Somebody help me with aperature...

Discussion in 'Beyond the Basics' started by C.Lloyd, Feb 12, 2009.

  1. C.Lloyd

    C.Lloyd TPF Noob!

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    NO, this isn't another "what aperature do I shoot an indoor wedding with.." thread. And I'm not new at this. I have no formal training, but I have a pretty good "feel" for how aperature and shutter speed work together.

    Here's my conflict.... Wikipedia defines the f-stop number as "the focal length divided by the aperature as measured in the same units". So lets take a regular "cheap" kit lens as our sample lens in this scenario. Say a 24-55 f3.5/5.6 is our example.

    If we take a focal length of 24mm and we know the f-stop is 3.5 max, at that setting, the aperature would be an actual size of 6.86 mm wide. (24mm focal length / 6.86mm aperature opening = f3.5)

    At the other end, if we take a focal length of 55mm and we know that the f-stop is 5.6, then the aperature would be an actual size of 9.8mm. (55mm focal length / 9.8mm aperature opening = f5.6)

    My question is thusly: If the aperature can physically open up to 9.8mm at a 55mm focal length, then why can't it open up to 9.8mm at a 24mm focal length? (Which would give us an f2.4 and a faster lens)

    Do lens manufacturers limit these lenses artifically? Or is there some physical limitation within the lens that I don't know about since I've never taken one apart? (And don't plan to). Does it take more lens elements (and a more expensive lens) to effectively pull off having that large of an aperature at that short of a focal length?

    If this makes sense to anyone, please help me wrap my head around why this works this way.
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    I can't fully explain it...but take note that the aperture measurement is not actually the physical size of the aperture opening...but of the 'entrance pupil'

    http://en.wikipedia.org/wiki/Entrance_pupil
     
  3. C.Lloyd

    C.Lloyd TPF Noob!

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    Great... so now I gotta find out what that is!
     
  4. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Your question isn't all that uncommon though. Many people have asked why cheap lenses don't have larger apertures at the wide end, when it seems (on the surface) like an easy thing to do. Or people have asked why 'constant aperture' zoom lenses don't have an even larger aperture at the wide end. Surely a 70-200mm F2.8 lens could have a bigger aperture at 70mm than 200mm.

    My guess is that is would require more advanced/complicated optics to deal with the wider aperture at shorter focal lengths...which might make the lens bigger and/or more expensive. Remember that with most of these variable aperture lenses, cutting size, weight and cost is an important factor.
     
  5. C.Lloyd

    C.Lloyd TPF Noob!

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    And how does one find the size of the "entranc pupil" on a particular camera? And is it a fixed size, or does it vary like the aperature?
     
  6. C.Lloyd

    C.Lloyd TPF Noob!

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    I knida figured that to get the wider aperature at longer focal lengths would require more (more expensive?) optics, I'm just trying to find out why. Part of my quest to understand my camera better. :blushing:
     
  7. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Head down to a library and pick up a book or two on optics...it will really help you (if you are having trouble falling asleep)
     
  8. 37fleetwood

    37fleetwood TPF Noob!

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    ok, let's see if I can get this across and have it make sense. the aperture on the lense a full open is the same at both focal lengths. the thing that changes it the ratio caused by the lengthening of the lense.
    lets take a 50mm and say a 100mm lense. lets say for our purpose they have the same diameter aperture at full open. though the apertures are the same size, the longer lense will have a smaller fStop rating because the smaller angle of view will allow less light through.
    here's a diagram that I found and altered for our purpose:
    [​IMG]
    notice on both lenses the aperture (indicated by the oval in the lense drawing) is at full open, but on the telephoto view the limitations of the lense only allow a small portion of the aperture to be effective while the wide angle lense allows more use of the same aperture.
    as you lengthen the lense you narrow the angle of view and automatically make the aperture less effective.
    after typing all this I found this site which goes into pretty good detail if you're interested.
    A Tedious Explanation of the f/stop
     
  9. 37fleetwood

    37fleetwood TPF Noob!

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    somehow you guys seem to have gone on while I was typing so let me see if I can catch up a bit.
    the reason lense makers use the fstop they use on cheaper lenses is that to increase the fstop they have to increase the size of the actual lense elements and that's more expensive to build. as lense elements get larger certain things get more critical so you don't introduce interference and abberations. so it's a compromise they go as big as the expense of the lense dictates.
     
  10. C.Lloyd

    C.Lloyd TPF Noob!

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    And I guess all of that makes perfect sense if the aperature only moves half the distance of the outer lens element when zooming. (The outer element moves an inch and the aperature only move half that distance due to the mechanics of the lens.


    BUT... that doesn't explain (to my feeble mind, anyhow) why the aperature at a shorter focal length would have to be a smaller physical opening than at a .....

    Now I'm just confusing myself....

    OFF TO THE BOOKSTORE!!!
     
  11. C.Lloyd

    C.Lloyd TPF Noob!

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    And thank you all for the instructions! you guys are great.
     
  12. 37fleetwood

    37fleetwood TPF Noob!

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    are you asking why the aperture would have to be smaller at a shorter focal length to get the same fstop? if so it is simple, since the wider angle lets more light in you have to compensate by stopping the lense down to get the same value.
     

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