f stop vs Front Element Diameter

Discussion in 'Digital SLR' started by J. Clarke, Sep 20, 2009.

  1. J. Clarke

    J. Clarke Guest

    I've seen numerous wrangles in which it was asserted that this, that, or the
    other thing MUST be true because the f/stop is _always_ based on the
    _diameter_ of the front element. So I decided to get out the ruler and see
    if the facts supported this argument.

    Methodology was to measure with an engineer's scale, convert to millimeters
    (1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
    round up to 1 decimal place.

    What I found was: (note-list was formatted in Courier New)

    Minolta
    28-56mm f/4-5.6 25mm diameter f/1.2-2.3 based on
    diameter
    56-170mm f/4.5-5.6 35mm diameter f/1.6-4.8 based on
    diameter
    50mm f/3.5 15mm diameter f/3.3 based on
    diameter

    Canon
    10-22mm f/3.5-4.5 50mm diameter f/0.2-0.5 based on
    diameter
    17-85mm f/4-5.6 46mm diameter f/0.4-1.8 based on
    diameter
    24mm f/3.5 43mm diameter f/0.6 based on
    diameter
    50mm f/1.4 36mm diameter f/1.4 based on
    diameter
    100mm f/2.8 44mm diameter f/2.3 based on
    diameter

    Seems that while none of the lenses are _faster_ than what would be
    indicated by the front element diameter, most of them are a good deal
    _slower_.

    Anybody want to add to the list?
     
    J. Clarke, Sep 20, 2009
    #1
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  2. That's simply not true, at least not for modern lens.
    Here's what Canon's own Chuck Westfall said about
    that when we asked for some data about constant
    aperture zoom lenses:
    --
    (o)(o)
    "As for Photoshop", the LORD continued, "thou shalt not allow heavy
    pixelation, for pixelation is detestable. And thee who might be having
    trouble cropping, shouldst thou push aspect ratios greater than 20%
    or so, thou shalt be stoned to death, for that is an abomination."
     
    Guybrush Threepwood, Sep 20, 2009
    #2
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  3. J. Clarke

    Robert Coe Guest

    : I've seen numerous wrangles in which it was asserted that this, that, or
    : the other thing MUST be true because the f/stop is _always_ based on the
    : _diameter_ of the front element. So I decided to get out the ruler and see
    : if the facts supported this argument.
    :
    : Methodology was to measure with an engineer's scale, convert to millimeters
    : (1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
    : round up to 1 decimal place.
    :
    : What I found was: (note-list was formatted in Courier New)
    :
    : [Meaningless measurements omitted]
    :
    : Seems that while none of the lenses are _faster_ than what would be
    : indicated by the front element diameter, most of them are a good deal
    : _slower_.

    That's because the quoted (maximum) f-stop is based on the maximum diameter of
    the aperture that lets in the light. That can't possibly be greater than the
    actual diameter of the lens; but it may be much less, depending on the lens
    design (i.e., of the glass elements and the aperture setting mechanism).

    Bob
     
    Robert Coe, Sep 20, 2009
    #3
  4. J. Clarke

    J. Clarke Guest

    You know that and I know that, but the people who argue on that basis
    apparently do not.

    Yep. So why not get out your ruler and add your lenses to the body of
    physical evidence?
     
    J. Clarke, Sep 20, 2009
    #4
  5. J. Clarke

    Ofnuts Guest

    Especially some long telephoto lenses...
     
    Ofnuts, Sep 20, 2009
    #5
  6. J. Clarke

    Guest Guest

    On Sun, 20 Sep 2009 05:13:11 -0400, "J. Clarke"

    ....

    When I worked professionally many years ago, I often used the
    8X10 camera. The lens was an f8 as I recall and was was about the
    size of a dinner plate. Well maybe not quite.

    Sorry but the formual is a little more scientific than that.
     
    Guest, Sep 20, 2009
    #6
  7. If that is what is being asserted then it is obviously wrong. The
    f-stop is based on the diameter of the pupil and the focal length. The
    diameter of the front element is almost always larger than the pupil
    since, except in unusual cases, the stop sits behind the front element
    which must be large enough to accept all FoVs through the stop.
     
    Kennedy McEwen, Sep 20, 2009
    #7
  8. J. Clarke

    J. Clarke Guest

    But that is not the purpose of the experiment.
     
    J. Clarke, Sep 21, 2009
    #8
  9. J. Clarke

    Miles Bader Guest

    So.... what you want is the _projection_, onto the front element, of the
    wide-open diaphragm?

    It makes a lot of sense that there'd be a bit of extra "unused" space
    around the edges of the front element, to give a margin of error, and
    avoid lower image-quality at the extreme edges (something which seems
    common). [Indeed for the non-zoom lenses, the differences he measured
    seems to be just around right to account for such things, e.g. "f/3.5 vs
    f/3.3"]

    -Miles
     
    Miles Bader, Sep 21, 2009
    #9
  10. J. Clarke

    Guest Guest

    the pentax 50mm f/2 was *not* the same lens as the f/1.7 with an added
    baffle. it had a slightly different optical formula with one more lens
    element (5-5 v. 6-5).
     
    Guest, Sep 21, 2009
    #10
  11. J. Clarke

    Paul Furman Guest

    Thanks for this experiment! I've done that with some old fast primes and
    the numbers were fairly close. Supposedly the stated focal length &
    aperture are subject to (sometimes significant) rounding 'errors'.

    Yep. The word 'telephoto' actually means the lens is designed to not be
    as long as the math implies. I have an old non-telephoto 500mm f/4.5 and
    it's a full 500mm long with a 111mm front element.

    --
    Paul Furman
    www.edgehill.net
    www.baynatives.com

    all google groups messages filtered due to spam
     
    Paul Furman, Sep 23, 2009
    #11
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