The x4 is not a measure of magnification. Its simply a ratio between the max and minimum focal length. By itself it does not tell you how wide it starts or how far in it zooms. Eg Lens 10-100mm = 100/10 = x10.

Lens 20-200mm = 200/20 = x10.

Lens 50-500mm = 500/50 = x10 Its also a handy way to sell superzoom point and shoots to Joe Public, he knows nothing about focal length etc, so telling him the latest superzoom is a x20 or x24 camera, means it's much better than that other camera thats only x10 or x12.. Of course as with most lenses, the higher the ratio the more compromises the lens has, the worse the optics tend to be (which is why primes are the best optically, as they are x1..), but Joe Public does not need to know that, just as he does not need to know, stuffing more and more pixels into a sensor will put more "stress" on the sensor and likely lead to more noise and a worse picture than if he had bought a camera with a lower number of pixels.. Joe Public just needs to know that the latest camera has more x?? than last years and more pixels so must be better, so he better get his wallet out or be left behind the neighbours.. If you assume that 50mm is roughly a "normal" or base view, then a 300mm lens is (on a full frame camera) roughly 300/50 = x6 magnification, or on a crop body, 300x1.5/50 = x9..

I am considering buying a D90 or D300. Can one mount Nikon/Nikkor lenses produced in late 70's and early 80's on the D90? How about the D300?..

Non-CPU lenses (pre AF) will mount on the D90, but it won't meter with them. The D300 will, as it has a mechanical aperture coupling in addition to the CPU interface...

Thanks. That's clear. The reason for my question is an offer "Nikon D40 Continental Package w/Nikon 28-80mm & 70-300mm and More!" and wen I saw the specs it said for the 70-300mm 1:4 magnification. So I assumed 4x. But from what you say, if the nominal lens is 50mm, then the zoom lens specified should start at 1.4x and have a magnificatin of 6x. Am I correct?..

No. Magnification is not related to zoom ratio. Magnification (or reproduction ratio) indicates how large an image can be reproduced on film or the sensor compared to the actual object. A true macro lens will give a 1:1 ratio; that is, the image on the film or sensor is identical in size to the actual object. A 1:4 magnification means the image can be up to 1/4 the size of the actual object. So it's an indication of the macro capabilities, not zoom ratio.

BTW, I'd avoid the 28-80mm for an APS-C sized sensor like the D40 has. Because of it's 1.5x crop factor, that lens will behave like a 42-120mm lens on the D40, which isn't very wide. If you do any landscapes, group shots or indoor shots, you'd be better off getting an 18-55mm or an 18-70mm lens to go with the 70-300mm. The 28-80mm is an inexpensive kit lens designed for 35mm film Nikons. All the official kit lenses (lenses bundled with the camera by Nikon) for their digital SLRs start at 18mm (18-55mm, 18-70mm, 18-105mm, 18-135mm, etc.), because they translate to 27mm at the wide end...