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Thread: What determines diameter size of lens?

  1. #1
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    Default What determines diameter size of lens?

    I've been wondering for some time now, what determines the diameter size of lens?
    Is it the focal length, glass elements, or what?
    My lens diameter is huge and I dun like it that way cos then filters are VERY EXPENSIVE!
    Also makes the lens bulky.
    The best photographer is one who is inspired by the innate nature of his subjects.

  2. #2

    Default Re: What determines diameter size of lens?

    Its mainly aperture and the lens construction itself. Manufacturers also sometimes make a lens range have the same diameter for professionals and users to have easy filter interchangability. Generally, today's auto-focus lenses in the professional range like the f/2.8s have 77mm over all of the zooms like 16-35mm, 24-70mm, 70-200mm etc... In the past, Nikon's prime lenses had a standard 52mm filter size for as many as they could make it for. Large aperture lenses need large front elements, some front elements are so big, there isn't much commercially avaliable filters for it like say the Canon EF 300mm f/2.8 or 400mm f/2.8.
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  3. #3

    Default Re: What determines diameter size of lens?

    Basically, the MINIMUM diameter is max range/biggest aperture.

    So a 50mm f1.4 will have a minimum lens diameter of 35.7 mm.

    A 300mm f2.8 will have a minimum diameter of 107.14 mm

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    Default Re: What determines diameter size of lens?

    longest focal length / biggest aperture = minimum front element size

    most 70-200 have 77mm front elements
    200mm / f2.8 = 71.4mm
    minimum is 71.4mm.. but to adhere to industry standards, they choose 77mm since 71.4 is the minimum size
    Last edited by yanyewkay; 17th September 2007 at 02:58 PM.
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  5. #5
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    Default Re: What determines diameter size of lens?

    Quote Originally Posted by yanyewkay View Post
    longest focal length / biggest aperture = minimum front element size

    most 70-200 have 77mm front elements
    200mm / f2.8 = 71.4mm
    minimum is 71.4mm.. but to adhere to industry standards, they choose 77mm since 71.4 is the minimum size
    Correct me if I am wrong..
    The focal length / biggest aperture formula only applies for 200mm and above.. (Canon 200mm f2.8 = 72mm diameter, wider lens with large aperture also have large diameter, eg 24mm f1.4 = 77 diameter)
    There is another formula when lens with short focal length kicks in, not sure of the formula myself. Maybe someone else can provides ?
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  6. #6

    Default Re: What determines diameter size of lens?

    Quote Originally Posted by majere2sg View Post
    Correct me if I am wrong..
    The focal length / biggest aperture formula only applies for 200mm and above.. (Canon 200mm f2.8 = 72mm diameter, wider lens with large aperture also have large diameter, eg 24mm f1.4 = 77 diameter)
    There is another formula when lens with short focal length kicks in, not sure of the formula myself. Maybe someone else can provides ?
    nope... Still applies from what I can see. Take a look at the 50mm lenses, etc. We're just referring to minimum diameter of the lens; if the lens construction or light diffraction limits require wider glass, they'll use wider glass.

    But in pure theory, a 24mm f1.7 only needs to be a minimum of 17.14mm wide. Doesn't mean the image will be useable.
    Last edited by Rashkae; 19th September 2007 at 09:01 PM.

  7. #7

    Default Re: What determines diameter size of lens?

    Quote Originally Posted by majere2sg View Post
    Correct me if I am wrong..
    The focal length / biggest aperture formula only applies for 200mm and above.. (Canon 200mm f2.8 = 72mm diameter, wider lens with large aperture also have large diameter, eg 24mm f1.4 = 77 diameter)
    There is another formula when lens with short focal length kicks in, not sure of the formula myself. Maybe someone else can provides ?
    The front diameter applies for all tele lenses including medium teles.

    For wide angles, because retrofocus (reverse tele) is used, the large front element is to cover the angle and the rear elements are to project the rays to the sensor. Each element has an aperture but it's the element with the longest focal length which dictates the aperture.
    Last edited by lsisaxon; 20th September 2007 at 12:51 PM.

  8. #8
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    Default Re: What determines diameter size of lens?

    Quote Originally Posted by lsisaxon View Post
    The front diameter applies for all tele lenses including medium teles.

    For wide angles, because retrofocus (reverse tele) is used, the large front element is to cover the angle and the rear elements are to project the rays to the sensor. Each element has an aperture but it's the element with the longest focal length which dictates the aperture.
    Hmm... is there a formula for wide-angle lens then?
    My lens is 17-40 f/4, diameter 77mm! So i'm very doubtful the same formula for medium-tele lens applies.
    The best photographer is one who is inspired by the innate nature of his subjects.

  9. #9
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    Default Re: What determines diameter size of lens?

    Actually the formula applies to ALL focal lengths: it stipulates the MINIMUM diameter, and a 77mm diameter for a 24/1.4 certain "obeys" the rule.

  10. #10

    Default Re: What determines diameter size of lens?

    Quote Originally Posted by ST1100 View Post
    Actually the formula applies to ALL focal lengths: it stipulates the MINIMUM diameter, and a 77mm diameter for a 24/1.4 certain "obeys" the rule.
    Yes. I see what you mean.

  11. #11

    Default Re: What determines diameter size of lens?

    Quote Originally Posted by WuffRuff View Post
    Hmm... is there a formula for wide-angle lens then?
    My lens is 17-40 f/4, diameter 77mm! So i'm very doubtful the same formula for medium-tele lens applies.
    Like what ST1100 mentioned, yes, it still obeys the MINIMUM diameter required is (focal length / smallest f-number) 77mm > 10mm (40/4).
    Last edited by lsisaxon; 21st September 2007 at 12:50 AM.

  12. #12
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    Default Re: What determines diameter size of lens?

    Quote Originally Posted by WuffRuff View Post
    Hmm... is there a formula for wide-angle lens then?
    My lens is 17-40 f/4, diameter 77mm! So i'm very doubtful the same formula for medium-tele lens applies.
    You won't want to buy a L lense with only a 10mm front element would you
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