From: jacobson@cello.hpl.hp.com (David Jacobson)
Subject: Re: Lens Focusing (was Re: lens tests..)
Date: Sat, 26 Sep 1992 04:33:46 GMT
Distribution: na
Organization: Hewlett-Packard Laboratories
Keywords: lens tests patterns

In article <1992Sep25.224817.22960@pasteur.Berkeley.EDU> dbailey@ICSI.Berkeley.EDU (David R. Bailey) writes:
>
>Yes!!!
>I noticed the same thing [focus inaccuracy] when I added a used 10s
>to my 650.  I didn't
>notice it right away, but I wouldn't say that you need to go to
>"extraordinary lengths" to detect it.  I first noticed it in a
>people-picture where the person was about 15 feet away, using a Tokina
>20-35 probably at 24 or so.  So it was not even a low-depth-of-field
>situation.  I could clearly see that the face was fuzzy.  The lens was
>new as well as the camera, and being non-Canon, it was the first
>suspect.  But then I noticed the same effect with my trusty old 35-135
>USM.

For most AF systems, and I assume the Canon, the focus accuracy, to a
first order, does not depend on the brand of the lens.  Basically
there is a sensor that optically equivalent to being right in the film
plane.  This sensor can detect whether an image is in front of,
coincident with, or behind it.  Its output drives a servos a motor to
drive the image to be exactly coincident with the sensor.  This basic
principle does not depend on the lens very much at all.  There are
some second order effects like coasting, backlash and "gain", that
might have a small effect on where it actually comes to rest.  But
none of these apply in focus-confirmation mode.

...
>Later it occurred to me that the problem could equally well be with
>the viewfinder end of things.  So I performed these experiments, with
>all three of my lenses: (1) I autofocused on a target using the 650
>(looks sharp); switched the lens to the 10s; and observed that the
>current focus-setting satisfied the 10s's AF, but it looked fuzzy.
>(2) I manually focused on the 10s until it looked right; verified, in
>MF mode, that the 10s AF sensor wasn't happy with the focus (no green
>dot); and switched back to the 650, whose AF sensor was also unhappy
>with the focus, _and_ it looked fuzzy in the viewfinder.
>
>Knowing my 650 to be good, I concluded that the 10s viewfinder was at fault.

Comparisons between cameras are not useful.  Suppose the
mount-to-film-plane was .1 mm different between the two cameras, but
otherwise they were perfect.  Then if you took a lens off one and put
it on the other camera, you would see things a tad out of focus, but
it doesn't matter.


What what you care about are that
	the film plane
	the focusing screen and
	the AF sensor are all optically in the same plane.  

I've discovered that with some cameras they are not right on.  With my
old 1962 Pentax (no AF of course) the focusing screen and the film
plane are a little out.  I put a microscope slide with a scratch on it
in the film plane, shown a light on it, and focused the scratch on a
plane surface (kitchen cabinet door with a strip of black electrical
tape).  Then I closed the shutter and focused using the focusing
screen.  There was about a 1/2 mm of inconsistency as measured on the
circumference of the focusing ring.  My brother claims that the AF
sensor and the focusing screen of his 8008 are just a tad out.

These are bad things, and I think your 10s has one of these bad
things.  Your test 2, first part, shows that the focusing screen and
AF sensor are not in the same plane.  I wonder which one, if either,
lines up with the film plane?

  -- David Jacobson
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