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Old Sep 30, 2010, 9:21 AM   #1
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Default Question about long zoom telephoto lenses

I've read a lot about DSLR lenses about 75-300mm or 135-300mm lenses.

However unlike a bridge camera with a super zoom where it often states the times of zoom (like fz100 is 24x, sx30is is 36x), I'm curious, how many times zoom those 300mm lenses can provide?
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Old Sep 30, 2010, 9:40 AM   #2
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The 'X' ratio is simply the longest focal length, divided by the shortest focal length, so a 75-300mm lens would be 4X. Of course, it starts at a longer focal lenght than most of the superzooms, so at the 300mm setting it is a considerable telephoto.
Many cameras don't specify the actual focal length of their lenses, so it is difficult to compare unless you look it up in a review. Usually, focal lengths are given in 35mm equivalents, because so many of us have experience with 35mm cameras. In those terms, 28-35mm is considered wide angle, 45-60mm is called normal, 75-135mm is mild telephoto, and 200mm and greater is telephoto. Lenses shorter than 28mm(eq) are either specialty wide angle or fisheye lenses. Those over 300mm(eq) would be considered super telephoto.
The definitions are imprecise, and mostly my own opinion, so anyone who disagrees is welcome to their own.

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Old Sep 30, 2010, 11:13 AM   #3
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As VTphotog has said, the Zoom (X) number is simply the ratio of the shortest focal length to the longest. That's all that matters with P&S digicams because they only have one lens. With cameras that have interchangeable lenses, that number doesn't mean anything. For instance a 100-300mm lens is a 3X zoom, but so is an 18-55mm lens, but their angles of view are very different.

You're trying to take one thing you learned about shooting with a P&S and relate it to dSLRs and it doesn't work the same way.

If you want to be able to relate what you get with a superzoom to what you get with an APS-C dSLR, you need to use the "35mm equivalent focal length". For instance, your superzoom may have a maximum 35mm equivalent focal length of 432mm, and when you put a 300mm lens on an APS-C dSLR, it will have a 35mm equivalent focal length of 450mm. This is cumbersome, but it's the only way to associate the angle of view you get from one size image sensor with the angle of view you get with a different size image sensor.
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Old Sep 30, 2010, 11:21 AM   #4
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Nice, succinct reply Brian and pretty much matches my wide, normal, tele, and super-tele definitions.

I suspect the reasons this question comes up so frequently is that normal consumers equate zoom ratio as spec'd on P&S cameras (simple focal length ratio as you explained) to visual magnification as spec'd on binoculars and terrestrial telescopes which is apparent image magnification compared to the naked eye.

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