I have a Nikon F55 fitted with a NIKKOR AF 28-100mm lense I wish to upgrade to digital. Can I use this lense on a digital body?

Model 1919.

No Nikon lens ever zooms automatically, so I assume you mean focus.
The D5100 lacks an autofocus motor, requiring the lens to have one. The older 70-300mm lenses lack this motor, expecting the camera body to have one.
The D5100 will only autofocus with lenses designated AF-S or AF-I. It will not autofocus with older AF-D and AF-G lenses.

The sensor on the D80 is smaller than a piece of 35mm film. This means the camera will only use the center portion of the image cast by the lens. This reduces the angle of view, effectively increasing the focal length. The 35-70mm will give you an angle of view similar to a 52-105mm lens on a film camera. Likewise the 70-210mm lens will give you an angle of view similar to a 105-315mm lens.

Lens mount Nikon F mount (with AF coupling & AF contacts)

Lens compatibility • D-type / G-type AF Nikkor (except IX-Nikkor): Autofocus and all functions possible
• PC Micro-Nikkor 85mm f/2.8D: Usable in Manual exposure mode
• AF Nikkor other than D-type (except AF Nikkor for F3AF): All functions except 3D Matrix Metering possible
• AI-P Nikkor: All functions except 3D Matrix Metering and autofocus possible
• IX-Nikkor: Cannot be used
• Non-CPU: Usable in Manual exposure mode (exposure meter cannot be used)
Electronic Rangefinder usable with lens with maximum aperture of f/5.6 or faster.

Almost all Nikon-mount lenses made in the past half-century will work on the F55. With old lenses, you need to make sure they're AI or adapted to AI. Among newer lenses, you don't want to use the DX lenses designed specifically for digital cameras. DX lenses will work, but you will get vignetting (dark shadows in the corners). You will also lose the autofocusing capability with AF-S lenses.

The d60 needs Auto focus lenses(Ones with motors inside the lense) it doesn`t autofocus on other lenses. this was done to reduce the price of the camera body

Hope this helps

Mirror lenses have no aperture nor AF, so they mount via a T mount. A $30 item that screws on the end of a mirror lens that fits your camera. A mirror lens has it's physical likes and dislikes against a glass 500mm. A glass 500mm will give a better image if you can stop down to F8 F16. Then you have shutter speeds to consider and those lenses are usually connected to the camera's contacts. So they can be specific to a single body. There are older Tamron lenses that take Adaptall mounts, they are old and hard to find. But they solved the multiple body problems for "auto" program lenses. None had AF, when AF camera around, we were back to individual lenses per body.

Is the switch on the side of the lens set to manual or auto? Sometimes it's the little things like this that we don't notice.

You need AF-S or AF-I Lenses. These have the autofocus motor built in to the lense. The D40 and D40x does not have an autofocus motor in the camera body itself, while the D80, D300, D3 Etc to have the motor in the camera. You can use most old lenses on your D40x, but you will have to focus them manually.

See Ken Rockwells page for a lot of D40 / D40x info
http://www.kenrockwell.com/nikon/d40.htm

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