P3D eyeballs and f/numbers

[bercov@xxxxxxxxxxx (John Bercovitz)]

Possibly there is some confusion out there on the subject of 
f/number and depth of field.  Depth of field goes with the linear 
aperture, not the relative aperture (f/number), so the depth of 
field you get with a certain f/number and a 50 mm lens is not the 
same depth of field you get with an eyeball at the same f/number.

If you don't have Rudolf Kingslake's book, "Optics in Photography" 
(ISBN 0-8194-0763-1, SPIE), then shame on you!!  (And no wonder 
you're so confused.)  8-)  From pages 102 and 103:

(3) For the same magnification on film (not the same subject 
distance) and the same f/number, lenses of all focal lengths have 
the same depth of field, except when the lens diameter is very 
small.

(4) For the same subject distance, the depth decreases with 
increasing focal length.  A 35 mm camera negative, enlarged to 
the size of a large format camera contact print, shows greater 
depth than the latter, provided both are taken at the same 
distance and relative aperture.

(5) For prints enlarged to equal size, all lenses of the same 
linear aperture (not relative aperture) have the same depth.  A 
16 mm cine camera with a 25 mm lens at f/1.9 has the same depth as 
a large camera with a 135 mm lens at f/11; at these settings, the 
effective linear aperture of both lenses is [approximately] 12.5 mm.


DOF: Remember that depth of field is in object space and depth of 
focus is in image space.  Depth of focus primarily tells us how 
accurately we have to build the camera to keep the circle of 
confusion under the agreed-upon size for the in-focus object's 
image.

John B


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