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03-06-2010, 04:04 PM   #1
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f stop problem

ok so i have the k-x body lens which is a DA L 18-55mm F3.5-F5.6

now the f-stop gets bigger then 5.6 but wont go lower then 4.0 how do i get to F3.5?

03-06-2010, 04:11 PM   #2
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Zoom out to 18mm. The maximum aperture at 55mm is f/ 5.6.
03-06-2010, 04:12 PM   #3
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The lens will only go to 3.5 on 18mm As you zoom the max aperture will keep getting smaller until it's 5.6 at 55mm.
03-06-2010, 09:04 PM   #4
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f-stops are fractions

Well, f/5.6 isn't bigger, and f/4.0 isn't lower. These are fractions. At f/4, the aperture opening distance is 1/4 the focal length. At f/8, it's 1/8 the focal length. And 1/4 is bigger than f/8, right?

What's happening is, when you push the lens down to 18mm (the shortest focal length, a moderate wide-angle), the aperture can open as far as f/3.5. When you pull the lens out to 55mm (the longest focal length, a portrait telephoto), the furthest the aperture can go is f/5.6, which is a smaller opening. So you can get an aperture almost 1/3 the focal length at one end, which lets in more light than the other end, where you can only open up to around 1/6 the focal length.

If you were in a situation where you wanted to stop-down (close the aperture) to its smallest setting, you'd find something similar. At 18mm, the smallest f-stop (aperture setting) you can get is f/22. At 55mm, the smallest f-stop is f/38. There are reasons NOT to be at the smallest f-stop, but don't worry about that now.

Anyway, this is just how this zoom lens (and many others) are designed. It's economical; it's confusing at first; but you'll get used to it. Now go shoot something!!

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