Hell, Steve, I'm usually the one in error! Thanks for checking! Now I'm going to have to go back to confusing myself - as usual! Regards, Bob... From: "Steve Hodges" <shodges@wantree.com.au> Bob Blakely wrote: > > Well, there's the math. Further my own experience says no. > >[Burp!] 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.
