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

[Joseph Tainter]

I bought this lens about a year ago. It got 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 bothered me, so I did 
systematic tests to see if/where there was a problem. I report the 
results here.

The subject was the old (1870) church in Corrales, New Mexico. The 
exterior is plastered in mud mixed with bits of straw; these give small 
details from which to judge sharpness. There is also a small plaque at 
9:00 indicating that the church is on the National Register of Historic 
Places. From a distance the lettering in the plaque is good for testing 
sharpness. 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 subjective 
classification: good (very sharp) / okay (acceptably sharp) / 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. None 
was in-between. At 35 mm, 3 were good and 3 were okay. 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 selling the lens. Yet it is so sharp at its best 
apertures that I will probably keep it and use it at those apertures.

Thanks,

Joe