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

Subject: [OM] 18mm is no substitute for a 24mm shift
From: "Paul Farrar" <farrar@xxxxxxxxxxxx>
Date: Thu, 3 May 2001 13:34:03 -0500 (CDT)
Since the 24mm shift and the 18mm have the same angle of view, I've 
heard people say that one can get the equivalent of a 24mm shift by 
using an 18mm and selecting an 18*27mm subframe. I've agreed in my 
own mind with this, without really thinking about it. But a few days 
ago, out of the blue, I had a theophany and realized it couldn't be 
true. Two days ago I tried it out with both lenses and found the 
differences were quite dramatic, especially for rise and fall 
(shifting across the narrow dimension). The 24mm shift has a 
tremendously greater coverage.

The reason is this:
Both lenses have the same ratio of (image circle radius)/(focal length),
so the 24mm is like a scaled up 18mm. The 18mm fits a 24*36mm frame 
into the image circle so that the corners just touch the image 
circle. The 24 has a 33% large circle and moves a 24*36mm frame 
around within it. To get a 24mm equivalent with the 18mm, you take an 
18*27mm subframe and shift it around. The problem is that the 
subframe is limited to within the 24*36mm full frame.  This mean that 
you can only get a 3mm rise or 170f the frame height. On the 24, 
however, the 24*36mm frame is limited by the image circle, not the 
boundaries of a slightly larger frame. This means you can get a 10mm 
rise before your frame corners get to the image circle, or 400f the 
frame height.  The difference is quite dramatic. The 18mm doesn't 
come close to the ability of the 24. For lateral shifts, the 
difference is much smaller, but detectable. This is because on the 
sides the 18mm full frame can get much closer to the image circle.

You can get a bigger rise with the 18mm by turning it vertical and 
shifting a 24*16mm frame for a 10mm rise, or 63%. But you can do the 
same trick with the 24 shift and get a full 100% rise.

What lens is equivalent to the 24 shift? At full rise, the top of the 
frame is 22mm above the axis, roughly the same as the focal length. 
For a subframe confined to a 24*36 full frame, this means you need a 
12mm focal length, with a 12*18mm subframe. You will get more lateral 
shift with the 12mm. Only Voigtlaender makes a 12mm -- but it's 
cheaper than a 24mm shift, but by then you're getting close to Pocket 
Instamatic format size.

The 24mm shift can do all sorts of tricks, not just prevent 
keystoning.  For example you can control the elliptical distortion of 
3D objects near an edge (present even on a perfectly "nondistorting" 
rectilinear).  You shift the object towards the center of the image 
circle. You get a corresponding increase of distortion on the other 
side of the frame, but if there is a person's head, or some item of 
interest, you can choose for it to have low distortion.

Unfortunately, it looks like the 24mm shift is no longer available 
new, at least in the US. It's not too hard to find used, but prices 
have risen a lot in the last few years.

As a true Zuikoholic, you really neeeeeed this lens! Show the above 
to a sceptical spouse -- but, ummmm, don't mention my name, 
especially if you live near the Gulf Coast.

Paul Farrar

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