John Lind's mention of Markerink's web site was the opportunity to have
another browse - and not just for IR (or UV) photography. It's recommended.
In doing so, I found that at
http://www.a1.nl/phomepag/markerink/shifcalc.htm Willem has attacked a
question which has arisen on the list in the past months, namely "Is the
angle of view of the 24mm/35mm shift lens greater than the normal
24mm/35mm?".
Olympus describe the 35mm as having an image circle equivalent to that of a
24mm - but that is misleading. The specs in the Sales Info File say:
"35mm shift AoV 63 degs (83 degs with 62mm image circle)" - this suggests
that it is just the image circle which is bigger.
"24mm shift AoV 84 degs (100 degs at max shift)" - this says something
different!
In truth, as Markerink discusses with applied trigonometry aplenty, the
angle of view of a shifted lens (in the direction of the shift) *is* greater
than the angle of view of a normal lens of equivalent focal length.
Visualizing how this can be is not easy - but it occurred to me there is an
obvious way to show the effect, using two sheets of office paper. No pen,
protractor or ruler needed. You don't even have to be able to count.
1 Fold one sheet of paper in half across the long side to find the
midpoint of the long side. Unfold it and make one fold from the midpoint on
one side to each corner on the opposite side. The angle between the two
diagonals is the angle of view of our notional normal lens.
2 Fold the other sheet of paper along a diagonal, from corner to
corner - the angle at the corner is the angle of view of a notional shift
lens, fully shifted.
3 Now put one sheet of paper over the other, and you will see that the
angles are different. The first "normal" lens has a wider angle of view
than the "shift" lens, but they both cover the same subject plane. In other
words, a fully shifted 35mm lens has the same coverage (parallel to the film
plane) as a 24mm normal lens.
That had been bugging me for months. Thanks to John Lind (again) for the
pointer that put it to rest!
Piers
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