F numbers on lenses

Discussion in 'Digital Discussion & Q&A' started by Timothy, Jun 27, 2009.

  1. Timothy

    Timothy TPF Noob!

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    i was just wondering if someone would be able to let me know what the f numbers mean on lenses, such as: 18-55mm f/3.5-5.6

    i am aware that it relates to the apatures, but not sure in which way.

    if you can oly shoot between those apatures, or for best results.

    any way, Thanks in advance


    Timothy
     
  2. Josh66

    Josh66 Been spending a lot of time on here!

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    It is the maximum aperture that lens is capable of producing.

    When it shows two apertures (like your example), the smaller one is the max aperture at the wide end, and the larger one is the max aperture at the long end. You only see that (two apertures listed) on zoom lenses.
     
  3. Dwig

    Dwig TPF Noob!

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    As an additional note, in years past it was conventional to mark the maximum aperture as a ratio rather than as a fraction or just the denominator of the fraction. On older lenses you would see "1:1.8" instead of either "f/1.8" (fraction) or "1.8" (denominator only). No matter which way its marked, the meaning is the same.
     
  4. Josh66

    Josh66 Been spending a lot of time on here!

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    Canon still does it this way. At least, that's how all of my lenses are marked (ratio).
     
  5. Andrew Boyd

    Andrew Boyd TPF Noob!

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    Sounds like you've got some typical confusion about aperture numbers and what it all means...this is one of the most difficult things for beginning photographers to get straight--the relationship between aperture, shutter speed and ISO.
    I've got an article on my blog that explains all of this that you might want to check out:
    The Discerning Photographer - Essays, Tutorials and How-To Tips

    Look under the "Featured Categories" on the left and you'll see the story.

    Good luck!
     
  6. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Just to try and explain things a bit more.

    When the lens lists 2 numbers it means the maximum aperture the lens is capable of is variable as a function of focal length.
    In other words at 18mm the maximum aperture is f/3.5. As you zoom to 55 mm the aperture gets smaller such that once you get to 55 mm the aperture is now at the new smaller maximum of f/5.6.
    So, at 55mm you can use from f/5.6 down to the smallest aperture for that lens, say f/16 or f/22. Conversely at 18mm you can use apertures from f/3.5 down to the same smallest f/16 or f/22.

    When a zoom lens has only 1 number like f/2.8 the maximum aperture stays constant if you zoom. So, you can use f/2.8 down to the smallest aperture of the lens regardless the focal length you are zoomed to.

    Remember, the larger the number the smaller the lens opening (aperture).
     

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