Archived posting to the Leica Users Group, 2000/11/06

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Subject: [Leica] why MF AF resolve more than AF AF....
From: "Birkey, Duane" <dbirkey@hcjb.org.ec>
Date: Mon, 6 Nov 2000 09:14:10 -0500

The obvious answer is that a slightly out of focus image..... which is what
you will normally get shooting wide open with an AF camera ....  will not
and can not resolve as much as a completely in-focus image...  POP
photography did a test on it with different systems a while back showing
just how much difference there is between an AF camera in AF mode and the
same camera and lens focused manually.  Kind of scary actually.  

I should add this bears out my personal experience..... I get far better
results manual focusing my EOS 1n than relying on AF.....  I use a 70-200
f/2.8 for portraits and often use it at near 200 at f/2.8 or f/4 to blur out
the background... with AF I was getting pictures where the focus point was
often behind the eyes or on their hair etc... everywhere but the eye....  If
you want a really bad looking portrait....  take one where people's ears
appear sharper than their eyes... or their forehead is critical sharp but
their eyes aren't..... anyhow.... you might not notice the cheek or nose is
sharper than the eye as much with a shorter lens at a greater distance and a
small enlargement.... But it is probably there.   So.....I' personally have
stopped using AF for all of my day to day work as I've been burned too many
times with it. I've been tempted numerous times to ditch EOS altogether....


If you want to see it for yourself, switch your AF to MF and find some text
somewhere about 4 or so feet away ... start with the lens at a closer focus
setting and focus the lens slowly until the AF light comes on...... (As I've
just done this again with the 28-70L here in my office...)  The AF light
comes on before the text is anywhere near perfect focus and I can visually
see the image go out long before the AF light goes off again.  Don't take my
word for it.... do it yourself and you'll better understand why AF AF can't
resolve as much as MF AF can and why AF AF it is near useless for critical
work or situations where DOF can't cover it's inaccuracies

Duane Birkey
HCJB World Radio
Quito Ecuador
Duane's Photographs of Ecuador
http://duane_birkey.tripod.com

Martin Howard wrote:

Now, I could go into saying that I understand how this explains grave
focussing errors (hunting) on the part of AF systems, but not how you can
get higher lp/mm from MF lenses compared to AF lenses, but I won't... ;)