Sigma 55-200mm F4-5.6 DC

That would depend on what you mean by "ok." Assuming you get the lens with a Nikon mount, it will fit onto the D40 and meter properly. However, the D40 lacks an autofocus motor and thus this lens will not autofocus. You can still focus manually.

It's just not compatible. You needed to order the Sigma lens for Nikon.

Assuming you get one with a Nikon mount, yes. Sigma makes their lenses with a variety of mounts to fit different makes of cameras. A lens with a Canon mount, for example, will not fit on a Nikon. A lens with a Nikon mount is fully compatible with the D50.

Sigma makes some lenses for Nikon cameras-- though the Nikon D40 requires a lens with its own internal focusing mechanism, since it does not have an autofocus motor. This entirely depends on what Sigma lens you have, i.e. if it were a Sigma lens made for a Canon camera, yes there would be an adapter for that-- but the likelihood of success is less with the limitation of the D40 needing an internal focus motor on the lens.

The Nikon D40 does not have an autofocus motor, relying on the lens having one. This Sigma lens does not have such a motor, relying on the camera having one. Nikon designates lenses with an autofocus motor AF-S, Sigma calls theirs HSM.

Yes, it will fit your camera perfectly as Sigma only supply this particular model with the Nikon mounting (see Sigma website and also their lens compatibility chart).

I hope that you have found my reply to be of assistance and in return ask only that you rate my answer.

You have to make sure you bought the sigma lens with a NIKON mount. Sigma makes a Sigma mount as well as a Canon mount, of course neither of those mounts will work with your camera. Also you must also make sure your lens is the Sigma 70-300 HSM (/Hyper Sonic Motor) or it will not AF on the D40. I just bought this lens from Abes of Maine and I love it on my D40.

hi ! iam too a D40 holder,u can opt for AF-s versions or HSM sigma,both of them autofocus on d40,can go for 18-200 AFs VR or if u need still longer end go for 70-300 but it is costly than the sigms 70-300APO DG HSM MACRO..,its a much better reviewed lens and dont forget the cost..anyway do some research before considering..good luck

You should be able to use any Nikon AF-S or Sigma HSM lens on your D40. If you want to be 100% sure, email or call Sigma from www.sigmaphoto.com. They are generally very helpful with such issues.

Unfortunately, this lens is not supported by the D40 due to Nikon's unfortunate decision to leave out the autofocus motor from this camera.  This is why I tell folks not to waste their money on the D40 - access to the many, many, many lenses that Nikon has made over the years, many of which are available used for a substantial savings, is lost on this camera.  The only Nikon lenses that work are AF-S lenses, and some third party lenses that have their own internal motors.  I got a list of Sigma lenses that are compatible with the D40 for you:
The list of compatible Sigma lenses for the Nikon D40 as of the current date is as follows:

• 10-20mm F4-5.6 EX DC
• 12-24mm F4.5-5.6
• 17-35mm F2.8-4 DG
• 50-500mm F4-6.3 DG
• APO 50-150mm F2.8 EX DC HSM
• 70-200mm F2.8 DG MACRO
• 80-400mm F4.5-5.6
• 100-300mm F4 DG
• 120-300mm F2.8 DG
• 300-800mm F5.6 DG
• 14mm F2.8 EX
• 30mm F1.4 EX DC HSM
• 150mm F2.8 EX DG
• 180mm F3.5 EX DG
• 300mm EX DG
• 500mm EX DG
• 800mm f5.6 EX DG

from here:  http://www.imaging-resource.com/NEWS/1177682545.html
Sorry for the bad news.  The cheaper thing at this point may be to sell the D40 and get a D80.  I personally have a D70S, but that is no longer made.