Chuck Norcutt wrote:
> If you have PhotoShop just use filter->distort->lens correction. ...
>
> If you don't have PhotoShop and your editor doesn't support perspective
> correction you can buy PTLens for $25. ...
I don't use the Lens Correction filter for these purposes. (Partially
misnamed, by the way, as perspective is not a lens abberation.) Neither
do I use PTLens for perspective correction. I've tried using the PS
filter on a couple of multiple perspective problems. This is an example,
where I had to shoot this skylight from a funny angle, as it is above
the stairwell in the prior gallery image so I had to be way off center
and possible sight lines were limited by the large light standards on
the corner posts.
<http://galleries.moosemystic.net/Brooklyn/Things/All%20Things/slides/_MG_3244corcr.html>
I've simply not able to completely square up shots like this with the
simple, two axis controls provided in the Lens Correction filter. And
not for want of trying; I spent way too much time on trying to learn how
to make it do the job. PTLens is worse for perspective correction. It
uses the same tool model as PS, assuming simple, two axis distortion. I
don't remember the details, but I seem to recall that it also does (or
did) a weird, slidey thing that loses more of the image than necessary.
My tool of choice for correction of perspective and also rotation when
there's no strong line to correct, is Edit=>Transform=>Rotate, Skew,
Distort, Perspective & Warp.
An interesting thing about these tools is that one may use various of
them in one, single edit, simply right click and choose another of the
tools. That means actual application to the image doesn't happen once
for each tool, with the resulting likelihood of artifacts from multiple,
sequential changes. Rather, all the changes done to the screen image are
saved up and applied in one pass to the underlying image.
Perspective acts somewhat like the Perspective tool in the Lens
Correction filter, except that it only acts on one end of the image at
once - and the center of that action may be controlled. I consider this
to already be more flexible. The two say tools simultaneously expand one
end and compress the other. With Edit ... Perspective, you have your
choice of either or both. You can, for example, only use expansion in
order to lose a distracting element in one corner. Or compress only on
large images to avoid any possibility of upsampling artifacts, although
I've yet to notice any.
Distort is my most used of the bunch. It acts on any corner in both
directions, expanding and/or compressing as you push-pull the corner.
This tool makes correcting complex perspective distortions quick and
easy. Very powerful for almost completely correcting vertical and
partially correcting horizontal in a lot of common situations that look
quite natural that way. I say "almost" because our vision systems are
quite forgiving of slight under correction and quite unforgiving of over
correction.
Skew is like Distort, but restricts movement to one axis at a time.
Warp is seldom needed for what I do, but incredibly useful when needed.
One thing it can do is visual correction of lens distortion manually,
separately for each "side" and with control points to allow asymmetrical
corrections. I've mostly needed it when extensive Distortion correction
leads to slightly bowed lines along edges. There's probably a way to
avoid that, but I'm not that adept yet, and Warp fixes them right up.
Rotate is dead simple. Grab a corner and rotate it. Works well and can
be used interactively with Prespective and/or Distort wen it becomes
clear that rotation is needed.
Notes on use.
- CTRL-double quote turns a reference grid on and off.
- The grid characteristics are adjustable in Edit=>Preferences.
- It won't work on a locked layer or the Background layer. Create a new
work layer or rename the Background layer.
---------------------------------
The other, quick, very useful and accurate way to correct rotation is
the Ruler tool. On the Tool Palette, right click on the Eyedropper and
select the Ruler. Click on one end of a line that should be horizontal
or vertical, drag the tool to the other end and let go of the button.
The characteristics of the resulting line are in the tool bar at the
top, including the angle.
If you then select Image=>Rotate Canvas=>Arbitrary, the correction
necessary for H or V is already filled in. Click OK, and you are straight.
> It real value is in correcting barrel and pincushion distortion customized by
> distortion profiles for specific lenses.
The Lens Correction filter in PS and other, similar tools has a HUGE
flaw. It assumes that the distortion is simple. Most really wide lenses
and many zooms even at moderate WA, have much more complex linear
distortion. It's usually called "waveform", but mostly is what I'd call
"donut", although it also might look something like an Airy disk viewed
from above and without the central peak. (Heck, maybe it is the same
sort of function, but the central peak is so narrow we don't see the
tiny point distortion?)
PTLens' methodology based on actual measurement does a wonderful job on
this kind of distortion. I'm sometimes amazed how squirmy lines
straighten out. It's also way faster and easier. In most cases, it picks
up camera, lens and focal length from EXIF data and does its job without
any input/adustment on the user's part.
I see it's gone up to $25 now. Worth every penny, one of the great
bargains in incredibly useful tools.
> If the database doesn't include your lenses you can send the author test
> images (instructions for doing so included). The author will calibrate the
> lens from your test shots and add it to the database.
> <http://www.epaperpress.com/ptlens/index.html>
>
He used to be fussy about MF lenses, but these days, you can submit a
shot taken with the MF lens on your DSLR and he'll profile it.
I mostly use PTLens for vignetting correction or addition. It seems to
me it worked better than some alternative, but I don't remember if it
was the PS Lens Correction filter. It's only one click away in my PS
set-up and it works, so why try anything else?
Pedagogical Moose
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