Normal Lens And Its Smallest Aperture

Discussion in '35mm Cameras' started by Steven Woody, Jul 18, 2005.

  1. Steven Woody

    Steven Woody Guest

    i noticed, most 50mm lens (for 35mm film) get a smallest aperture of f11
    only. why equip a f22? i think its highly desired. i believe that there must
    be a reason and i want to know it.

    -
    narke
     
    Steven Woody, Jul 18, 2005
    #1
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  2. This is the result of a typical unavoidable engineering trade-off
    between maximum aperture, cost, and lens barrel size. A 50mm f/1.4 has
    to have a clear opening with the aperture blades retracted of 36mm.
    The outside of the blades has to be within the lens barrel. To close
    down to f/11 the blades have to stop down to 4.5mm while still covering
    the 36mm field stop with their outer edges. This defines the minimum
    barrel diameter for a given number of blades. You also need a
    symmetric aperture so each blade needs to move in the same amount. To
    get to f/22 the aperture is 2.3mm. This requires larger blades and a
    larger lens barrel or more blades with added cost and complexity. To
    make a nice hexagonal (or heptagonal etc.) aperture at 2.3mm the blade
    movement has to be controlled twice as well as at f/11 meaning the
    build quality and mounting/cam accuracy has to be twice as good for the
    same aperture symmetry. More bulk, weight, and money. By the same
    token, it's pretty easy to get a 55mm f/2.8 macro lens to stop down to
    f/32 since the outer blade edges only have to cover a 20mm field stop
    and there's plenty of space for blades that will cover 20mm stopped
    down to f/32. There's also plenty of room outside the 20mm field for
    the blade support / guide mechanism to make a clean symmetric f/32
    aperture opening.
     
    bob.kirkpatrick, Jul 18, 2005
    #2
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  3. Most high-speed 50mm lenses stop down to f/16. Smaller apertures give
    too much diffratcion.
     
    uraniumcommittee, Jul 18, 2005
    #3
  4. Most high-speed 50mm lenses stop down to f/16. Smaller apertures give
    too much diffraction.
     
    uraniumcommittee, Jul 18, 2005
    #4
  5. Maybe it's trying to fit all the autofocusing equipment into the lens
    barrel? My Pentax SMC-A 50mm f1.4 has 8 blades which go from f1.4 to f22,
    and the new one's exactly the same, so is the 50mm f.1.7, so I guess you
    must be talking about different makes (maybe ones with motors in the lens).
    Duncan.
     
    Duncan J Murray, Jul 18, 2005
    #5
  6. Steven Woody

    Paul Furman Guest


    My Nikon 45mm 2.8P manual focus goes down to 22, it has a very curious
    aperture blade configuration, the tips narrow down like fingers and they
    spill back up around each other making a sort of flower when stopped
    down. The reason may be to maintain a rounded aperture rather than
    polygonal. It is an extremely small lense also.
     
    Paul Furman, Jul 18, 2005
    #6
  7. I have never seen a 50mm lens for a 35mm camera that only closes down to
    f/11. Most of them go to f/16.

    It is worth pointing out that when you stop down beyond f/16, you start
    losing resolution due to the Dawes limit. Interestingly, this is true
    regardless of the focal length of the lens -- so if you have, say, a 300mm
    lens that stops down to f/32, you will find the images considerably less
    sharp at f/32 than at f/11 (unless it's a crummy lens to begin with).
     
    Andrew Koenig, Jul 18, 2005
    #7
  8. Steven Woody

    Steven Woody Guest


    yes, i did mean f/16, that's a typo.

    -
    narke
     
    Steven Woody, Jul 19, 2005
    #8
  9. Steven Woody

    Steven Woody Guest

    thanks, very clear!

    -
    narke
     
    Steven Woody, Jul 19, 2005
    #9
  10. Steven Woody

    Steven Woody Guest

    i am talking about Carl Zeiss T* Plannar f/1.4 50mm lens, which is a manual one
    and has 6 blades.

    i think Bob did a good explain, more blades you can get more smaller apperture.

