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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