[roscot]

In addition to all the other arguments for f2.8 glass, assume for the moment you can shoot at f4.0. An f2.8 lens stopped down to f4.0 will return better and sharper images than an f4.0 lens wide open, everything else being equal.

[falconeye]

One thing that will stay always different is DOF. f4 will never be f2.8 ...

As i have written elsewhere, the hyperfocal distance H (multiplied by d/f to get the same magnification of a subject at hyperfocal distance) is:

H d/f ~= 1730 a

(f: focal length in mm, a: aperture in mm, d: sensor size in mm)


So, whatever be the lens and sensor, it is the mere diameter (in mm) which creates the DoF effect on a given subject (when moving forth and back to get the same print size of the subject -- and I do not say the same perspective ).

And tele lens have large apertures (in mm) anyway. My 75mm limit may be only 300mm f/4.0 but it would be 50mm f/0.67 for a normal length!!

So you tell me that you need less DoF than this? Cool!