Autofocus on 300mm 2.8 IS not working with 50D

Sorry, no. This lens relies on the camera having an autofocus motor, while the D3000 will only autofocus with lenses having their own autofocus motor.

Yes.

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.

The D5000 does not have a focus motor in the body, relying on the lens to have one. The old 70-300mm G lens does not have a focus motor, relying on the camera to have one. Thus, this combination will not autofocus.
The newer 70-300mm AF-S G lens, as well as the even newer 70-300mm VR AF-S lens, do have the autofocus motor and thus will autofocus on the D5000.

That lens is not an AFS type lens. The D40x camera only uses AFS compatible lenses. You could still mount and use it in M, but it will never give you autofocus or TTL flash.

You will need the interupter replaced the iris is closed down so the a/f will not lock on

Get the Canon EF 70-300mm IS USM about $500 and EFS 17-85mm IS USM about $300. The 75-300 all of them I would not use them 4 a body cap. Camera is fine or step up to XSI.

Yes and yes. Bear in mind that not all features may work. You probably won't get autofocus or metering with the Nikon D90, for example.

Try SONY

Here is Nikon's specs of lens compatibility for the D40X:


Compatible Lenses*: Nikon F mount with AF coupling and AF contacts Type G or D AF Nikkor:
1) AF-S, AF-I: All functions supported;
2) Other Type G or D AF Nikkor: All functions supported except autofocus
3) PC Micro-Nikkor 85mm f/2.8D: Can only be used in mode M; all other functions supported except autofocus
4) Other AF Nikkor*â¹/AI-P Nikkor: All functions supported except autofocus and 3D Color Matrix Metering II
5) Non-CPU: Can be used in mode M, but exposure meter does not function; electronic range finder can be used if maximum aperture is f/5.6 or faster
6) IX Nikkor lenses cannot be used
*â¹ Excluding lenses for F3AF

They do not recommend 3rd party lenses, ever