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[OM] Re: Shift lenses

Subject: [OM] Re: Shift lenses
From: Vaughan Bromfield <Vaughan.Bromfield@xxxxxxxxxx>
Date: Mon, 26 Jun 2000 12:20:15 +1000
A bit of a rant, delete if necessary.

John Lind wrote:

> The 35/2.8 is really a 24mm lens with an image circle much
> larger than the film gate (rectangular hole where film gets exposed).

Sorry to be anal-retentive, but the 35mm shift lens *really is* a lens 
with 35mm focal length. Not 24mm or anything else. Thirty five 
milimetres.

A lens for a full-frame 35mm camera needs to make an image circle 43mm 
in diameter -- the length of the diagonal of the film format. Most 
lenses for 35mm cameras make a circle which is just big enough to cover 
the film, and nothing more. The Shift 35mm is different from a "normal" 
lens because its image circle is quite a bit bigger than the film 
format. The 35mm shift lens allows 12mm movement maximum, so it has an 
image circle of about 43+24=67mm. It is still a lens with 35mm focal 
length though.

Getting a sharp and contrasty image over such a large image circle is 
hard, and the retrofocus deisgn needed to fit the SLR design makes the 
job even harder. Hence shift lenses for 35mm are usually ruinously 
expensive, and rarely as sharp as their fixed mount siblings because 
they are harder to design, and  harder to make what with the moveable 
mount and all.

That's why large format is ideal for this kind of stuff: there is no 
reflex mirror to worry about so the lenses can use symetrical designs 
(our beloved 50mm and 40mm lenses are symetrical). Symmetrical designs 
are the easiest to correct, so the designers have room to either make 
them fast (like the 50 f1.2) or make them cheap (the 50 f1.8) or make 
them small (the 40mm f2) or make them sharp (50 f1.4?) or whatever 
combination of these qualities the designer feels like.

A modest large format camera with 90mm wide angle lens will be 
significantly cheaper than the 24mm shift lens alone. (90mm in 4x5 is 
equivilent in angle of view to 24mm in 35mm.) If you want the Zuiko 24mm 
shift lens to complete the set, then get it. If you wanna do the best 
architectual photography, get large format. Horses for courses.

> With the shift, the 35mm angle of view you crop out of the 24mm image 
circle ends up on a full frame of film.

Me being anal retentive again... the normal 24mm lens has *exactly* the 
same image circle as any other normal lens -- 43 mm.

Sorry to rant, but the "photo teacher" in me cannot stand this confusing 
and counter-productive lexicon. Forget angles of view and focal length, 
think image circles. Makes it so much easier.


Vaughan Bromfield
Education Consultant
Information Technology Division
University of Technology, Sydney
Australia

tel: + 61 2 951


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