At 10:14 AM 7/5/04, Piers Hermy wrote:
>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?".
[snip]
The 35mm Shift's true focal length is 35mm. Same with the 24mm Shift; it's
24mm. The "secret" is in the wider acceptance angle that creates a larger
image circle. Another way to think about it: a shift lens for 35mm small
format is actually somewhat like a 35mm focal length designed for a medium
format SLR with a larger image circle and lens cell farther from the focal
plane using reverse tele element[s] on the back of the lens cell. The
mechanical design uses this additional distance in front of the lens mount
for a shift mechanism that allows moving the larger image circle around the
smaller film gate. I should pull the lens out of the bag to verify this,
but IIRC there are some baffles to reduce flare from light bouncing around
from the unused portion of the larger image circle. The 24mm Shift (which
I don't have :-( ) is the same design concept.
I also have a 50mm f/4 Shift for medium format. Like the OM lenses, it is
much bigger. Indeed, the 50/4 is much bigger and heavier even if scaled
from 35mm to 645, with shift mechanicals like the Nik*n Nikk*r AI/AIS
shifts with rotating lens cell (with detents every 30 degrees) and a jack
screw to move it laterally relative to the mounting flange. The jack screw
allows more precise shifting and fine adjustment of it is easier, but it
does add to the bulk.
Acceptance angle for a lens and its focal length are independent. FOV is
determined by the geometry of its focal length and the dimensions of the
film gate (portion of its image circle diameter used on the film at
infinity focus). If you ever work with large format technical cameras,
their lenses are nearly all designed like shift lenses for 35mm and medium
format with acceptance angles much wider (and image circles much bigger in
diameter) than needed to fill the film plane. This allows the tilt and
shift "adjustments" of the lens board (and film plane if that can also be
adjusted).
An aside:
The acceptance angle and resulting image circle diameter for practical
"normal" (non-shift) lenses is intentionally made a tad bigger than the
film gate diagonal. This reduces the effect of cos^4 light intensity
falloff at the edges of the image circle. There is a practical design
tradeoff between how much larger than the film gate diagonal the image
circle diameter is made at infinity focus to reduce cos^4 falloff effects
and the risk of flare resulting from the extra light that can bounce around
inside the light box (the space between lens flange and film plane) at the
design's closest focus distance (for nearly all primes; see next paragraph).
For prime lenses designed with a lens cell and focus helical that moves the
entire cell farther from the focal plane to focus closer than infinity, the
image circle grows in diameter as focus distance is decreased. The
greatest amount of extra light bouncing around in the light box is at the
lens design's closest focus distance.
-- John Lind
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