Full frame (FX) lenses will work perfectly fine on your D40.
Crop frame (DX) lenses do not work well on full-frame bodies or 35mm film bodies because they are designed to be more compact and cover only the smaller DX frame that they need to. DX lenses are cheaper to make simply because they require less materials.
On 35mm film, DX lenses will cause nasty black corners. On Nikon FX digital, the camera knows when you have a DX lens attached and crops the sensor accordingly, but you'll only get 5MP and it kind of nullifies that whole point of spending big bucks for an FX body.
If you can afford FX glass, go for it. There are craploads of old manual Nikon 35mm lenses on
eBay before DX was ever conceived, so they will work on both DX and FX digital as well.
what lenses have a motor in them and work with a full frame. Is it only nokkor lense that are like this or sigma ect like this too.
I assume this is a second question. Whether or not a lens has a built-in AF motor has nothing to do with a lens being DX or FX. That said, however, most of Nikon's new FX glass is also AF-S simply because it's better and faster than the old AF screwdriver system.
The D40 will only autofocus with AF-I and AF-S lenses from Nikon, HSM lenses from Sigma and a few lenses from Tamron, but I forget what Tamron's nomenclature is for "built-in AF motor".
So a 50mm lens on a d40 looks like a 75mm on a D3.
Correct. The crop factor of an image sensor is inversely proportionate to the focal length compared to a 35mm frame. That's why you need a miniscule 5mm length for point-and-shoot cameras with tiny sensors to equal a moderately-wide 28mm on a 35mm frame.
Four years is a long time. I wouldn't worry about upgrading right now, just buy the best lens for your camera.
Indeed.
Although, the alternative and
much cheaper way to go full-frame is to use 35mm film
