Re: how far is the horizon and OT photography

Paul Saunders <pvs1@xxxxxxxxxxxxxxxxxxxxx> wrote:
Fellwalker wrote:

pentax just announced they are not keen on going for FF after all,
looks like my FF capable lenses are going to be a waste of weight and
money for the foreseeable future, I'm tempted to get a Sigma 10-20
APS to gain wide angle and save weight, its "only" 300 :-(

WARNING! WARNING!

A friend of mine has the Sigma 10-20 and it's not well corrected. I think
the sharpness is okay but there's a distinct curve to straight lines, and
people bulge and distort on the edges of group photos.

First, the curve to straight lines is barrel distortion and causes
*less" widening of bums on the edges of wide angle images than does
completely correct rectilinearity. Second, distortion correction
software with good lens distortion models (such as PTLens) corrects
the Sigma 10-20mm geometric distortion instantly and simply with one
button click, reading the correct focal length from the EXIF data.

So if you want width-sensitive people not to bulge too much at the
edges of wide angle photographs, use the lens uncorrected. Or in
extreme cases add more barrel distortion. On the other hand, if you
want perfect straight lines for architectural photography, use lens
distortion correction software.

It can be corrected
in Photoshop if you're willing to spend the time fiddling about, but you
need to be aware of it. It's very obvious.

Photoshop's native corrections here are rather primitive and very
fiddly indeed to use. Much better and much easier to use is
specialised geometry correction programs or plugins (PTLens for
example is available as both a stand alone program or Photoshop
plugin).

On the other hand, my Canon 10-22 is excellently well corrected. Straight
lines remain straight, even though there is unavoidable stretching in the
corners and edges at the widest settings.

It's not unavoidable if your principles permit the use of software
with with a variety of lens projective geometry transformations :-)

That's why the Canon cost 650 and the Sigma 300.

So for perfect wide angle architectural rectilinearity you can spend
650 on the Canon or 310 on the Sigma and PTLens. I'm very happily
using the PTLens-corrected Sigma 10-20mm for architectural
photography. I'm delighted by how well the combination works, much
better than I expected it would before trying it.

Of course, you'd need to buy a Canon camera to make use of it, but that
would be no bad thing with the latest 500D and the forthcoming 5D MkII (if
you want full frame). But of course, that's a lot of money to invest in a
new system.

If you're going for full frame and >20MP images you're probably also
going to have a buy a more powerful computer to digest the larger
image files at reasonable speed, unless you're very patient or don't
do much image editing :-)

--
Chris Malcolm



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