I often hear photographers say that using a longer lens e.g. a 105mm lens, creates less depth of field, or alternatively, using a shorter lens e.g. an 18mm lens, creates more depth of field. Or in other words, depth of field is inversely proportional to focal length. Well that is not necessarily the case. And while this discussion is most valuable to an SLR photographer, it applies to any lens.
1. For any given focal length, the depth of field varies directly with the aperture selected. That is, the wider the aperture (e.g. f2.8) the shallower the depth of field. Does that mean that we should always use f22 or f32 to maximize depth of field? Not necessarily. Remember that a lens will be the sharpest at about 2 or 3 stops down from widest aperture, that is at f5.6 or f8, so we sacrifice sharpness if we arbitrarily maximize depth of field. And of course bokeh is also changed as we widen aperture (minimize the f-stop number e.g. f2.8). Depth of field is a tool and should be used accordingly. There ARE times when you have to maximize depth of field for a given subject but shooting at your smallest f-stop every image, is a way to sink into mediocrity.
2. For a given f-stop and lens (holding f-stop constant) combination, the closer one gets to the subject, the shallower the depth of field. What this means is that for a 105 mm macro lens at f8, as we approach shooting one to one (1:1), the depth of field will decrease (as the magnification is increasing).
3. At a given working distance, the depth of field is inversely proportionate to focal length of the lens. This means that a longer lens WILL have shallower depth of field vis a vis a shorter lens for the samedistance to subject. (The magnification will be greater with a longer lens).
4. If image size and aperture remain the same (same magnification), all focal length lenses give the same depth of field. This means that shooting 1:1 with a 60 mm, 105mm, or 200mm lens will have the same depth of field since magnification will be the same. It also means that if shooting at 1:1 with a 13mm lens (probably infeasible because of minimum focus distance), depth of field would also be the same as for a 200mm lens. The working distance to achievethis magnification will be significantly different for the 200mm (much farther) than for the 200 mm lens.
So, in other words, for a given magnification, depth of field for a given selected aperture is constant and is independent of focal length. It takes a while to think through this concept, but understanding it will be worth while.
Copyright Steven N. Norvich 2001