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What determines a lenses maximum aperture?

Discussion in 'Lens Lust' started by wgilles, Sep 28, 2008.

  1. wgilles


    Apr 25, 2008
    Well I've been on the hunt for a new lens recently and settled on the Nikon 105mm 2.8G. I own a 18-200 3.5-5.6 Nikkor lens and a 50mm 1.8 Nikkor. While I was looking at different lenses, I was taking notice of the wide range of maximum lens apertures through the Nikon and Sigma assortment.

    I'm just curious as to what gives a lens its maximum lens aperture. If they wanted to, couldn't they just make every lens a 1.8 f/stop? Does t have something to do with the actual construction of the lens? Is it impossible to make every lens a 1.8, 2.8, etc? I'm sure that making lenses affordable has something to do with it, but for example, why not make my 18-200 a 2.8-3.5? Is there a reason people wouldn't want a fast lens with a large maximum aperture? Going back to the affordable issue, if they made every lens fast, it could possibly mean they would all be the same prices they are now (just a thought).

    I'm just really curious as to the decision process that Nikon, Sigma, Tamron, Canon, etc. makes when they make a lens and determine it's aperture range...
  2. f Number = Lens Focal Length/effective aperture

    To make the number f numerically smaller the size of the aperture has to get larger. That requires a larger diameter front element which obviously makes the lens bigger (and more expensive).

    You also notice the equation shows that longer focal length lenses require bigger elements than smaller focal length lenses for any given f stop.

    A lot of zoom lenses are variable aperture and if you vary focal length (keeping aperture the same) in the equation above you can see why. In this case the diaphragm doesn't change as the lens is zoomed.

    Some zoom lenses have a fixed aperture (like most pro glass) which means the diaphragm is changing as the lens is zoomed.

    You can find more about it here.

    I think part of the initial design requirements for any lens is focal length, cost, and physical size. Bigger apertures (lower f numbers) mean more money and larger size.
  3. PeteZ28


    Oct 5, 2007
    Newtown, PA
    Basicly stated; aperature = focal length / diameter.

    Of course there is a lot more involved than that, but it starts at the front objective.

    For example: 50mm F2 or 50/2=25mm or roughly a 1 inch front element to start with.

    500mm F2 or 500mm/2=250mm, almost 11 inches!!! Now you see why there are 50mm f2 lenses and NOT 500mm f2 lenses!!!
  4. sheesh


    Sep 28, 2008
    that would make it real easy to have really fast and short lenses, but in practice the fastest lenses seem to be about 50mm? any ideas?
  5. wgilles


    Apr 25, 2008
    Thanks, that explained quite a bit.

    I wonder if it has anything to do with it being the "most normal" lens?
  6. AviSys


    Mar 31, 2008
    Placitas, NM
    The attempts to answer the question in a crisp, simple manner resulted in some potential misinterpretation of what's really going on.

    The impact of increasing the lens aperture by even one stop is relatively humungous in terms of design and manufacturing costs.

    It's much, much more than the size of the front element -- although the size of that element gives a visual clue as to what's going on and represents a significant part of the cost of the lens. (The real aperture is actually the ~effective~ size of the iris diaphragm, not the front element.)

    Actually, the size of EVERY element of the lens is impacted as the aperture is increased. And, with faster lenses, more elements are typically involved to correct for aberrations and distortions, and more expensive optical materials are required, adding to cost and reflecting also additional design cost.

    That's just one set of reasons why an 18-105mm f/3.5-5.6G weighs 16 ounces and costs $400 and a 28-70mm f2.8 weighs 35 ounces and costs $1600 even though its focal range is less than half the prior. (There are mechanical differences other than optics in this admittedly gross comparison.)

    As for "short vs. long" lenses vs. aperture, wide angle lenses are more expensive to produce because of more complex design, and "normal" length lenses tend to be easier to produce with wider apertures.

    A 50mm f1.8 lens has 6 elements at $109
    A 24mm f2.8 lens has 9 elements at $309
    A 14mm f2.8 lens has 14 elements at $1300

    So, basically, as lenses depart from "normal" lengths, the cost rises significantly out of proportion to relative length. And as the aperture increases, there is also that significant increase in cost.

    As for 50mm being "most normal," these days the 50mm is probably one of the least purchased lenses in the lineup, suggesting that volume vs. cost is not the real issue here.
  7. wgilles


    Apr 25, 2008
    Thanks, that was super simple
  8. Good clarifications. As you said, I was attempting to keep it simple.
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