Archived posting to the Leica Users Group, 2001/05/31
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on 5/31/01 10:12 AM, Pete Su at psu_13@yahoo.com wrote:
> A Nikon F100 will focus a 100mm f2 lens in low light on
> someone's eyelashes in much less than half a second. Even my Hexar AF usually
> focuses faster than I can.
I have no idea whether that's true or not BUT as I have found out to my cost
if you are focused on the eyelashes with very thin DOF the iris will be out
of focus.
For example, working close up so that a face fills the frame, tests I shot
over the weekend show that all the following lenses have DOF wide open that
is LESS than the length of a standard :) eyelash.
80mm @ f/2.8 on 6x6
135mm @ f/5.6 on 4x5
203mm @ f/8 on 4x5
I haven't tested it but I would suspect that a 50/1 and a 75/1.4 and a 100/2
would all have the same problem in 35mm, if you were using a good enough
film that you could tell, of course.
You really have to focus on the iris, not the eyelash. And of course that
really IS the problem with AF because it can be hard to be really sure what
you've focused on. It's hard enough on ground glass with a loupe.
- --
John Brownlow
http://www.pinkheadedbug.com