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Old Jun 24, 2009, 9:51 AM   #1
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Default K2000/Km aperture question


I apoligize if this seems like an stupid question but still trying to get used to doing my own settings with DSLR. If I'm using the lens that came with my K2000 (18-55mm F3.5-5.6) in manual mode, why does the camera only allow a low end F of F5.6? Is this pertaining to the camera or the lens itself? I want to get better at doing my own settings and need alittle education on this! Thanks in advance!
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Old Jun 24, 2009, 10:19 AM   #2
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When you are at 55mm the widest aperture is f5.6 you can only get f3.5 at the wide end so zoom out and you will be able to select a wider aperture, however as soon as you zoom it will change. This causes problems with full manual settings as it will mean you exposure changes when zooming if you start wide with f3.5.

Hope that helps.
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Old Jun 24, 2009, 10:20 AM   #3
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The 3.5-5.6 designation is the maximum aperture value available at the widest to longest zoom. So, with your lens at 18mm, 3.5 is the max aperture available. As you zoom out, that max possible value will decrease to 5.6. By the time you are at 55mm, f5.6 is the maximum value the lens is capable of. At ANY focal length, you can always select a narrower aperture. So, f8,f11,f16 etc are available at all focal lengths. It's just the widest aperture (lowest f-number) that changes as the lens zooms. More expensive lenses will often retain their maximum aperture throughout the entire focal range. I say more expensive because it requires better optics and more complicated design to retain a same maximum aperture throughout the zoom.
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Old Jun 24, 2009, 10:21 AM   #4
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LOL, beat you John!
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Old Jun 24, 2009, 10:28 AM   #5
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The lens that your using sets the minimum aperature based on the focal length it is set to. So for the 18-55mm F3.5-5.6 lens at the 18mm focal length the minimum aperature is f3.5 and is progressively increases with the focal length to f5.6 when the lens is zoomed to 55mm.

Its all based on the optical design of the lens. There are two types of zoom lenses in terms of aperature design. One is a constant aperature. For instance the 16-45mm f4 lens, has a minimum aperature of f4 through out its entire focal length range. The other is the variable aperature that varies with the focal length.

So its nothing that has anything to do with your camera. It all has to do with the design and physical construction of the lens, as to how large the aperature of the lens is able to open at the desired focal length. Here is an experiment. Zoom out to 18mm and the minimum aperature is f3.5, then zoom to 55mm and the aperature will change to f5.6.

I went to the kitchen to get some breakfast - a bowl of cereal while I was replying and ......

Last edited by interested_observer; Jun 24, 2009 at 10:32 AM.
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