Stupid question?

Discussion in 'Beyond the Basics' started by Jimmy The Bat, Feb 16, 2010.

  1. Jimmy The Bat

    Jimmy The Bat TPF Noob!

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    I'm sure this is a stupid question but i dont understand,Its about f stops on lenses.I have an 18-55mm f/3.5-5.6 lens, I understand that at 18mm it is f/3.5 and at 55mm it is f/5.6. But my question is I can adjust my command dial on the camera to f/22. the largest aperture is f/3.5, but is the smallest f/5.6, or can I go to f/22? I hope this is understandable,I had a hard time trying to explain what I'm asking. anyway,thanks in advance for any help.
     
  2. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    The largest aperture is a tricky thing because its backwards:

    At 18mm your largest aperture is f3.5 - your smallest f22
    At 55m your largest aperture is f5.6 - your smallest f36

    The blades themselves won't actually adjust, but the act of zooming changes the properties of the lens - reducing light entering the setup and creating the same situation as if the blades had been closed.
     
    Last edited: Feb 16, 2010
  3. Dwig

    Dwig TPF Noob!

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    Incorrect as stated since you failed to define "it". This lack of definition is, perhaps, a big part of your confusion.

    Correct statement would be "... at 18mm the maximum aperture is f/3.5 and at 55mm the maximum aperture is f/5.6...".

    Neither of these apertures give any lead as to what the minimum aperture is. Your command dial does not "go to f/22". The minimum aperture can be a limit set by the lens. You may find that the lens limits the minimum aperture to the same f/stop regardless of focal length and you may find that the minimum aperture varies with focal length.
     
  4. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    *tests this*
    ahh you're right at the 55mm end the lense does change the minimum aperture as well - to f36 (at least on my canon 18-55mm I suspect nikon performs similar). my reply above edited now to reflect this

    I have to admit though that I never go that small with the aperture - ever. Past f13 diffraction starts to become a problem and that will result in softer images by far no matter how well you shoot (diffraction being a factor in lens construction).
     
  5. Jimmy The Bat

    Jimmy The Bat TPF Noob!

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    thank you so much guys, You really cleared this up for me, Overead, I never realized that the 55mm lets you close the aperture to f/36, Just tried it on my Nikon. Anyway thanks again for the help.
     

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