A 75-300 lens is said to have a 4X zoom range, not a 4x magnification. The 4X zoom range actually means that 300mm is 4X greater than 75mm (75x4=300). A 30mm to 300mm lens would have a 10X zoom range, but it would not bring anything closer than the 75 to 300mm lens (with a 4X zoom range), since the maximum focal length in both is the same. It is generally accepted that for a 35mm film camera, a 50mm lens most closely approximates what our eyes see (in reality it should be 42mm - always using the 35mm format). By dividing the focal length of your lens by 50, you can estimate the actual magnification. A 300mm lens (on a 35mm film camera or a full frame digital camera) would have a magnification factor of 7X (300/42=7.1). When used on a Nikon D90 (smaller sensor than a full frame camera - 1.5 crop factor) then you would have a magnification of 11X (300/28=10.7). You could say that a 300mm lens on a Nikon D90 is roughly the same as using a 11X telescope.. A 1200mm scop...