P3D depth calculations

[rmt <rmt@xxxxxxxxxxxxxxx>]

In response to my suggestion that Tom's chart emulates DOF calculations,
Tom replies:

>All possible combinations?  Simply untrue.  For instance, with a near >point
>of three feet, the f5.6 guess says the "permissable depth" is 12 inches
>deep, where the curve says 26 inches.  Since when does 26 inches equal >a foot?

>Or to put it another way, the rudimentary f5.6 guess would give you >0.67mm
>of film deviation, nowhere near 1.2mm.

>The curves give mathless access to accurate geometry.  Accent on >"accurate.
>It's okay if you don't want to use them, but don't discourage others >from wanting to be a bit more precise.

As I stated in my original post, I compared results from a Kodak
Depth-of-field-computer to samples from Tom's graphs.  In the example he
gives, a near-point of 3 feet on the computer shows a far point of 5.1
feet, or 61 inches, or a depth of 25 inches, quite close to the 26
inches his graphs indicate.  However, his reply sent me to my Realists,
and I must concede that the DOF scales are probably not accurate enough
for close-up work, particularly since 4 Realists  indicate 4 different
readings, though they all sport 35mm lenses.  
Checking further, the Revere agrees with the DOF computer, and the Kodak
appears to, although its hard to tell as the focusing scale is not
marked down to 3 feet.

Dick Twichell


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