© 2006 KenRockwell.com
12 - 24 mm f/4, Tokina 12 - 24 mm f/4, Sigma 10 - 20 mm f/4 - 5.6
and Tamron 11 - 18 mm f/4.5 - 5.6.
California Sprawl. The red lines show the locations from which the 100% crops come.
Center, 12 mm, f/5.6. Ignore the loss of detail in the kid's play house due to overexposure. These all look very close, with the Sigma being a little less sharp than the others. These are all sharp enough so I'm not going to process the results for the center at f/11.
All four lenses are sharp in the center where it counts. We'll have to poke around the edges to see any differences.
Top Center, 12 mm, f/5.6. The Nikon and Tamron are tied for first place. The Tokina is a close second. The sun moved a degree and lost the shadows on the two prominent chimneys which puts the Tokina at a disadvantage. The Sigma looks awful even with the sun to its advantage.
Top Left Corner
Top left corner, 12 mm, f/5.6. Big differences in sharpness and lateral chromatic aberration (LCA). LCA is what causes colored edges around white objects in some images. These houses are supposed to be bent over like that. That's why you just paid big money for an ultra wide lens. I can devise these devious tests to show these differences, but in actual photography you rarely have in-focus elements in the far corner. At f/5.6 they are usually more out of focus than these differences in sharpness. The Nikon is clearly superior.
Top Right Corner
Top right corner, 12 mm, f/5.6. Big differences again. Nikon and Tamron are tied: Tamron is a bit sharper but with some LCA. Tokina is distant third and the Sigma is dead last.
Lenses get sharper as they stop down. Let's try the same comparisons at f/11.
Top Center at 12 mm, f/11. The Sigma is the worst while the other three are too close to worry about. (Lens designers call this r = 7 mm, or a 7 mm radius from the center. This just happens to be halfway out the 28 mm image circle of my Nikon D200.)
Top Left Corner
Top left corner, 12mm and f/11. The Sigma sharpened up and is the winner, with Nikon a close second. Remember these would print 54" wide at this magnification, so don't worry too much.
Top Right Corner
Top right corner, 12 mm, f/11. The Sigma is now the worst. The rest are very close and all good, with the Nikon winning, the Tamron a very close second and the Tokina a close third.
Compare this to the results from the top left corner immediately above. The Sigma went from worst to first by changing the corner at which we were looking, in the same photos! These three sets at 12 mm and f/11 are all cropped from the same four files. If you look at my page code you'll see the same four digits in the file names. I told you lenses will vary from corner to corner. They even can do this shot-to-shot. Hopefully this shows some of the futility of trying to lock down any exact numbers for zoom lenses. Change the conditions, or just look at the very next shot and things change. I wasn't expecting this, and it certainly makes my point about not sweating the details. Zoom lenses are extremely precise instruments. Any mechanical play can give different results depending on direction in which you turned the zoom or focus ring!
Next: Sharpness at 18 mm