Why do fixed-lens digital cameras often have more depth of field than DSLRs?
Asked 6/29/2013
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In Understanding Exposure, the author says fixed-lens digital cameras can produce much greater depth of field than DSLRs, giving an example like f/5.6 on a fixed-lens camera being equivalent to f/22 on a DSLR. I assume he means equivalent in depth of field, not exposure. Is this mainly because many fixed-lens cameras use smaller sensors and therefore much shorter focal lengths for the same angle of view? If so, how does that make depth of field deeper?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
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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.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
13y ago
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Yes—he’s generally assuming a smaller sensor, which was true for most fixed-lens digital cameras when that was written.
The key is that for the same framing/angle of view, a smaller sensor uses a much shorter focal length lens. Depth of field depends on focal length, subject distance, and the physical aperture size. Since the f-number is a ratio (focal length ÷ aperture diameter), a shorter lens at the same f-number has a smaller physical aperture opening.
That combination gives more depth of field on the smaller-sensor camera for the same composition.
So when he says something like “f/5.6 on a fixed-lens digital is equivalent to f/22 on a DSLR,” he means approximately equivalent in depth of field, not that the exposure is the same only because of sensor size. The exposure at f/5.6 is still f/5.6; the comparison is about how much of the scene appears in focus.
“Fixed-lens” itself is not the reason—sensor size is. Some fixed-lens cameras have larger sensors, so the statement is not universally true.
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