    -
    narke
     
    Steven Woody, Jul 19, 2005
    #10
  11. Steven Woody

    Steven Woody Guest

    yes, it's a typo, should be f/16.

    i dont desire f/32, i just dream for a f/22, which could gives me more DOF :)

    -
    narke
     
    Steven Woody, Jul 19, 2005
    #11
  12. Steven Woody

    Steven Woody Guest

    if it was a f1.4, i think you will not get f22 in the same complexity. -- Bod
    pointed this.
     
    Steven Woody, Jul 19, 2005
    #12
  13. Steven Woody

    Bandicoot Guest

    Bob K has answered the why, so I'm just posting to note that all my Pentax
    50mm lenses go to f22 (including the f1.4 and f1.2), except the later design
    macros, which go to f32.

    As Bob has said, it's easier to get smaller minimum apertures on slower
    lenses, so f32 on the f2.8 macros is less surprising than the f22 on all the
    others. I think there's also a presumption amongst many buyers that that
    small aperture is especially useful in a macro for the DoF, though in fact
    diffraction may mean it is best used sparingly...

    I was interested to see your post, being used to my Pentax 50mm lenses with
    their f22 minimum apertures and very nice round diaphragm openings - and as
    a child even my old Soviet Jupiter 58mm had an f16 minimum. So I was
    surprised to hear that other lens makers are now wimping out at f11 - all
    down to cost cutting, I suppose, as Bob has indicated the need for greater
    build quality enforced by going for smaller minima.



    Peter
     
    Bandicoot, Jul 19, 2005
    #13
  14. Steven Woody

    Bandicoot Guest

    aperture blade configuration, the tips narrow
    The diaphragms in some of my Pentax lenses work this way too, though they
    are arranged the other way around, so the 'flower effect' you describe is
    seen from teh back of the lens, not the front.

    Given Pentax's concern about bokeh, and the fact that a rounded aperture is
    _part of_ the bokeh equation, I think you may be right about the reasoning.
    That Niikon lens is one of theirs that is known for nicer bokeh than many.


    Peter
     
    Bandicoot, Jul 19, 2005
    #14
  15. The Canon EF 50/1.4 and 50/1.8 lenses both have a minimum aperture of
    f/22.

    David
     
    David Littlewood, Jul 19, 2005
    #15
  16. Steven Woody

    Don Stauffer Guest

    Mechanical precision needs are higher for smaller apertures. I think
    the mfgs just believe no one uses small apertures any more. They think
    the great majority of their customers use auto exposure and don't really
    care which specific aperture or shutter speed results. They are
    probably right. Still, it WOULD be nice.

    I know some pundits will say, hey, with a 35mm format, at f/22 you are
    losing resolution due to diffraction.

    Yes and no. The shape of the blur- the spot profile- is different for
    diffraction than for defocus. The peak is smaller in diameter and more
    of the energy is distributed in the wings, way out. The result is more
    of a softening and overall loss of contrast than an obvious loss of
    resolution. It is nice for the photographer to use his judgement on what
    effects he wants.

    It is nice to keep our really old cameras- some have neat features like
    this that can be used for special circumstances.
     
    Don Stauffer, Jul 19, 2005
    #16
  17. Steven Woody

    Don Stauffer Guest

    Hey, I have some old 35s that DO go down to f/22.
     
    Don Stauffer, Jul 19, 2005
    #17
  18. Steven Woody

    Steven Woody Guest

    sorry. what is 35s?

    -
    narke
     
    Steven Woody, Jul 19, 2005
    #18
  19. Steven Woody

    Steven Woody Guest

    :( Cannon lens cann't mount on my carmera.
     
    Steven Woody, Jul 19, 2005
    #19
  20. Steven Woody

    Paul Furman Guest

    Paul Furman, Jul 19, 2005
    #20
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