hey guys, i've had a question on my mind for a while and i tried looking for the answer but didnt come up to anything solid.
when dealing with SLR lenses we choose ranges for the lenses 1855mm, 70300mm, 50mm etc.... what i would like to know is how does one go about converting those values in the magnification factor that you see on the compact cameras. sorry if this is such a newbie or stupid question but its just been bugging me :p. what i gathered from looking at the sony R1 and canon 350D is that the R1 has a max zoom of 5x at 120mm (35mm equivalent) so would that makea lens like the 70300mm (480mm equivalent)give a 20x magnification factor on the 350D?or am i going about it all wrong?! 
Tyrant wrote:
Quote:
The 'times' factor you see on fixed lens cameras is thenumber of times the shortest focal length (widest angle) will go into the longest. So, the Sony R1 has a lens that starts at 24mm and zooms to 120mm. Which is 5 times. A Dslr lens that states it is 70mm300mm is actually 4.2X. Not 12.5X by your calculations as the times factor here is based on the shortest focal length of 70mm. Why you thought it was 20x is because most Dslr cameras do not have a full frame sensor, that is, 35mm. So the majority have what is known as a 'crop factor'. The example you gave of a 70300mm (480mm equivalent), is because the Canon Dslr cameras have a crop factor of 1.6x. So, 300mm X 1.6 = 480mm. It is still a 4.2x lens as the 70mm becomes 112mm with the crop factor. There is a little more to this crop factor issue, but this is in simple terms so you understand the difference and why the figures are usually also quoted as 35mm equivalent But of course this doesn't make the Sony's 5x zoom the longest as it is only 120mm at the longest end compared to 300mm. You need a shorter focal length to take 'wide angle' shots, so this will be of benefit. But when it comes to 'reaching' your subject, then the 70300 will by far and away outdo the Sony. Simply looking at the X factor will not tell you if a given lens is longer or shorter than any other. You have to look at the shortest and longest focal lengths. 
Stevekin thanks for your reply :). it was very informative and cleared up quite a few things :).

You're welcome Tony.
As I said, there is more to the crop factor, or focal length multiplier, to give it the correct terminology. This site will help to understand better. Steve. 
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