Aperture question newb style!

Discussion in 'Photography Beginners' Forum' started by Wisconsin_Bob, Feb 20, 2008.

  1. Wisconsin_Bob

    Wisconsin_Bob TPF Noob!

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    I apologize in advance for this terribly newbish question. If you have a constant aperture lens, is it possible to decrease the aperture (i.e. on the Canon EF 70-200 f/2.8L, can I bump it up to f/4 or f/5.6)?
     
  2. Offbeat

    Offbeat TPF Noob!

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    Yes, up to f/22 i think maybe more.
     
  3. alabama1980

    alabama1980 TPF Noob!

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    Not trying to hijack the thread...but can you explain that a little?

    How do you get f/22 out of a fixed app lens?
     
  4. Mystwalker

    Mystwalker TPF Noob!

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    I think I got it to f/32 while messing with "M"anual mode.
     
  5. evo5gsr

    evo5gsr TPF Noob!

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    The minimum aperture depends on the camera, IIRC.
     
  6. reconstyle

    reconstyle TPF Noob!

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    It's not necessarily fixed...

    On a lot of the zoom lenses the aperture will be smaller at the long end of the lens. Such as the kit lenses for the d40.XT. They are 18-55mm f/3.5-5.6. But you can still get up to like f/22 with them


    With the 70-200 f2.8L you can obviously get 2.8 at 70 and 200, but you can get a smaller app as well
     
  7. Socrates

    Socrates TPF Noob!

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    There is no such thing as a "fixed aperture lens." All lenses are identified by their largest aperture (smallest f-number). Zoom lenses typically have two apertures indicated, the largest aperture at minimum zoom and the largest aperture at maximum zoom. In any event, the lens can always be stopped down from the largest aperture. The question is how far can it be stopped down. Some to f/16, some to f/22 and some to f/32, a lot depends on the maximum aperture.
     
  8. Battou

    Battou No longer a newbie, moving up!

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    Actually yes, there are, just not in modern production or use to my knowledge.
     
  9. Helen B

    Helen B TPF Noob!

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    It does sound wrong, and 'constant aperture' might be a better term. Fixed aperture used to mean fixed aperture. Now that fixed aperture doesn't mean fixed aperture, what do we call fixed aperture lenses? (Later edit, having read the recent posts: Fixed aperture lenses do exist, they are still manufactured, and they are quite common. There is no need for an adjustable aperture in many applications.)

    Constant aperture zooms hold their f-number as the focal length is changed, so if you set f/4 at the wide end then zoom in you will still have f/4 at the long end. You can still change the aperture: the lens still has adjustable aperture blades.

    Remember that the f-number (which can be written as 'f/') is the focal length of the lens divided by the diameter of the entrance pupil, not the physical diameter of the aperture. The entrance pupil is the image of the aperture blades (I'll call the physical aperture the iris) you see when looking through the front of the lens - ie the image of the iris formed by the lens elements in front of it. It can be magnified or reduced.

    As the lens zooms, the magnification of the iris will change. Lens designers can arrange things so that the magnification of the iris changes in proportion to the focal length of the lens, so that the ratio of focal length to entrance pupil diameter stays constant even though the diameter of the iris itself stays the same.

    If you think about an 18 mm to 55 mm f/3.5 to f/5.6 zoom, the entrance pupil diameter must be 18/3.5 = 5.1 mm when the lens is wide open at 18 mm and 55/5.6 = 9.8 mm when the lens is wide open at 55 mm. The diameter of the iris has not changed, only its magnification. To maintain a constant aperture, the entrance pupil diameter would have to be magnified to 55/3.5 = 16 mm at 55 mm.

    Best,
    Helen
     
  10. Battou

    Battou No longer a newbie, moving up!

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    That explains why this whole thread confused the tar out of me.
     
  11. Socrates

    Socrates TPF Noob!

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    Examples, please.
     
  12. Battou

    Battou No longer a newbie, moving up!

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    aprature on any box camera is fixed. more reasent boxcameras had some adjustment to them but that was minimal at best. Still nothing more than a hole drilled in a metal plate.
     

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