Monday, May 9, 2011

Lens Distortion

I have been testing the MK Direct Photo e-Box 1419. In order to shoot the entire specimen, I have been using the EF17-40mm f/4L USM lens that I rented from Adorama Camera.

The pictures look fantastic, but unfortunately, the lens - even though it has very very little distortion, still causes a bit of a curve around the edges.
Curvature is most noticeable at the bottom of the image. 

To test this, I printed a test pattern of 1 cm squares, photographed it using the 17-40mm lens at various focal lengths from 30mm-39mm. I found the least distortion at 32mm.
The black lines are the original test pattern, the red lines show the distortion at 32 mm.

These results are consistent with the excellent distortion chart found at SLR

While this may seem trivial, when you are dealing with text, any curvature becomes noticeable. It looks like I am going to have to stick to the Canon EF 50mm f/2.5 Macro. Here is how this lens stacks up:
Again, the black lines are the the original pattern, the red lines show the distortion from the 50mm Macro.

The 50mm Macro shows virtually no distortion. But, as with everything in photography, there is a trade off. To cover the entire specimen with a 50mm lens, the camera has to be about 30" above the shooting surface. With the camera this high, the opening in the top of the MK Direct Photo e-Box is too small making it look like I am shooting specimens through a keyhole.

Peephole photography

So, it looks like I am going to have to get out my band saw and cut a bigger hole in the top of the box. Stay tuned...

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