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Super wide-angle and distortion
written by Fred Miranda

NOTE: The image samples are 100% crops of full images. They were shot with a 1D using the standard curves settings, Matrix 4 (AdobeRGB) and default sharpening setting. The images were brought into Photoshop and assigned to AdobeRGB. I then corrected for barrel distortion/perspective, resized, and converted to sRGB before posting.

A few months ago, I had an opportunity to review the new Canon 16-35L super wide-angle zoom lens and make a few comparisons with the 17-35L.
I have received many emails asking me how well the new 16-35L compares to the 17-35L in regards to barrel distortion.

I tested the older 17-35L and from my analysis, I found severe distortion at about 5.5-6 % at its widest focal length (17mm). I decided to test the new 16-35L for distortion as well, at the following focal lengths: 16mm, 20mm, 24mm, and 35mm

I used my new Canon 1D to shoot a grid target.
Then I used an excellent Photoshop plug-in from Helmut Dersch, called Panorama Tools, to calculate the amount of distortion compensation required.

Here are the results of my tests:

16-35L at 16mm: (distortion of ~2.1%)

16-35L at 20mm: (distortion of ~1.3%)

16-35L at 24mm: (distortion of ~0.7%)

16-35L at 35mm: (insignificant)

These results were acquired by taking a picture of a target at the selected focal lengths and guessing the distortion levels until the target lines became straight (free of barrel distortion).

 

Example 1 (before and after barrel distortion correction) - 1D, f/8, 1/200s, 16-35L at 16mm

 

Example 2 (before and after barrel distortion correction) - 1D, f/3.2, 8 seconds, 16-35L at 16mm

 


How to use the free Panorama Tools Plugin:

- Download PanoTools: PC | Mac
- Place the file pano12.dll into the same directory as Photoshop.
- Place the file correct.8bf into the Plug-ins folder for 'Filters'.
- Restart Photoshop. You should find the Tools in the Filters submenu.

Here is a screenshot of the Filters submenu:

 

Next, check "Radial shift" and make sure the other options are unchecked. Click on Options.

 

Set 'a' and 'c' to zero for all colors, choose a value for 'b' and choose 'd' so that d + b = 1. For barrel distortion compensation 'b' must be negative. For the 16-35L at 16mm, use "a = 0", "b = -0.021", "c = 0", and "d = 0.021" (2.1%) for all colors (red, green and blue).

 

Finally, choose your interpolation method. I recommend Spline 36Pixels for great quality and performance. If you have time and your files are not too large you could choose Sinc 256Pixels for the best image quality.

 

Keep in mind that the above settings are custom made for the 16-35L. If you want to use the above technique for your own lens, you'll need to calculate and customize your settings based on your target grid. When tilting your lens upwards or downwards, you may have to correct the perspective as well. You can do that easily using Photoshop.

Here is how:

Open you image. Select all (Ctrl-A). Edit/Transform/Skew

Then move your points until lines that should be parallel get parallel.

Here is an extreme example corrected for barrel distortion (panorama tools) and perspective (transform/skew):

 

 

Update (April 6th, 2002):

Photographer Dale Kaechler, tested his Sigma 15-30mm f/3.5-4.5 EX Aspherical DG DF zoom lens and calculated the following distortion settings using the above method.

Here are his findings:

15-30mm Sigma at 15mm: distortion of 1.8%
15-30mm Sigma at 20mm: distortion of 0.9%
15-30mm Sigma at 24mm: distorition of 0.3%
15-30mm Sigma at 30mm: insignificant

Example (before and after barrel distortion correction) Sigma 15-30mm f/3.5-4.5 @ 15mm

image by Jerry Eakes

 

Update (November 1st, 2002):

Jakob V Christensen tested the D60 with Sigma 17-35 EX for barrel distortion using PanoTools and found the following:

b/d value to correct at x mm

mm. b d
17mm -0,008 1,008
18mm -0,007 1,007
19mm -0,006 1,006
...
...
24mm -0,001 1,001

(0,001 step down per mm step up)

To discuss this tutorial, visit the Canon digital SLR board. Click here for a full review of the Canon 16-35L.

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