I'm reading Understanding Exposure and ran across some discussion of apertures in fixed-lens digital cameras as compared to SLRs (page 54 in the 3rd edition). The author says that fixed-length digital cameras give much larger depth of field than SLRs, and gives some examples, but never explicitly says why the depth of field is so much greater. For example, he says that f/5.6 on a fixed-lens digital is equivalent to f/22 on a digital SLR. It's not clear to me what "equivalent" means here, but I'm pretty sure he's talking about depth of field (DOF).

I think this discussion is just saying that fixed-lens digital cameras have smaller sensors (and thus greater DoF), but I'm not sure. Can anyone help clarify or explain what the author is talking about here?

  • Good related discussion: photo.stackexchange.com/questions/9467/…
    – Charlie
    Jun 30 '13 at 2:03
  • I do think the "bigger sensor" question is closely related, but to me the question here is a bit different, because it's not a given in the question that the determining factor is the sensor size.
    – Charlie
    Jun 30 '13 at 17:19
  • So maybe the question could be titled: "When Peterson says 'fixed-lens digital cameras have greater DoF, is he assuming a smaller sensor?"
    – mattdm
    Jun 30 '13 at 17:36
  • I changed the question as suggested and think it's no longer duplicate-y, but I'm also fine with it being closed.
    – Charlie
    Jul 5 '13 at 4:36

In this case, the author makes the assumption that fixed lens cameras have smaller sensors. This no longer true for all cases but is for the majority of digital cameras. If you read small sensor instead of fixed-lens, the following will make more sense.

The physical aperture size and focal-distance determine depth-of-field. With smaller sensors, focal-lengths are much shorter and therefore so is the aperture. This part where most people get mislead is that aperture is measured as a ratio of focal-length over the diameter of the lens opening.

Just an example using very round numbers, if you have a 200mm lens on a full-frame DSLR with a F/4 aperture, the diameter opening is 50mm (200 / 4). On a small-sensor camera with a 5X crop factor, a 40mm lens would give the same field-of-view but at F/4, it's aperture would measure 10mm (40 / 4). You would therefore have much more depth-of-field with the small-sensor cameras because smaller physical apertures gives a larger depth-of-field.

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