Back Focus Problem

rajul

I owned Nikon D50 for more than 2 years now. Recently i bought Nikkor 17-35mm f2.8 which i found sharp at most focal length even at f2.8. But i discovered that this lens is soft when i shoot at approx 1m distance. I found the background, at about 0.3m from the subject, is sharp. Is this what is called back focus? For your info, when i receive this lens, i immediately return for exchange due to similar problem but even worst i.e. very soft at most focal length at f2.8. Therefore this lens is my second copy but still find this problem but better than the first copy. I don't think this lens is defective again after the exchange. I have no problem with my other five lenses. Has anybody having this experience? Is there any tips to shoot wide open? Pls heip.

ziggy53

Minimum focus for that lens is supposed to be 1 ft. It is not clear whether that is at 17mm or at 35mm, but generally minimum focus is better at the wide end of a telephoto lens.

It would help us to help you if there were full-res samples, with EXIF, for us to review.

rajul

Thansk ziggy, sorry for the late reply

pls check the test images from this link http://rajuladli.smugmug.com/gallery/3841223

At 17mm f2.8, subject (trolley toys) is soft and background is relatively sharp depending on distance between the subject.

At 17mm f5.6, subject is better but still soft.

At 35mm f2.8, subject is sharp.

I found my kit lens 18-55mm f3.5-5.6 performed better in term of sharpness with the same setup but at 18mm f3.5. If this thing is normal, it's ok, i will not return this item for exchange again. I just need confirmation esp from the 17-35mm owner.

Your help is very much appreciated

ziggy53

Be careful with this sort of test. The subject you chose has rounded corners and, if you look at a grayscale conversion, you will see relatively low contrast as well. The background, by comparison, has sharp edges and is easier for the focus sytem to lock on.

When you zoom in, it may be that the subject is now large enough for the focus system to find the edges of, for instance, the speaker opening (I think there is a speaker opening in the center of the toy.)

There is a pretty good explanation of how autofocus works here:

http://en.wikipedia.org/wiki/Autofocus

rajul

Thank ziggy for your input.

I have no prolem having tack sharp focus of the toys in the same setup using my kit len s 18-55mm, 28-200mm, 70-200mm and other primes.

This is simple test, anyone can do this test even using fully auto mode. What i really want to know whether this lens is defective or inherently soft at 17mm at f2.8 - f5.6 within 1-3m shooting distance from the subject.

Could someday who own this kind of lens give some input please. I need to decide whether to return it or keep it.

Thank you