Defishing the Canon 10D and Sigma 15mm fisheye lens

If you use a digital SLR and enjoy shooting wide angle, you've probably lamented the fact that current digital SLR's with a reduced size frame have a crop factor that essentially magnifies the effective focal length of your lens. Digital SLR's such as the Canon 10D and Digital Rebel have a crop factor of 1.6. This effectively turns your 28mm lens into a 45mm lens. Even a 24mm lens becomes 38mm - hardly wide angle. Canon's widest lens is 14mm, but at over $1799 not very popular. Optically high quality fisheye lens are vastly easier to design and manufacture as compared to normal rectilinear wide-angle lenses. I purchased my Sigma 15mm fisheye for $399 from Allen's Camera. Software can be used to defish the image if desired.

To defish, I use the incredibly powerful and versatile program called Panotools by Helmut Dersch. Besides defishing, you can also use it to create stitched panoramas from multiple overlapping images. Best of all it's free!

First Example

After defishing with Panotools to transform into a rectilinear image. This is equivalent to 20mm lens on a full frame camera. Tiles now appear straight. The next step is to crop out the empty areas at the top and bottom. I added the red lines to show the area to be cropped away.

After cropping away the top and bottom.
Move your mouse over the image to see before and after

Notice that part of the image in the corners get cropped away after defishing and the aspect ratio changes slightly. You'll want to frame the image a little loose to compensate for the corner cropping if you plan to defish later. Transforming from fisheye to rectilinear also causes pixels in the center to become denser and pixels at the corners to expand.

Another Example

After Panotools to transform into a rectilinear image.

Cropping away the top and bottom...
Move your mouse over the image to see before and after

I don't know about you, but I prefer the unprocessed fisheye image in this case. Fisheye lenses introduce less distortion in wide angle three dimensional images but at the cost of bending straight lines that don't go through the center. If your image does not have straight lines, or by placing straight lines through the center of the image, you can actually achieve less distortion with a fisheye lens.

Panorama Tools Directions

You can use Panotools either standalone or as a Photoshop plug-in. I have Photoshop so this example uses screenshots from the Photoshop plug-in. The standalone version is similar.

First, download and install Panotools here and follow the easy installation directions.

Now that you've installed Panotools, open the fisheye image in Photoshop and go to Filters and select Panorama Tools|Adjust. If Panorama Tools is greyed out, you need to convert your image to 8 bit.

Next, choose Set

The Canon 10D/D60/Rebel produce files that are 3072 x 2048 pixels and with a 90 degree Horizontal Field of View (HFOV) when shot horizontally. If your image is in portrait orientation then use 2048 x 3072 and HFOV=60 degrees. Repeat values in the Panorama section.

Wait for the progress bar to reach 100%, and voila, you converted your fisheye image into a rectilinear image. Overall, I really like this technique as I have a wide angle lens for my cropped sensor SLR and since designing and manufacturing fisheye lenses is vastly easier and cheaper than rectilinear lenses, I don't have to pay a lot to achieve a high quality result.

Optional extra step for ultimate image quality

Before performing your defish steps, you can set the interpolation algorithm that Panotools uses internally. Better quality interpolation yields better results but is slower. Go to Filters and select Panorama Tools|Adjust and click Prefs

Click More

Choose the quality of interpolation. I use the best quality, Sinc, for prints.

I hope you enjoyed this article. If your want to purchase your own fisheye lens, I suggest the Sigma 15mm fisheye over the Canon fisheye. Both achieve equally great results, but the Sigma at $449 costs less.

A great place to purchase lenses for international buyers is www.bhphotovideo.com. I suggest US buyers try www.canogacamera.com in California or www.allenscamera.com in Pennsylvania. I've purchased gear from all three places and recommend them all.

This site explains Correct and Remap in Panotools to achieve similar, but slightly different results.