Bill Pearce wrote:
> I have both the 18 and the 21, and I use the 18 almost exclusively for
> interiors.
>
> The software solutions for perspective distortion are decent, and the ones
> that correct for fisheye and extreme WA are quite remarkable, but in the
> correction process, quite a bit of the photo is lost, as it is cropped to
> have straight borders.
I think you have over generalized here.
- The distortion correction I demonstrated for the Tamron 17-35 @ 17 mm
does not, by itself, lose any image area. At 35 mm, correction does pull
in the very farthest corners a little, so minimal cropping would be
necessary. However, the distortion is so minor that I can't imagine many
shots where correction would be needed.
- Perspective distortion does indeed lose image area. However, it loses
only image area that simply wouldn't be captured in the first place
using shift/tilt. So from a theoretical composition perspective, there
is no loss. From a practical perspective, the stretching of some areas
and dropping of others may reduce ultimate detail resolution compared to
doing the same thing optically.
- Some software works better than others in this regard. Although I love
PTLens for linear distortion correction and it's perspective distortion
correction is easy to use, it seems to gratuitously chop off image area
that needn't be lost. I do the perspective correction in PS.
- There is an interesting trick that often works to minimize image loss.
Make a duplicate layer and apply the perspective correction, pulling
corners that are too wide in, rather than their opposites out. This
leaves long, skinny, triangular corner areas empty. But if you take the
corresponding area of the underlying, uncorrected layer and move it
around until the area where they meet matches, it will often nicely fill
the missing area. A little application of eraser or mask adjustment
makes the line disappear and no cropping is necessary.
I discovered this more or less by accident while working on some
interior shots of Hearst Castle. It works well on even complex detail to
the extent that even at full pixel, the patch is unnoticable.
- I don't have a fisheye, but I think the same considerations as for
perspective correction apply. Nothing is "lost' that wouldn't be lost by
using a linear lens, but the process may affect IQ, depending on the
degree of fisheye distortion and size of image display.
> I wonder if you don't throw so much away that you could have used a longer FL
> lens all along.
>
As I said above, this isn't an issue with simple linear distortion
correction with linear super WAs. Also, of course, use of a longer FL to
capture the same subject coverage often isn't physically possible, as in
the Elfin Forest shots I posted. And even when sneaker zooming is
possible, changing FL changes the apparent size relationship between
subject and background, which may be an important part of the image
composition.
Moose
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