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Thread: Is a lense's physical length nearly its focal length?

  1. #1

    Question Is a lense's physical length nearly its focal length?

    Let's ignore EF70-300 DO or crop factor for the timebeing. For normal 35" camera & lense,
    is the focal length the lens operate on nearly the physical length of the lens at the time?

  2. #2
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    Default Re: Is a lense's physical length nearly its focal length?

    From what I know, the physical length has to be at least equal to the focal length. The diameter is also at least equal to the (focal length) / (min aperture size).
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  3. #3

    Default Re: Is a lense's physical length nearly its focal length?

    longer barrel length, the further the image, hence longer focal length...

    can't rmb the formula liao

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    Default Re: Is a lense's physical length nearly its focal length?

    yes u are right in a sense, the physical length of the lens is roughly equal to the focal length..
    but the focal length should be measure from the tip of the lens to the "focal plane" of ur camera. To know where's the focal plane, it is inside ur manual if u are using nikon camera like D50 and D70s.
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    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by kcuf2
    yes u are right in a sense, the physical length of the lens is roughly equal to the focal length..
    but the focal length should be measure from the tip of the lens to the "focal plane" of ur camera. To know where's the focal plane, it is inside ur manual if u are using nikon camera like D50 and D70s.
    If you see -o- symbol on your camera, that's focal plane mark of the camera.

    Regards,
    Arto.

  6. #6

    Default Re: Is a lense's physical length nearly its focal length?

    Not talking about 35 mm, but whether a lens's physical length is nearly its focal length.

    Pardon the quality of the picture. Here are two lenses.

    The longer one is a 210 mm lens for the Contax 645.

    The shorter one is a 229 mm lens for a 4x5 camera.

    Let us say that there is "no difference" between the 229 and 210. The image characteristic (such as depth of field for a given aperture) of these two lenses should be the same.

    But it is clear that lenses with the "same" focal length do not have to have the same physical length.


  7. #7
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    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by student
    Not talking about 35 mm, but whether a lens's physical length is nearly its focal length.

    Pardon the quality of the picture. Here are two lenses.

    The longer one is a 210 mm lens for the Contax 645.

    The shorter one is a 229 mm lens for a 4x5 camera.

    Let us say that there is "no difference" between the 229 and 210. The image characteristic (such as depth of field for a given aperture) of these two lenses should be the same.

    But it is clear that lenses with the "same" focal length do not have to have the same physical length.

    agreed.

    dunno if i am right or wrong, but i think the lenses inside will distort into thinking the focal length right? since focal length is also closely linked to angle of view.
    Logging Off. "You have 2,631 messages stored, of a total 400 allowed." don't PM me.

  8. #8

    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by student
    Not talking about 35 mm, but whether a lens's physical length is nearly its focal length.

    Pardon the quality of the picture. Here are two lenses.

    The longer one is a 210 mm lens for the Contax 645.

    The shorter one is a 229 mm lens for a 4x5 camera.

    Let us say that there is "no difference" between the 229 and 210. The image characteristic (such as depth of field for a given aperture) of these two lenses should be the same.

    But it is clear that lenses with the "same" focal length do not have to have the same physical length.

    example... a picture speaks a thousand words...
    makes sense.
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  9. #9

    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by student
    Not talking about 35 mm, but whether a lens's physical length is nearly its focal length.

    Pardon the quality of the picture. Here are two lenses.

    The longer one is a 210 mm lens for the Contax 645.

    The shorter one is a 229 mm lens for a 4x5 camera.

    Let us say that there is "no difference" between the 229 and 210. The image characteristic (such as depth of field for a given aperture) of these two lenses should be the same.

    But it is clear that lenses with the "same" focal length do not have to have the same physical length.

    Hi,
    I think that may be because the 4x5 camera got a focal plane far more behind compare to a SLR camera.

    Anyway, it'll only a useful and valid comparison when you compare only between SLR lenses and not compare a SLR lenses and a 4x5 lenses.... it like compare between apple and watermelon to see which one got more juices.

    By the way, those mirror lenses will have a much shorter physical length than it focal length.

    Have a nice day.
    Last edited by weixing; 15th September 2006 at 11:35 AM.

  10. #10

    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by weixing
    Hi,
    I think that may be because the 4x5 camera got a focal plane far more behind compare to a SLR camera.

    Anyway, it'll only a useful and valid comparison when you compare only between SLR lenses and not compare a SLR lenses and a 4x5 lenses.... it like compare between apple and watermelon to see which one got more juices.

    By the way, those mirror lenses will have a much shorter physical length than it focal length.

    Have a nice day.
    Your point that we should not compare 35 mm to 4x5 is valid.

    I may do some studies on 35 mm lenses. It may take some time, but meanwhile I had taken more images of different 4x5 lenses.



