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Re: [OM] 18mm is no substitute for a 24mm shift

Subject: Re: [OM] 18mm is no substitute for a 24mm shift
From: "Paul Farrar" <farrar@xxxxxxxxxxxx>
Date: Fri, 4 May 2001 09:31:22 -0500 (CDT)
> 
> Sorry, Paul,
> 
> but you must be wrong. Within a circle that covers a rectangle of 24x36mm
> you can draw a rectangle 27mm broad with a greater height than 24mm!
> A mathematician better than you and me may tell us the useable height.
> 
> Greetings
> Jochen (Hans-Joachim)

That's precisely my point. You CAN draw a 27mm rectangle higher than 24mm
within the 43mm image circle of the 18mm lens. What you CAN'T do is take 
a picture with it on the 18mm because your 27mm wide * 18mm high subframe 
is imprisoned within the 36*24mm frame of the 18mm lens. If the 18mm was
a shift you could do it.

Here is a diagram that shows the difference:

http://www.datasync.com/~farrar/shift.htm

The image circles of the two lenses have been scaled to the same size.
The 24mm is 33% larger in real life. Note the large unusable areas of the
18mm image circle. They have image in them, but you can't put film there.

BTW, in a minimum full frame image circle a 27mm wide rectangle can be
34mm high.

2*SQRT((24/2)**2 + (36/2)**2 - (27/2)**2)

Paul Farrar

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