> DOF is a function of reproduction scale and f stop

> only. Consequently a 21mm lens and a 500mm have identical DOF for a

> given f stop at a given reproduction ratio.

I'm affraid this is not completely correct. I was also surprised to discover,

that DOF is a function of reproduction scale and _absolute_ --- not

_relative_ aperture (that is f-stop).

Because magnification ratio equals approximately F/D, where F is a focal

length and D is a distance from camera to a subject, and hyperfocal

distance equals (F*F)/(c*f), where c is a circle of confusion, and f is

an f-stop, you get M=F/(cf), or equivalently f=F/(Mc). So at given

focal length F, magnification ratio M and circle of confusion c, you

need to stop down your lens to F/(Mc) to get everything from your

subject to infinity sharp (that is to obtain COC c at infinity). Now assume

that M is constant, and you double the focal length. To get the same

DOF you need to double f-stop!

So to obtain the same DOF at the same magnification, you should stop down

21 mm lens to, say, f/2.8, and 500 mm lens to about f/64. Strange, but

seems to be true...

This also explains, why medium format cameras have relatively shallower

DOF. If you take two standard lenses, say 1.7/50 (for 35mm camera) and

2.8/80 (for medium format), then they have nearly the same DOF when

wide open.

This is a real problem with small sensor digital cameras. Recenty Sony

introdued a new model equiped with CZ Sonnar 2-2.8/7.1-50 which, as

they say, corresponds to 28-200 lens for 35mm camera. In fact, when

DOF is concerned (not the amount of light reaching the sensor), it

corresponds to 8-11/28-200 lens. For such a small sensor, to have a

lens comparable with, say, 1.4/85, you need a 0.35/21 lens. I'm not sure

if it is physically possible to build such a fast lens.

Assuming that that what I wrote above is true, I think the main reason

for building full-frame digital cameras will not be compatibility with existing

systems, but problems with limiting DOF when using small sensors.

Tomasz