first reach for tilt, which can definitely create crazy shallow bokeh effects, and else fake bokeh. – Kelley van Evert May 31 '16 at 19:00
  • Yes, with a 1.4 you shouldn't have any trouble. – Chris H May 31 '16 at 21:03

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