Corrected for keystoning and barrel distortion.
This is trivial with Photoshop CS2, CS3, CS4 and CS5. This particular feature isn't in earlier versions of Photoshop, but I'll expalin how to do this with them at the bottom.
This works with any image, JPG, RAW, a film scan, or anything.
Here's what this filter, included free with Photoshop CS2 and later, can do to a bad digital camera image.
The original image, top left, is from a Nikon D70 with an 18-70 mm lens set to 18 mm. At 18 mm the 18-70 mm not only has barrel distortion, it has complex barrel distortion which is difficult to correct. As you can see even this nonlinear sort of barrel distortion is very well corrected. Most other lenses are even easier to correct. I chose this as the toughest example I could find.
Barrel distortion is what has made the top of the hotel look curved. In the corrected image, made by setting the slider I'll explain below to +6.00, the roof is now straight.
Most amateurs would be happy just getting the curving lines straightened out. Professionals also want the verticals fixed as well. Because I had the camera pointed up the building looks like it's falling backwards. I detail how to correct the converging perspective lines in the final large image here.
Photoshop CS2, CS3, CS4 and CS5
From the menu at the top of your screen in CS2 simply choose: FILTER > DISTORT > LENS CORRECTION. Be sure to have the grid turned on for reference.
As of CS5, it's in FILTER > LENS CORRECTION.
There are three ways to select the amount of correction:
1.) Type it in the "Remove Distortion" box, or
2.) Move the slider, or
3.) Use the Remove Distortion Tool (D) at the top left of the screen. Select it and grab the middle of a distorted line. Drag it until it looks correct. Release.
Feel free to fine tune as you see fit.
Most lenses only need values between +5 and -3, so go easy here. In my lens tests I'm now listing values to type into the box to save us all time.
Hit OK when you're done fine tuning. Voila!
Before Photoshop CS2
Back in the dark days of Photoshop CS and before I usually used the "Spherize" command under FILTER > DISTORT > SPHERIZE. Increase your canvas size 50 - 100% first so you correct the whole image.
CS2's filter works perfectly for simple first-order linear distortions. These simply curve lines into broad curves.
More complex distortions bend lines into more complex shapes like a moustache. CS2's Lens Distortion filter can't correct those completely.
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