Thanks for the tips. Although it's been staring me in the face I've
never used Edit=>Transform=>Distort before for perspective correction in
PhotoShop. I just tried it and see that the underlying layer must be
made invisible first in order to see better what you're doing.
Although I didn't mention it, since I doubt many people have it, my
favorite tool for perspective correction is not in PhotoShop but Picture
Window Pro. In PW Pro Transformation=>Geometry=>Warp does the same
thing as PS Edit=>Transform=>Distort except... it automatically puts up
the grid and continuously displays in a preview window what the cropped
version of the image will look like. There are also some usage options.
For perspective correction one would normally use it with the "free"
and "perspective" options checked. "free" allows simultaneous
correction of both horizontal and vertical perspective as well as
horizontal and vertical resizing. Switching from "free" to "rigid"
causes the grid to become rigid but allows grid repositioning including
rotation.
Thanks for the tip on the ruler tool. Never knew that. I'd always
wondered why PhotoShop didn't have something as simple to use as the
straighten tool in ACR. Still not as simple but close.
Chuck Norcutt
Moose wrote:
> 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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