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There are multiple ways to increase depth of field (DoF) in macro photography. I know about focus stacking, but I have a question about which way is best for a single-shot. Two common options are:

  1. Reduce aperture. The depth of field is essentially proportional to the F-number. This trades image quality for DoF because of diffraction.

  2. Increase the distance to the subject, and then crop the image (i.e. zoom digitally to compensate the distance to subject). This also obviously trades image quality for DoF (less pixels, defects of the picture amplified by cropping if displayed/printed at the same size as the non-cropped image). This is more or less the same as taking the picture with a camera having a smaller sensor.

My question is: in which condition should I favor one or the other? In other words: at constant DoF, how does image quality vary if I change the distance to subject and aperture? Or, equivalently, at constant optical resolution (i.e. constant size of the airy disk relative to the size of the picture) changing distance and f-number, where will I get the maximal DoF?

Thanks,

  • Haven't you answered your own question with your note in #2? I.e., given your desired resolution, back up until you have dropped to that with crop. Beyond that you have to focus stack. OTOH: If you're asking for a specific equation between aperture and "effective resolution" you should make that explicit, but I believe that question has already been dealt with here. – feetwet Aug 20 '15 at 13:21
  • What are "defects of the picture amplified by cropping"? When you crop, you take some pixels away. There is no amplification. – null Aug 20 '15 at 13:30
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    @null: when you display (or print) the cropped picture, for a given display size, the more you crop, the more you need to rescale the picture, and the more you amplify the defects of the picture. – Matthieu Moy Aug 20 '15 at 15:07
  • @feetwet: no, I haven't answered my own question. Sure, I can back up until I have something satisfactory (i.e. apply #2), but I can also reduce the F-number until I have something satisfactory (i.e. apply #1). – Matthieu Moy Aug 20 '15 at 15:10
  • ok, you were implying a modified resolution due to scaling up the cropped image, got it – null Aug 20 '15 at 15:11
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The pragmatic answer is: If possible, you make your aperture smaller to achieve the desired DOF. That way you do not trade in resolution.

Stepping back and cropping in is equivalent to choosing a longer focal length and stepping back (apart from the resulting resolution). It probably is an unpractical way to influence the DOF.

Mathematically, I think none of them is superior regarding the final DOF effect.

  • make your aperture smaller [...]. That way you do not trade in resolution.: Reducing aperture does reduce quality. As soon as you get close and have a non-flat subject, you need an aperture around f/16 in macro, and diffraction is dominant. Reducing the aperture increases diffraction (try taking a sharp shot at F/32 or more to see what I mean). – Matthieu Moy Aug 24 '15 at 8:16
  • Stepping back and cropping in is equivalent to choosing a longer focal length and stepping back: not in macrophotography. The DoF is almost only a factor of the magnification ratio (read e.g. dpreview.com/articles/3064907237/…). At magnification ratio 1:1, you'll get the same DoF with any focal length, even though the distance to subject will vary. OTOH, stepping back and cropping is exactly what you do if you decide your full-frame camera has a too shallow DoF, and use the same lens with an APS-C camera instead. It does increase DoF. – Matthieu Moy Aug 24 '15 at 8:22
  • I still believe what actually influences the DOF is distance. It is similar to how I initially thought, focal lengths affected the compression of a scene (which they only indirectly do, since they force you to move to keep your composition). The distance to the subject is what determines the DOF in combination with the aperture. – J0hj0h Aug 25 '15 at 1:25
  • You are not considering the fact that DoF decreases with long focal length. The distance increase is compensated by the focal increase. If you want more details, read en.m.wikipedia.org/wiki/Depth_of_field (DOF formulae, Close-up). – Matthieu Moy Aug 25 '15 at 5:22
  • I just do not find any statement that contradicts mine: Focal length indirectly affects DOF by generally forcing you to move. – J0hj0h Aug 25 '15 at 18:56

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