Bob Blakely wrote:
>
> Well, there's the math. Further my own experience says no.
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Well I did the maths, and you're right Bob. Trust your experience.
Your numerical example was the spur I needed to redo the maths. Even
going back to the book, it's still possible to read it as suggesting
that DOF is the same, but on careful re-reading, and analysis of the
equations shows my error.
Now, my practical experience has always been of incresed DOF with a
wider lens, but obviously this reduces on enlargement.
I've also seen examples of negs being blown up to demonstrate the same
perspective at different focal lengths. Invariably these have a
different point of focus due to the difficulty of focussing a wide angle
lens as compared with a long lens.
In fact if I had ever been bothered to read to the end of the section in
my favorite reference I would have seen:
"...the depth of field will be seen to remain essentially constant when
the actual diameter of the aperture of the lens remains unaltered. Thus
combinations of 100 mm at f/11, 50 mm at f/5.6, and 25 mm at f/2.8, all
have the same depth of field for a fixed viewpoint."
And I've verified this mathematically, *and* found an error in the
formulae printed in the 8th edition of Jacobson, Ray, & Attridge!
Sorry Bob, Ýzzet, et al.
Steve
p.s. The maths as far as it went in my previous tome was correct. The
fact that it dealt with only a single ray meant that issues of DOF were
glossed over. Thus the conclusion number 1 should exclude DOF.
