| Not to barge in, but being a phycisist specialized in optics I feel that maybe I should provide some input on DOF calculations...
A macro lens is just a lens, and it's important to remember this. Its only special capability is to allow closer focusing than a regular lens. As with any lens, the standard equation applies for magnification. If O is the distance between lens and subject, and I is the distance from the lens to the image (the sensor or film, in the case of a camera), then magnification is M = I / O.
Focal length is linked to those parameters by the so-called "lens equation":
1 / f = 1 / O + 1 / I
I'm neglecting signs, for the sake of simplicity.
Depth of field is mildly more complex. It involves the circle of confusion (a measure of resolution) among other things. There is an aproximation of the real DOf value that is often used which is only valid if the subject is far from the lens. This approximation shows that DOF is inversely proportional to focal length, but proportionnal to subject distance.
There is another approximation that is used when the subject is close, as with macro shots. This is the interesting part. I'll quote wikipedia because it's very well expresed there:
for a given magnification, DOF is independent of focal length. Stated otherwise, for the same subject magnification, all focal lengths give approximately the same DOF. This statement is true only when the subject distance is small in comparison with the hyperfocal distance, however.
The reason for this is that although the 50 mm lens has a shorter focal length, it also has a shorter minimum focus distance, and those two parameters cancel each other.
Hyperfocal distance for a 50 mm f2,8 lens is 44 meters, roughly. For a 100 mm lens at f2,8, hyperfocal is at 178 meters.
To summarize, if two lenses are used at 1:1 magnification, AND if they are used very very far from the hyperfocal distance (as in any macro situation) then they will both yield similar DOF.
I hope this helps. |