Telephoto Lenses and Aperture Size

Discussion in 'Photography Beginners' Forum' started by ElizaMM, Mar 27, 2016.

  1. ElizaMM

    ElizaMM TPF Noob!

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    Good morning. I have been looking at telephoto lenses (for a Canon 7d) on-line today and they all seem to be limited to large apertures (4.5-5.6). Why is this?


     
  2. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    To make them affordable.

    Yea, they can make tele lenses faster, but they are more expensive, heavier and larger. And that makes them unsellable.
     
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  3. SCraig

    SCraig Been spending a lot of time on here!

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    Because the numbers used for aperture size (the "F" numbers) are the ratio of focal length to the size of the entrance pupil of the lens. In other words, a 500mm F/2.8 lens would have an entrance pupil of 500/2.8 = 178mm = 7.03 inches. The glass would be enormous and enormous glass is expensive.
     
  4. Ido

    Ido No longer a newbie, moving up!

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    For examples and reference to what the others have rightly explained, look for following lenses—both their physical size and weight, and their price:
    • Any 70-200mm f/2.8
    • Sigma 120-300mm f/2.8
    • Any 300mm f/2.8, 400mm f/2.8
    • I think either Sigma or Tamron has a 200-500mm f/2.8
     
  5. Derrel

    Derrel Mr. Rain Cloud

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    Hi Eliza! Good question, but just want to let you know that you're using the term "large aperture" completely backward. In photographic parlance, the terms "large aperture" means a large, wide aperture, something in the f/1.4 to f/1.8 range for wide-angle lenses, normal lenses, and short to moderate telephoto lenses. In zoom lenses of most types, the large aperture lenses have a maximum aperture of f/2.8, and moderate aperture zooms are f/3.5 or f/4 maximum apertures.

    Zoom lenses that have variable maximum apertures ranging from f/4~5.6 or from f/4.5~5.6 and often referred to as "slow" or "slow maximum-aperture" zooms, or ,"Small maximum aperture zoom lenses." It's important to realize that when taking about the lens aperture, we refer to it as "large" when it is a large, wide, big hole, and we refer to the small, narrow apertures as being either small, or "slow."

    If you want to refer specifically to the f/number, then it makes sense to say, "Use a high f/number, like f/16," or "For this situation, select a low f/number value, like f/1.8 or f/2.2, to get the background to be blurry."
     
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  6. SCraig

    SCraig Been spending a lot of time on here!

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    Click Here (or use Google) to see images of the Sigma 500mm F/2.8 lens and then maybe you'll understand.
     
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  7. beagle100

    beagle100 Been spending a lot of time on here!

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    f5.6 is affordable, f2.8 in the long telephoto lens ..... maybe not so affordable
     
  8. table1349

    table1349 Been spending a lot of time on here!

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    Yep, the same reason there are Ford's, Chevy's, and Dodge's, not everyone can afford a Lamborghini or Ferrari.
     
  9. wfooshee

    wfooshee No longer a newbie, moving up!

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    To gather more light, the lens has to be larger. Simple as that.
    A 50mm f:1.4 lens is easy to build. The iris needs to be about 35mm across, which easily fits into the lens barrel.

    What makes long focal-length lenses different from nifty-fifties is the telefoto construction needed to make a compact lens. A 400mm focal-length lens mounted on a camera is obviosuly not 400 millimeters out from the image sensor. (That's what focal length means for a simple single-element piece of glass: how far away from the image surface the lens is.) "Telefoto" doesn't actually mean simply a large focal length, it means that optical magic is done to "compress" the focal length into a smaller body. (Similarly, short focal length lenses have "retro-focus" construction: the lens may have a focal length of 18mm or 24mm, but it's certainly much farther than that from the image surface.)

    That telefoto construction can also give some leeway into faking out the aperture's physical size while still behaving properly for giving f:4 exposure, but only up to a point. To get really large iris openings (low f-number) in a fast long lens, you simply have to have room for a large iris, and you have to have very large front elements on the lens. Big glass costs a lot more to make than little glass. not everyone can spend $10,000 to get a 300mm lens, but 500 or a grand is pretty easy for most folks.|

    It's kinda like cars, motorcycles, boats, and airplanes: speed costs!
     
  10. chuasam

    chuasam Been spending a lot of time on here!

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    AFAIK (and that's plenty)
    there is an 85mm (which is a telephoto) that sits at f/1.2 :)
     

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