Depth Of Field & Digital Cameras
Q) I've shot 35mm for years and only recently ventured into the digital field. One thing I miss is the depth-of-field control that I had with my 35mm. Even though my digital has ƒ/1.8 lens capabilities, I don't notice much change in depth of field from, say, ƒ/5.6 to ƒ/1.8. Is this inherent with compact digital cameras, in general, or is it a problem with the shorter fixed lenses?
A) Depth of field is usually defined as the range of distance from front to back in which the objects in the scene are in focus. In reality, there's only one plane that's in focus, but the depth of field is the range in front of and behind the focus plane that's acceptably sharp in a photograph.
Depth of field is a function of four things: ƒ-stop, distance, focal length and the size of the print. Distance to the subject and the print size aren't affected by the digital camera, but it's worth remembering that the closer you are to a subject, the narrower the depth of field gets, and as print size increases, depth of field decreases.
With small digital cameras, the size of the sensor influences the size or focal length of the lens used. Lenses start getting very small. A lens with a 35mm equivalent of 28-200mm might actually be 7.2-50.8mm. In addition, lens designers have found ways to make these lenses extremely small and compact (especially compared to 35mm).
This affects both ƒ-stop (apertures) and focal length for depth of field. Shorter focal lengths ("wide-angle" compared to 35mm) will give more apparent depth of field, so these very short lenses will give more depth of field than typical 35mm lenses.