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On the front of the camera body, below the lens release button you should find a two position switch labeled "AF" & "M" This is the "Auto Focus - Manual" selector switch. Make sure the selector is set to "AF" for auto focusing.
Your lens may have a similar focus selector switch on its barrel. There are two types of switches used - depending on the lens. The first is labeled "M/A" and "M" The "M/A" position indicates it is in "Auto Focus" mode with a "Manual" focus over ride capability. You can simply twist the focus ring to change the focus to a point other than that chosen by the camera's auto focus system. The "M" position is Manual Focus mode. No focusing operations will be performed by the D80 auto focus system - focusing must be done by the photographer.
The second type has "A" & "M" positions. These represent Auto and Manual focus. The difference between this type switch and the "A/M & "M" switch is that when in "A", the camera will no allow the focus to be changed manually. There will be opposition to twisting the focus ring in the position. Forcing it will likely result in damage. The selector switch must be changed to "M" to change the focus.
If either switch is set to manual, the auto focus system will be disabled, so it is very important to have them both set to "A" or "A/M".
How new is the lens? In or out of warranty? If out or warranty most likely solution is to find a Sigma authorized repair shop because your description sounds like the auto-focus motor gear has failed. If in warranty, check out Sigmaphoto.com for repair sites and also download the SIGMA_SERVICE_REQUEST.PDF. I've only had to deal with Sigma once on warranty matter and they were very helpful, provided UPS authorization for shipping on their nickel.
It will default to manual through the viewing screen. It will only work on automatic, using the viewfinder. Make sure you also set the focal length (lens to shutter distance). You might also have a mf / af switch on the lens. This also only works through the viewfinder.
Your 28-90 AF-D lens can't autofocus with the D40, which requires AF-S lenses with a focus motor in the lens. Sigma lenses with "HSM" (HyperSonic Motor) in the name also have the focus motor.
Did it ever work on this camera?
I have seen some Sigma lenses that are auto focus on canon film cameras but then when put on the digital they dont work right. If you have used it on this camera then make sure the lens is clean.sometimes dirt on the front wil make it try focusing on the dirt. Next try focusing in manual focus to make sure the focus is working. Then take the lens off and then put it back on and make sure it clicks into place.
Let me know if none of those things work. also make sure to try focus on something close and somthing far away.
(From Sigma lens literature) Capable of macro photography, this
lens has a 1:2 maximum close-up magnification at the 300 mm focal
length. It's the ideal high performance lens for portraits, sports
photography, nature photography, and other types of photography that
frequently use the telephoto range. It also has a switch for changeover
to macro photography at focal lengths between 200mm and 300mm with a
maximum close-up magnification from 1:2.9 to 1:2. The minimum focusing
distance is 1.5m / 59 in. at all zoom settings.
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