Architecture, Perspective Correction, and Photoshop

More of a comment than an answer:

Elliot I do not so much of this correction but if I do, the number crunching time bothers me. It helps to do all corrections on a smaller image version and later go through the entire process again with the full size image. With 6x9 you must get 120MB or larger files? I use the "Panorama Tools" plugin in PS. In the panorama tools you can dial in the number of degrees for correction. This again is easy to determine by the measure tool. ( Well, yes you need to substract two numbers if there is no perfect vertical alignment). I like the quality of the output but have never tested if your route will give better results.

BTW: In my images the most pleasing perspective is not always the "perfect square" of windows or other rectangular structures. Often this gives an "overcorrected" impression. On tele shots this is usually not the case or not so severe.

The aspect ratio of a building is preserved if you stretch the top and squeeze the bottom symmetrically (or vice versa). Afterwards, you must crop the image to remove the corners you just clipped.

Walter: My files are very big (400Mb 16 bit Tiffs). I have a fast computer (G5), but nevertheless certain methods of corrrecting perspective are too slow (i.e. Photoshop's Lens Correction filter). But Transform Perspective is fast. I understand what you're saying about the dangers of full correction - that it can look artificial and over-corrected. However my current project is about low level buildings, so none of the corrections I'm making are extreme. The buildings have an abstract quality which is enhanced if all horizontal and vertical lines are parallel to the edges of the photograph.

Edward: Thanks. I had already picked up that tip elsewhere, and that is the method I use. Pull out the top corners, and pull in the bottom corners by the same amount. I was just unsure about whether that was it, or whether further corections along the vertical axis were necessary.

I'm still interested in any further comments about using Photoshop to correct perspective distortion... I'd also like to ask whether there is any fundamental difference between doing it this way and doing it using shift with a large format camera? Some say there is, some say there isn't....

thanks

Elliot, I use the Perspective option with the crop tool, and use sets of ruler guides for alignment. In particular, this is handy for indoor shots: Please see the shot below which I corrected for another photog in the lab.

I might well pick the second; it looks more dynamic, informal, befitting the mood. There are times when parallel lines are not desirable, so it is best to keep an open mind.

'I use the Perspective option with the crop tool, and use sets of ruler guides for alignment'

I'm not sure how guides would help when using the perspective option of the crop tool, as there is no live preview - you just line up the crop box against the lines in your image that you want to be horizontal or vertical, hit return, and wait for Photshop to do its magic.

It's an amazing tool, but I prefer the live preview of Edit > Transform Perspective - you can see what is going to happen before committing to it. In this case, guides are very useful to help get everything lined up.

Now I remember why I don't like Edit > Transform Perspective: It's very jumpy, and is symmetrical.

Generally I apply the Perspective crop using Bicubic Sharper when I perform the main cropping, i.e. I first do a trimming crop (film edges), then global color correct, then final crop (i.e. 5 x 5 x 300 ppi) is done via Bicubic Sharper, with the perspective option to bring it in. Please see the scan from a 4x5 Tri-X sheet below, noting the ceiling line.

[Other photos are at http://users.snip.net/~joe/austin/pages/044.htm ]

Dan, I'm not sure why you're not just using rotate.

Err,

BECAUSE ROTATE DOESN'T CORRECT FOR PRESPECTIVE!