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Lens front element diameter - Photography

Why do different lenses have differing front element diameter? Is a lens with a bigger front element inherently superior than one with a smaller front element? Does a 77mm capture more light than a 58mm? - Siddhartha...

  1. #1

    Default Lens front element diameter

    Why do different lenses have differing front element diameter? Is a
    lens with a bigger front element inherently superior than one with a
    smaller front element? Does a 77mm capture more light than a 58mm?
    - Siddhartha

    Siddhartha Guest

  2. #2

    Default Re: Lens front element diameter

    Siddhartha Jain wrote: 

    A number of reasons. Some lens designs need a larger front element.
    Generally wide angle, zoom and larger aperture lenses call for larger front
    elements. Another issue is the use of filters. Often using a larger size
    filter mount will allow the use of stacked filters where a smaller mount
    would not.

    --
    Joseph Meehan

    26 + 6 = 1 It's Irish Math


    Joseph Guest

  3. #3

    Default Re: Lens front element diameter

    Do not confuse filter diameter with front element diameter, which you do not
    know in general.


    Dps Guest

  4. #4

    Default Re: Lens front element diameter

    Dps wrote: 
    do not 

    Yes, I think I got the two mixed up and assumed that filter diameter ==
    front element diameter!!

    My bad ...

    - Siddhartha

    Siddhartha Guest

  5. #5

    Default Re: Lens front element diameter

    "Siddhartha Jain" <co.uk> wrote in message
    news:googlegroups.com... 
    > do not 
    >
    > Yes, I think I got the two mixed up and assumed that filter diameter ==
    > front element diameter!!
    >[/ref]
    Front diameter can mean a faster lens in terms of f:stop. Let's assume the
    lens was a 200mm with a 58mm diameter. 200/58=3.44. There will be some type
    of bezel so the glass will be a bit smaller so this could be a 200mm f:4
    lens. Now the same 200mm with a 77mm diameter 200/77=2.59 this could be a
    200mm f:2.8. This is using the very basic math, lens designers may decide to
    use a 82mm element and a baffle to mask of the edge to correct for edge
    distortions. This is a simple explanation, the actual optical designs are
    more complex.\



    Darrell Guest

  6. #6

    Default Re: Lens front element diameter

    On 31 Jan 2005 03:30:08 -0800, "Siddhartha Jain"
    <co.uk> wrote:
     


    The front element has nothing to
    do with the lens' max aperture.
    Nor does it say much about the
    quality of the lens.

    I have a 90mm f/8 lens (a wide-
    angle for 4x5 format) with a
    huge front element -- at least
    45 or 50 mm diameter. By that
    measure it should be an f/2 lens!

    Similarly: a Canon zoom (for my
    10D) rated 17-40/f4, with a front
    element that's at least 60 mm in
    diameter.

    There are a zillion different
    lens designs. Wide angle lenses
    tend to have large front elements.


    rafe b.
    http://www.terrapinphoto.com
    rafe Guest

  7. #7

    Default Re: Lens front element diameter

    rafe bustin wrote: 

    No no, I did not confuse front element diameter with aperture.
    - Siddhartha

    Siddhartha Guest

  8. #8

    Default Re: Lens front element diameter

    rafe bustin wrote:
     
    >
    >
    >
    > The front element has nothing to
    > do with the lens' max aperture.
    > Nor does it say much about the
    > quality of the lens.
    >
    > I have a 90mm f/8 lens (a wide-
    > angle for 4x5 format) with a
    > huge front element -- at least
    > 45 or 50 mm diameter. By that
    > measure it should be an f/2 lens!
    >
    > Similarly: a Canon zoom (for my
    > 10D) rated 17-40/f4, with a front
    > element that's at least 60 mm in
    > diameter.
    >
    > There are a zillion different
    > lens designs. Wide angle lenses
    > tend to have large front elements.
    >
    >
    > rafe b.
    > http://www.terrapinphoto.com[/ref]

    Not only do wide angle ones, but so do low f/# (fast) lenses. Minimum
    diameter of front element is primarily determined by these two factors
    (f/# and fov). Also, distance from front element to front principal
    point is a factor. Grinding glass is expensive, so no one wants
    elements to be bigger than they have to be. But generally you do not
    want front element to act as an aperture stop, nor do you want
    vignetting. So there is a certain minimum diameter that the lens has to
    be. Modern lens design software takes care of determining that, however.
    Don Guest

  9. #9

    Default Re: Lens front element diameter

    In article <com>,
    rafe bustin <net> wrote: 
    >
    >
    >The front element has nothing to
    >do with the lens' max aperture.[/ref]

    That's not strictly true - it does place an upper limit on the maximum
    aperture. If you have a 50mm lens with a front element that's, say. 25mm
    across, it can't possibly be an f/1.4 lens, for example.

    As you point out though, it isn't necessarily an f/2 lens either.
    Chris Guest

  10. #10

    Default Re: Lens front element diameter

    Siddhartha Jain wrote:
     

    You've gotten several replies. I'll add to that that often a lens will have a
    greater than absolutely neccesary filter diameter so that the photographer can
    use a given set of filters. I have 3 lenses that are 72mm. I'm sure that for
    the 80-200 they could have used a smaller filter size. However, as one who owns
    an 80-200 f/2.8 is likely to own a 28-70 f/2.8, they've conscientiously made the
    dias the same. 9 current lenses by Minolta are 72mm.

