Increase in Focal Length =/= Increase in Barrel Length?? Huh??

Discussion in 'Beyond the Basics' started by Stradawhovious, Feb 25, 2016.

  1. Stradawhovious

    Stradawhovious Been spending a lot of time on here!

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    Ok, I just bought a new (to me) Sigma lens, the 28-70mm f/2.8, and it's making me scratch my head.

    I always thought that focal length was a function of barrel length, but this lens seems to show the opposite. With every other zoom lens I've had, as the focal length increased with the zoom function, so did the physical length of the lens. This makes sense to me.

    With the Sigma 28-70 however, as the FL increases from 28mm to 70mm, the physical length of the lens DECREASES, meaning the front element moves closer to the sensor, not away from it as my logic would dictate. This does not make sense to me.

    Is this a function of creative element clustering, or is there some sort of crazy voo-doo that makes this lens operate in reverse of what makes sense?

    I once made a thread that shows barrel length as a function of blowing out backgrounds. This physical anomaly kind of flies in the face of that "factoid".

    Anyone want to shed some light on this one for me? Will this affect my ability to blow out my backgrounds vs a lens that grows with FL?

    Also, Mods, if this is in the wrong place, feel free to move it.


     
  2. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    Google 'retrofocus lens design'.
     
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  3. FotosbyMike

    FotosbyMike No longer a newbie, moving up!

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    I would say it is part voo-doo part optical physics but I don't think barrel length affects background blow out because you are still increasing your focal length(narrowing field of view) when you are "zooming" which helps to increase bokeh.

    I would also think these companies makes these changes to avoid copyright infringement.
     
  4. Stradawhovious

    Stradawhovious Been spending a lot of time on here!

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    Well, that's far less magical than I had hoped... but makes sense of it.

    Thanks!

    Still not sure that I'm a big fan of it. I guess I will have to spend some time playing with it to be sure.
     
  5. Designer

    Designer Been spending a lot of time on here!

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    The Nikon AF-S 18-55mm is shortest at around 30mm, extending when zooming either shorter or longer.
     
  6. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    Well, without retrofocus you'd never be able to design a lens with a focal length shorter than the camera's Flange-Focal Plane distance. So if your camera's F-FP distance is 40mm, the shortest lens you'd be able to fit would be 40mm. That would really suck if you wanted an über-wide lens.
     
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  7. petrochemist

    petrochemist TPF junkie!

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    True telephoto designs also break the length/focal length relationship. These days long focal length lenses are usually called telephotos even when they're not...
     
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  8. Derrel

    Derrel Mr. Rain Cloud

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    I was taught that the definition of telephoto is a lens that has an effective focal length that is longer than the physical length of the lens, which many years ago was uncommon; long focus or long focal length lenses were very common,built in a long tube with only a few simple elements in the barrel, and it pretty much 500mm in length. A 500mm telephoto lens like the Nikon 500mm f/4-P manual focus is only 384 millimeters in its total, overall length. At one point, there was the term true telephoto lens, that was used to differentiate between the older long focus style of lens, and the newer, much more-compact telephoto designs.

    At one time, the specification telephoto factor was sometimes listed in the detailed specifications for a lens, so that potential buyers--who would be perusing specifications from catalogs or from magazines and such, could get an idea of just how much shorter a lens might be, in a relative sense. So, the Nikkor 500m f/4 Ai-P manual focusing telephoto would have a telephoto factor of 1.30, from 500mm divided by 394mm.

    As petrochemist noted, today some of the long focal length lenses that exist are called "telephoto", even if they are not true telephoto designs.

    A good article on retrofocus lens development is here:
    The Development of Wide-Angle Lenses
     
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  9. Stradawhovious

    Stradawhovious Been spending a lot of time on here!

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    Um... Math?

    Nope. I'll stick with magic thanks.



    [thanks for the article. ;) ]
     
  10. robbins.photo

    robbins.photo Yup, It's The Zoo Guy Supporting Member

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    So cutting the head off a live chicken, or long division.

    Yup..I'd go the chicken route myself. Good call.
     
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  11. Stradawhovious

    Stradawhovious Been spending a lot of time on here!

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    Easier. And makes more sense.
     
  12. wfooshee

    wfooshee No longer a newbie, moving up!

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    And better food!
     

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