DSLR – Zoom and Magnification
I was searching for formula to calculate Zoom ‘x’ for DSLRs, While searching I found this nice and detailed answer regarding this by some person named ‘Jim_T’
Answer was ->
Zoom only describes how much you can change the focal length of a lens. For instance, if you have a 28-135 lens that’s set to 135mm, and you’re too close to get your whole subject in the viewfinder, you can ZOOM OUT to a smaller focal length to make things fit.. It’s common to think zoom is designed to bring things closer.. but you have to consider you can zoom ‘away’ or ‘out’ as well.
When you twist a zoom lens in and out, your subject appears to rush towars you, or away from you… Or in other words, zoom close or zoom far.. Or zoom in and zoom out.. Zoom is a verb that’s used to describe this apparent motion caused by varying the focal length.
The amount a lens can zoom in (or out) is determined by dividing the maximum focal length by the minimum focal length.. 135/28 = 4.82 X (135mm is 4.82 times greater than 28mm)
A fixed lens (ie EF 1200 f/5.6L USM) has NO zoom. That’s right.. 1200mm and you have NO zoom. You have a fixed focal length, you can’t zoom in or out.. (You have to use your feet 
) You’ll never find a fixed focal length lens described as a zoom lens.
If you go to the Canon lens site.. You’ll see they do not call *any* of their prime lenses ‘zoom’ lenses.. http://www.usa.canon.com/eflenses/lineup/index.html
The magnification of a lens gets complicated. It depends on what format they’re designed for.. A 100mm lens for a medium format camera will have less magnification than a 100mm lens designed for a 35mm camera.
Generally, it’s accepted that for the 35mm film format, a 50mm lens approximates the human eye.. (Not exactly, but close). You’ll notice that the 50mm prime lens is described as a ‘Normal’ lens or a ‘Standard’ lens. As focal lengths decrease, you move towards wide angle and as focal lengths increase, you move towards telephoto.
Since 50mm is roughly what we see.. You can get an approximate of lens magnification by dividing by 50. So a 100mm lens would magnify like a 2X telescope. A 300mm lens would be like a 6X telescope.
The 10D/300D have a 1.6 crop factor, so you have to take that into account. 300mm nets the same field of view as a 480mm lens.. So a fixed 300mm lens on a 10D/300D will give you no zoom and 9.6X magnification..
If you have a 15-30mm lens, you have 24-48mm.. Despite the fact the lens has a 2X zoom rating, at the maximum of 48mm, it has no magnification at all..
Zoom and magnification are two different things.