    --
    -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
    -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
    -- [SI] gallery & rulz: http://www.pbase.com/shootin
    -- e-meil: there's no such thing as a FreeLunch.
    Alan Guest

  11. #11

    Default Re: Lens front element diameter


    "Siddhartha Jain" <co.uk> wrote in message
    news:googlegroups.com... 


    All things being equal (which, of course, they never are!), given two lenses
    of identical quality and similar focal lengths, I'd think the larger lens
    would have less image-plane distortion _for_ a_given_exposure_, since more
    of the refracted light reaching the image plane through the larger lens
    would pass closer to the optic axis, i.e., the most accurate part of the
    lens.

    Of course, the choice consumers probably face is deciding between cameras
    having smaller lenses of high quality and cameras with larger lenses of
    somewhat lesser quality. In such cases, it's a wash, I suppose.




    Paul Guest

  12. #12

    Default Re: Lens front element diameter

    Paul H. wrote: 

    The term 'distortion' has a specific meaning to a lens designer or
    optical engineer. I think you mean aberrations here. Many lens
    aberrations are a function of the diameter of the lens (aperture
    diameter). There are many aberrations which increase rapidly with
    aperture diameter. About the only 'aberration' that increased diameter
    reduces is diffraction blur. Spherical aberration, coma, astigmatism,
    etc. All increase with diameter.
    Don Guest

  13. #13

    Default Re: Lens front element diameter


    "Siddhartha Jain" <co.uk> wrote in message
    news:googlegroups.com... 

    I think it has a lot to do with the zoom and being a wide angle. On the
    other hand, anybody remember the good old days when almost all Nikon lenses
    used 52mm filters? I have a 28mm 3.5, an 85mm 1.8, and an 80-200mm 4.5 zoom
    that all use 52mm filters. Go figure.


    Sheldon Guest

  14. #14

    Default Re: Lens front element diameter

    Don Stauffer in Minneapolis wrote: 
    >
    > The term 'distortion' has a specific meaning to a lens designer or
    > optical engineer. I think you mean aberrations here. Many lens
    > aberrations are a function of the diameter of the lens (aperture
    > diameter). There are many aberrations which increase rapidly with
    > aperture diameter. About the only 'aberration' that increased diameter
    > reduces is diffraction blur. Spherical aberration, coma, astigmatism,
    > etc. All increase with diameter.[/ref]

    To me "distortion" (wrt a lens) means that caused by the basic optical
    mechanics resulting in pincushion or barrel distortion. Hence, as Paul
    said a larger diameter lens design for focal-length/aperture is more
    likely to be closer to flat. "Aberation" would include the others you
    mentioned.

    eg: distortion is geometric and aberation is most other optical shortcomigs.

    I may be wrong of course, but that's my take on distortion v. aberation.

    Cheers,
    Alan

    --
    -- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
    -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
    -- [SI] gallery & rulz: http://www.pbase.com/shootin
    -- e-meil: there's no such thing as a FreeLunch.
    Alan Guest

  15. #15

    Default Re: Lens front element diameter

    Sheldon wrote:
     
    >
    >
    > I think it has a lot to do with the zoom and being a wide angle. On the
    > other hand, anybody remember the good old days when almost all Nikon lenses
    > used 52mm filters? I have a 28mm 3.5, an 85mm 1.8, and an 80-200mm 4.5 zoom
    > that all use 52mm filters. Go figure.
    >
    >[/ref]

    In addition it depends on the length of the lens, and the distance from
    the front element to the aperture stop plane. The further from the
    aperture stop the front element front surface is, the larger it needs to
    be for a given FOV.
    Don Guest

  16. #16

    Default Re: Lens front element diameter


    "Don Stauffer in Minneapolis" <net> wrote in message
    news:net... [/ref]
    lenses [/ref]
    lens [/ref]
    more [/ref]
    cameras 
    >
    > The term 'distortion' has a specific meaning to a lens designer or
    > optical engineer. I think you mean aberrations here. Many lens
    > aberrations are a function of the diameter of the lens (aperture
    > diameter). There are many aberrations which increase rapidly with
    > aperture diameter. About the only 'aberration' that increased diameter
    > reduces is diffraction blur. Spherical aberration, coma, astigmatism,
    > etc. All increase with diameter.[/ref]

    True, but I think you're reading more into my comment than I intended to
    convey. If you have a ray of light approaching a spherical refracting
    surface of radius R parallel to and at distance H from the optic axis,
    Snell's law obtains: n1*sin(A)=n2*sin(B) where (n1, A) is the (index of
    refraction, angle to the normal in air and (n2, B) the corresponding
    characteristics of the refracting medium.

    sin (A)= H/R, so sin(B)= (n1/n2)*H/R, or for sufficiently small H,

    B=(n1*n2)*H/R

    The quality of the image at the image plane will depend greatly on the
    accuracy and consistency of exit angle B. The uncertainty in B, dB, is then
    roughly

    dB = -dR* (n1*n2)*H/ (R^2),

    i.e., it varies inversely as the *square* of the radius of curvature. By
    the way, I do know that "dB" usually denotes decibels--it's just a
    coincidence here, though.

    Furthermore, having ground a couple of telescope mirrors in my mispent
    youth, I can testify that larger radii of curvatures are easier to implement
    than smaller ones and also that grinding accuracy falls off precipitously
    the her one gets from the optic axis. Hence a portion of a larger, less
    curved lens would be cheaper to produce than a better engineered-smaller
    lens to achieve the same light gathering ability within a certain error
    range. Or so my thinking went.

    That's all I meant. If I had known there was going to be a test, I would
    have studied. :-)





    Paul Guest

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