Center of the Frame
Far Right of the Frame
Auto focus using right of center focus spot
These images have been somewhat over sharpened to accentuate the differences in focus. The center of the frame appears to be fairly close for both the left and right images. You can see the difference most clearly on the fence in the foreground of these images. The fence can be seen to be out of focus in the right image. Also if you look at the handle of the door, you can see that the auto focus image on the left is in better focus. If you look at the lower images from the far right of the frame, it is clear that the image on the right, for which focus has been manually corrected, is sharper. This clearly shows the field curvature, which is present in this lens. In addition, it also demonstrates the ability of the 17-55mm lens to give very good results wide open at the edges of the field if one is willing to sacrifice center sharpness.

This test clearly indicates that in order to obtain the best focus across the field with my copy of the 17-55 mm lens, one must manually adjusted the focus if a compromise between center and edge sharpness is desired. (My guess would be that this is true for most Nikkor 17-55mm lenses.) Of course, when the lens is stopped down some of this effect is mitigated. But then another problem creeps in, residual spherical aberration.

When my 17-55 is stopped down the focus is shifted to the front, giving rise to the appearance of front focus. Here is a case where results can be improved with one of the newer cameras such as the Nikon D3, which allows one to make focus corrections for specific lenses. Of course, this will only give the best results at one specific aperture. For critical results with stationary subjects, nothing will beat using manual bracket focusing.

The images below show the effect of focus shift due to spherical aberration. These images were shot at 35mm and show the effect of stopping the lens down to F4. Interestingly, stopping down the lens further does not result in much additional shift of focus.

Manual focus correction
photographs by bernard

Checking Auto focus and field Curvature

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When your lens does not seem as sharp as you expect, the problem maybe auto focus inaccuracy, field curvature or spherical aberration. Not being completely satisfied with results from my Nikkor 17-55mm F2.8 zoon lens, I did some tests and found several interesting problems.

I was first concerned that the auto focus on the lens or on my camera was off. Other lenses, such as a 105mm f2.8 Nikkor macro lens and a 35-70 Nikkor F2.8 zoom lens seemed to focus properly on my Nikon D2x but had slight back focus on my D100 (focusing at distance slightly behind the subject). On the other hand, auto focus with the 17-55 usually resulted in front focus with both cameras, though less so on the D100.

I subsequently adjusted the adjusted my D100 to eliminate the back focus. I then proceeded to carefully test the 17-55 at 17mm, 35mm, and 55mm by first using the auto focus and then manually adjusting the focus for slightly increasing distances. Doing this I discovered some interesting facts about the lens. The four images below (shown at 100%) were taken at 55mm, F2.8. The left shows the results using the center auto focus point set on the door in the center of the frame. The images on the right show the results after focusing manually for the best results at the edge of the frame. The top images show the center of the frame and the bottom images show the far right.