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Old Apr 15, 2008, 3:34 PM   #1
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Hi all, could someone explain in plain English what all the numbers mean regarding lenses.
For example what is the difference between a lens with the
canon 40D 17-85mm IS USM Lens and a lens with 18-55mm f/3.5-5.6 IS Lens
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Old Apr 15, 2008, 3:51 PM   #2
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17-85mm and 18-55mm refer to the focal length of the lens. In practical terms this is an indication of the view you would see when looking through the viewfinder.

http://www.tamron.com/lenses/learnin...comparison.php provides an easy to use way to see what view you will get from various focal lengths.

In your example the 18-55mm covers less view than the 17-85mm.

f/3.5-5.6 refers to the aperture of the lens or how much light gets to the sensor. The smaller the number, such as f/2.8 or f/1.4 the more light gets through. This allows you to use a faster shutter speed if required to stop action. You also use the aperture as a creative tool to control how much of the scene is in focus. We call this depth of field.
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Old Apr 16, 2008, 2:27 AM   #3
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Bob Nichol wrote:
Quote:
17-85mm and 18-55mm refer to the focal length of the lens. In practical terms this is an indication of the view you would see when looking through the viewfinder.

So if I was to make a choice between these two focal lengths , am I correct in assuming that I should go for the larger length lens for general photography ?




f/3.5-5.6 refers to the aperture
I completely get that part
Also, on my bridge camera I have a optical zoom, am I correct in saying that SLR's don't have optical zooms as such, the zoom is in the lens?
Many thanks for your reply, I will check out that link now :-)
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Old Apr 16, 2008, 5:13 AM   #4
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The term optical zoom refers to the longest focal length divided by the shortest. For example an 18-55 lens is typically referred to as a 3x optical zoom. So technically, if you are talking about a zoom dslr lens, yes they do have optical zoom. It's just that DSLR lenses are not characterized by their zoom factor. Even a prime lens (single focal length) could be referred to as a 1x lens. "X" factor is really a useless term created by marketers. For example, an 18-55 and a 100-300 lens are both 3x lenses...however they are obviously very different...the 18-55 is a slighlty wide to very short telephoto, while the 100-300 is a short tele to moderat tele. Just pay attention to focal lengths when looking at lenses rather than "x" factor. Shorter focal lengths are wider angle, longer a lengths give you more reach..

As for which lens to use for general photography, that depends on what you like to shoot. Start with one of the basic kit lenses (18-55, 17-85 etc) and find out how you are limited by that lens. More range is more convenient (less lens changes, less likely to have the wrong lens), but you give up some in image quality and speed with the longer range.
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