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Zoom lenses - fixed vs. variable aperture

Discussion in 'Open Discussion' started by ~tc~, May 4, 2011.

  1. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    So, if I understand right, the f/stop is the ratio of the diameter of the opening to the focal length of the lens.

    So, using the Panny 7-14 as an example, the opening of the aperture is the same diameter at 14mm f/4 as it would be for 7mm f/2.

    Which would you rather have - a fixed f/4 (more consistent/predictable) or a variable f/2-4 (faster for most focal lengths)?
     
  2. traveltrader

    traveltrader New to Mu-43

    8
    Apr 14, 2011
    I prefer variable. But I do see the benefits of a fixed aperature. The beercan is prized because it is a fixed aperature at F4. My "professional" video lenses are also fixed.
     
  3. usayit

    usayit Mu-43 Hall of Famer

    Depends on the image quality at varying apertures, size, weight, etc...
     
  4. I had wondered whther constant aperture zooms actually "choke" the aperture at certain focal lengths. I have an old Tokina 28-70 constant f4 where you can actually see this happen, but this is the only lens I have where this is visible. As you move towards 28mm the edges of the aperture blades are exposed and you never get a completely circular aperture wide-open at these focal lengths.
     
  5. Pan Korop

    Pan Korop Mu-43 Veteran

    479
    Mar 31, 2011
    Phare Ouest
    "Choking" the lens at shorter focal length is the usual way for constant f-stop.
    It is more convenient with manual exposure, or artificial light (auto strobes excepted).
    It is a feature working professionals tend to appreciate, but it is "costly" to the marketing people, since you can't boast of a higher max speed.
    It does explain why consumer-oriented zooms have a "floating" aperture, while high end often stick to the fixed.
    Quite obvious when you compare "pro" WA zoom in a range with its more affordable counterpart.
    I tend to prefer the fixed f/ but maybe because I tend to see the glass half-empty, not half-full: to me the Oly 9-18 is an 18/5.6 more than a 9/4.0.
     
  6. I think I similarly tend to treat a variable aperture lens (say f/4-5.6) as a fixed f/5.6 lens, unless in desperation I need a bigger aperture at the shorter focal lengths. For example if I want to stay 1 stop away from wide-open I'd use f/8 as this figure and take this to be true over the full zoom range.
     
  7. mauve

    mauve Mu-43 Top Veteran

    892
    Mar 9, 2010
    Paris, France
    Only in the case the lens is a single convex piece of glass (what I was told were called 'thin' lenses when I was doing optics 101 in highschool - maybe only a french expression). Otherwise, the calculations are much, much, much, more complex. Just take a caliper and divide the diameter of your front lens by the focal : there's very little chance it will match the maximum aperture.

    This number in fact isn't used to refer to physical properties, but is calculated to provide a common comparison ground between vastly different lens types.

    Cheers,

    ps. for example, the Pany 20mm f/1.7 has a front lens diameter of 20 mm, giving a theoretical maximum aperture of 1.0 instead of the true aperture of 1.7 - in fact, there's very little physical relationship between lens diameter and aperture.
     
  8. Pan Korop

    Pan Korop Mu-43 Veteran

    479
    Mar 31, 2011
    Phare Ouest
    Hmm... Try it another, old and proven way: same calipers; extend the lens at arms' length, the front of the lens facing you, the back against some white surface, or light; measure not the front lens diameter, but the entrance pupil i.e. what seems to be the diameter of the wide open iris of the lens (careful with these calipers, rest them on the filter thread!).

    I get:
    For the Pan 20/1.7: 11.3mm. So 20mm/11.3mm= f/1.77
    For the Pan 7-14/4 set at f=14 (easier measure than at f=7mm): 3.3mm. Hence 14mm / 3.3 mm = f/4.24
    For the Pan 45-200/4~5.6 set at f=45 (same reason, for here my arms were too short): 11.3mm. Now 45mm / 11.3 = f/3.98. Hey, close enough for an advertised f/4.
    :wink:

    In order not to complicate the thread, we'll avoid (for now...) :rolleyes: the issue of the T (transmission) scale marked on some cinema lenses. Nor will we indulge in reviewing the Pen-FT scale on the back of them later Pen-F lenses.:tongue:
     
  9. mauve

    mauve Mu-43 Top Veteran

    892
    Mar 9, 2010
    Paris, France
    Absolutely ! This is of course absolutely right, and normal because the pupil is the result of the optical formula. But we're already drifting away, I just wanted to point out that the aperture wasn't really linked to the physical diameter of the lens (but of course, you can't really have a wide aperture in a small diameter, there are physical limits to what opticians can do).

    Cheers,
     
  10. Orientator

    Orientator Mu-43 Regular

    48
    Dec 7, 2010
    The link between the max. aperture and the front lens diameter can be seen rather good in fixed length lenses >50 mm (equivalent)

    e.g. my 135 f2.8 lens has a front glas diameter of 47mm
    or the 50mm f1.8 with 28mm
    or one of my old 80-200mm zooms (f4) with the diameter 49mm

    But to be precise, isn't the f/4 label of the zoom a liar? The front lens diameter should be at least 50 mm!

    Stefan
     
  11. ~tc~

    ~tc~ Mu-43 Hall of Famer

    Oct 22, 2010
    Houston, TX
    By "opening" I meant the aperture opening, not the front lens.
     
  12. mauve

    mauve Mu-43 Top Veteran

    892
    Mar 9, 2010
    Paris, France
    Either case, Pan Korop is right : it's the apparent diameter of the aperture that counts (the pupil), not the real diameter.

    Cheers,