    Left: Nikkor 300mm
    Right: Nikkor 150mm



    Left: Nikkor 150mm
    Rght: Linhof 135mm

  11. #11

    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by Splutter
    From what I know, the physical length has to be at least equal to the focal length. The diameter is also at least equal to the (focal length) / (min aperture size).
    No. Not necessarily so. Nowadays, lenses are so complex. One good counter example is the use of a teleconverter. Just a short section but it can turn a 200mm lens into 400mm.

  12. #12

    Default Re: Is a lense's physical length nearly its focal length?

    Hi,
    Quote Originally Posted by student
    Your point that we should not compare 35 mm to 4x5 is valid.

    I may do some studies on 35 mm lenses. It may take some time, but meanwhile I had taken more images of different 4x5 lenses.



    Left: Nikkor 300mm
    Right: Nikkor 150mm



    Left: Nikkor 150mm
    Rght: Linhof 135mm
    I never use a 4x5 camera before... (I'm still a newbie in photography). But just wonder does the distance between the lens and the focal plane of different focal length lens differ by much when focus on a same object at same distance. My guess is that part of the camera body itself may act as an extension of the lens, so althought the phyiscal length is similar, the distance between the lens and focal plance may be different... and of course I may be wrong also, since there are various ways of increasing focal length without increase the physical length.

    Have a nice day.
    Last edited by weixing; 15th September 2006 at 12:47 PM.

  13. #13

    Default Re: Is a lense's physical length nearly its focal length?

    Got out some 35 mm lenses for my Leica M.

    Guess which one has the shortest focal length in this lineup?



    Answer: the extreme LEFT (the all black one) is a 35 mm lens. The rest are 50mm.
    Last edited by student; 16th September 2006 at 08:30 AM.

  14. #14
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    Default Re: Is a lense's physical length nearly its focal length?

    Hi Pals...

    The focal length method discussed from the first thread is fit only for simple single element type of focal length measurement.

    For our type of camera lens due to groups of glasses mated together for specific image correction results in some lens physically shorter or longer. Point of focal length measurement may lie anywhere in the lens barrel itself.

    As for the aperture, you can roughly take the focal length of the lens and divide with the opening of the lens(filter size) to guage its maximum aperture. So a 200mm lens with 55mm filter may be a f4 type of lens. So a 200mm with a 70++ filter give you about f2.8.

    Design it for physical or internal zoom. For those fast f2.8 zooms, the zoom is design with internal zoom.As there is also no change in physical length when Zooming, the f number is therefore fixed across the zoom.

    So the physical filter is set to the 77/72 mm size @ 200mm point e.g. 70-200mm f2.8. Where the 70m focal lengh enjoy the already fixed f2.8.

    Designing it for variable length zoom, for a variable aperture 75-300 f4.5-5.6 type of lens. The largest aperture is set for the longest focal length e.g. 50++mm filter. 300/50 gives you f6 which relates to the f5.6 aperture at 300mm. So when zoomed out at 75mm the physical length of the lens is shorter hence getting to buy a little more light gathering ability giving you the f4.5 at 75mm.

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    Default Re: Is a lense's physical length nearly its focal length?

    Quote Originally Posted by lsisaxon
    No. Not necessarily so. Nowadays, lenses are so complex. One good counter example is the use of a teleconverter. Just a short section but it can turn a 200mm lens into 400mm.
    Hi Is,

    Teleconverters are nothing but "optical croppers". it crops you image from the original main lens and then project it back to the image circle for your imager(film/ccd).
    Thats why you need a really good main lens to start with.

    If you physically look at a 1.4X TC vs a 2.0X TC, the glass on the front for the 2x TC is smaller - why? Because it basically takes a smaller part of the image and then "stretch it bigger" to form the image circle to your imager/film. Thats why it will degrade to a certain extent as compared to using the main lens without teleconverter or even to an image using the 1.4X TC.

    CHeers...happy shooting
    Last edited by sulhan; 16th September 2006 at 09:55 AM.

  16. #16

    Default Re: Is a lense's physical length nearly its focal length?

    Thank you all. It's inderesting to learn new knowledge.

  17. #17
    Senior Member +evenstar's Avatar
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    Default Re: Is a lense's physical length nearly its focal length?

    300mm 2.8 vs 300 f4...which one is longer?
    eat. drink. shoot

  18. #18

    Default Re: Is a lense's physical length nearly its focal length?

    Hi,
    Quote Originally Posted by +evenstar
    300mm 2.8 vs 300 f4...which one is longer?
    Depend... ha ha ha Should be same length if from same manufacturer, but 300mm f2.8 should be bigger in diameter than 300mm f4.

    Just my S$0.02...

    Have a nice day.
    Last edited by weixing; 20th September 2006 at 09:16 PM.

  19. #19
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    Default Re: Is a lense's physical length nearly its focal length?

    canon's 300mm length

    f2.8L IS: 9.9in
    f4L IS: 8.7in

    according to here
    eat. drink. shoot

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