Wednesday, February 10, 2010

Distortion Correction

(If you want to go straight to the tutorials, scroll to the bottom of the page.)

There are two types of distortion: lens distortion & perspective distortion.  

Lens distortion is an optical distortion caused by the lens. It is more likely to be noticeable when you are using a wide-angle lens, when you are very close to your subject, or when your subject is very large (such as a building).

Types of lens distortion include:

Barrel distortion
-associated with wide-angle lenses & at the wide end (zoomed out) of a zoom lens
-grid lines appear to bulge out from the middle (like a barrel)
-most easily seen around the edges of the image
-fisheye lens

Pincushion distortion
-associated with telephoto lenses at the tele end (zoomed in)
-lines appear bowed inwards or pinched in the middle
-most easily seen around the edges of the image

Mustache (or Complex) distortion
-a mix of both barrel & pincushion distortion
-not easily fixed by software
-Canon 24-105mm f/4L is known for its mustache distortion

Chromatic Aberration
-color fringes in high-contrast edges, sometimes worse in the outer part of the image
-simplified explanation:
    the lens is splitting the light (much like a prism does) which should not happen
-occurs more frequently in wide angle lenses
-less likely to occur at wide apertures

-darkening of the corners of an image
-caused by uneven illumination of the film plane or digital sensor
-occurs with:
   wide-angle lenses
   the use of converters
   sometimes with the use of a lens hood (particularly if it is not properly sized for the lens)

Perspective distortion is caused by the angle &/or distance from the camera to the subject (not by lens distortion), regardless of the lens used. It happens when the camera lens is not straight-on/parallel to the subject. The image itself is not distorted but it is perceived as distorted.

Types of perspective distortion include:

-image appears to be leaning backward or forward
-the problem exists anytime you need to tilt the camera back to get something tall into the photo
-most noticeable with buildings due to their straight parallel sides
-keystoning is one type of vertical perspective problem:
-gets its name from the keystone of an arch
-lines are straight but leaning inward at the top
-can also occur as an upside down keystone (straight lines that lean inward at the bottom)

-one side of the image appears closer to the viewer than the other side
-example: in a portrait where the head is turned & one eye is closer to the camera, the closer eye will appear larger

Horizon Misaligned (not truly a perspective distortion but I've included it here)
-horizon is not straight


Hardware solutions:
-Buy an expensive Tilt/Shift or Rectilinear lens

Software solutions:
-Photoshop CS2 & later (Lens Correction Filter)
-Photoshop Elements 5 & later (Lens Correction Filter)
-GIMP version 2.4 & later (Lens Correction Filter)
-Lightroom 3 (Lens Correction Filter)
-DXO Optics Pro - the ultimate lens distortion correction software - $199
-PTLens - use as a stand alone application or PS plug-in; good if you have earlier versions of PS/PSE w/o lens correction features) - $25

TUTORIALS (with some clickable links):
If you have GIMP or PS Elements watching the Photoshop videos below may help you.

Lightroom 3:
-Lightroom 3 Lens Correction tutorial 

I'm not familiar enough with GIMP to recommend a tutorial but I'm sure you can find some by googling "GIMP lens correction filter" 

Photoshop 7 & earlier:
-I was unable to find a workaround for the lens correction filter in these versions
-You can correct for vertical/horizontal distortion and keystoning (only if there is no barrel or pincushion distortion) by using the Transform Tool (EDIT > TRANSFORM or EDIT > FREE TRANSFORM)
-Here's a video tutorial on how to use the Transform Tool (the first 2 minutes are tutorial & the last section is a useless commercial)
-Try using PTLens as a standalone application (I have not used this program but it generally gets good reviews)

Photoshop CS:
-Try using the "Spherize" command to correct for barrel/pincushion distortion
FILTER > DISTORT > SPHERIZE    (Sorry... I couldn't find a tutorial)
(Tip: increase your canvas size by 50-100% first so you can correct the whole image & prevent Photoshop from crashing)
-Use the Transform Tool for vertical/horizontal distortion & keystoning
-Try using PTLens as a standalone application (I have not used this program but it generally gets good reviews)

Photoshop CS2:
-Tutorial here: Lens Correction filter in CS2 

Photoshop CS3 & CS4:
-Video tutorial here: Perspective Correction in PS

Photoshohp CS5:
-Video tutorial here: CS5 Lens Correction 
-There is a vastly improved Lens Correction feature in Adobe Camera Raw 6.1 (compatible only with CS5) It does not come pre-loaded in CS5 but you can download it HERE

Hope that helps! 
If you have any questions please feel free to email me: 

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