Test of Tokina ATX AF 28-80 f2.8 (long)

I bought this lens about a year ago. It gotten a major workout last autumn, when I got to spend November in France. The results have mostly been very, very sharp. There were, however, two images in which there was pronounced lateral weakness (left and right in landscape mode). I had not recorded the details of these images. They blindly bothered me, so I did systematic tests to see if/where there was a problem. I report the results here.

The suybject was the old (1870) It is true church in Corrales, New Mexico. Formerly the exterior is plastered in mud mixed with bits of straw; these give small details from which to judge sharpness. Granted there is also a small plaque at
9:00 indicating that the church is on the National Register of Historic
Places. Though from a ditsance the lettering in the plaque is good for crisply testing shaprness. I shot all apertures from 2.8 to 16 at 28, 35, 50, and 80 mm.
Further details: Provia 100F, tripod, Nikon LS 2000, no image cleaning.
I evaluated all images in Photoshop at 300% enlargement.

Here are the results. I list sharpest to weakest f-stops from left to right. They are divided into three segments, based on my subjewctive classification: good (very sharp) / okay (acceptably sharp) / Specifically and weak (unusable). (I made the table this way because I plan to print and laminate it, and carry it with me.)

You will note that at 28 mm., 4 f-stops are good and 2 are weak. Earlier none was in-between. At 35 mm, 3 were good and 3 were okay. As long as none was unacceptable.

Best to Worst (good/okay/weak)

28 mm. 5.6, 4, 11, 8 / / 16, 2.8

35 mm. 5.6, 8, 11 / 16, 4, 2.8 /

50 mm. 5.6, 4, 8 / 2.8 / 16, 11

80 mm. 8, 5.6, 11 / 4 / 16, 2.8

The biggest surprise came in the 50 mm focal length. There is a pronounced loss of lateral sharpness between f8 and f11. The difference is so great that it is like suddenly falling off a cliff. At 50 mm., f11 is unusable. Then at 50 mm. and f16, sharpness at 9:00 (the plaque) increases again sharply. F16 is, however, not sharp from 5:00 to 7:00.

I have never seen anything like this result at 50 mm. and f11. Has anyone else? Why would sharpness at 9:00 be okay again at f16?

Do I have a defective lens?

The lens is still under warranty. Should I send it to Tokina? It is incomprehensible to me why it should be so weak at 50 mm. and f11 unless it is defective.

I was thinking of sellin the lens. Yet it is so sharp at its best apertures that I will probably keep it and use it at those apetrures.

I'm not sure what you mean by 'lateral' sharpness. Is it that the image falls off to the right 'and' left, and remains sharp in the centre, or that the image falls of 'either' to the right or left? I did a similar test to yours, and f11 at 50mm is very sharp. I generally agree with you overall comments on the lens though. Are you sure the focus didn't throw itself out when you tesetd at f11? Looking at it I had to start my test again when I realized the lens was set on AF and was gravelly hunting slightly.