Re: Autofocus lenses fo IR?

[Ian D Wilson <idw@xxxxxxxxxxxxxxx>]

As with all these things, the criticality of focus depends on the type
of picture: in particular, the depth of field, focal length and camera
to subject distance.  I use Canon FD (manual focus) equipment, and can
report the following experiences (all with HIE and a 25 red).

With a 24mm, I work hand-held at f/16 or f/22, simply setting the
hyperfocal distance and shooting away - I've never had any problems with
focus.

With a 35-105, I work hand-held at f/11, and am always reasonably
careful about IR correction.  I test to use this lens for landscape and
architectural work, so I'm always a reasonable distance away from the
subject.  

If I need to work with a longer lens, or at a smaller aperture, I use a
tripod and stick religiously to the correction marks.

The only time I've had genuine focus problems was the first time I used
HIE to shoot studio portraits.  The lens I used was a Tamron SP 90mm
macro, with the lighting provided by 2 or 3 Courtenay studio flashes. 
Working at f/8 or f/11, I made the big mistake of focusing critically on
the eyes *and* forgetting to apply the IR correction.  The combination
of the focal length, the aperture and, most importantly, the camera to
subject distance of 1 to 1.5 meters, meant that all the pictures I took
appeared to have the plane of focus about in line with the ear-lobes!

-- Ian.

--------------------------------------------------------------
Dr Ian D. Wilson         |  Email:  iwilson@xxxxxxxxxxxxxxx
Principal Engineer       |  WWW:    http://www.telemedia.co.uk
Telemedia Systems Ltd.   |  Tel:    +44 (0) 1223-477000
Mount Pleasant House     |  DDI:    +44 (0) 1223-578378
Cambridge CB3 0RN        |  Fax:    +44 (0) 1223-506282
England                  |  Mobile:         0467-473594
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