Re: Pentax 20mm ...
Do you fully understand the distortion that an ultrawide lens will
introduce, especially on architectural photos?
Pentax made a 28mm PC Shift lens that would be a much better alternative if
you are shooting exteriors, because you can correct for convergence (the
optical effect that makes buildings look like they are leaning over). The
lens is pricey, and I have had much success in correcting for convergence
issues in Paint Shop Pro X, which has an excellent deformation tool. PS has
one also.
The advantage of correcting in the editing software is that any lens becomes
a PC shift lens, not just the 28mm.
The other disadvantage of using wide angle lenses for architecture is their
apparent perspective distortion. The relative sizes of the objects in the
photo get messed up. As an example, I took a shot of two buildings using my
24mm lens. One was 2 stories high, the other, about a half a block further
away, was 10 stories high. The lens made the 2-story building (which was
closer to me) look like it was half the height of the 10-story building.
Even people that did not understand apparent perspective distortion could
look at the photo and realize that there was something wrong. The 10-story
building should have appeared to be 5 times taller than the 2-story
building.
Wide angle lenses often are invaluable in shooting cramped interiors, but
they still introduce perspective distortion. Small rooms can be made to
appear larger than they actually are. If REALISM is important to you, you
may find that wide angle lenses are disappointments.
If you have a zoom lens, try a few shots at 43mm--which is dead normal focal
length. I use 50mm normal lenses for most of my outdoor architectural work.
The Pentax 50mm f/1.7 is ideal, because it has a flatter field than does the
f/1.4 version and it offers excellent sharpness. The spatial relationships
between the buildings will be the same as they appear to the eye. If I need
a wider perspective my first choice is to step back, if possible, to fit
more into the frame. If that is not possible, I typically mount a 35mm
lens, but I grow uncomfortable with even that moderate wide angle, because
it emphasizes foreground, and I start to sense that things just don't look
right.
A wide angle fits more into the frame, but that comes at a price. You need
to determine whether you can live with its limitations. I shoot for
realism, and I get uneasy over the prospect of apparent perspective
distortion. But my objectives may be different from yours.
That having been said, I think that you will be most pleased with a genuine
Pentax lens, rather than a non-OEM brand. Pentax lenses are renowned for
their sharpness, flare control, color saturation and generally excellent
bokeh. Try www.keh.com for a good selection of used Pentax lenses. Even
their "Bargain" level will have good optical qualities, although they may
show scuffs on the barrels. Don't buy anything with a dinged filter ring,
as that usually indicates that the lens has been dropped and there may be
de-centering problems with the internal elements. I do not believe that you
will find any non-OEM lens in a Pentax mount that will outperform a genuine
Pentax lens, so I again suggest that you mount only Pentax-branded lenses on
Pentax cameras. These days you can find tons of excellent Pentax lenses at
prices that are dirt cheap.
Jeremy
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