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Thread: Why isn't my 300mm 30cm long?

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    Why isn't my 300mm 30cm long?

    I have a Nikon 70-300mm telephoto zoom. At full extension it's still barely more than 20cm long. How can the distance between the sensor and whichever bit of the lens it's measured against possibly be 300mm?

    Or am I missing the point of focal length?

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    Moderator Dave Humphries's Avatar
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    Re: Why isn't my 300mm 30cm long?

    Quote Originally Posted by Xander314 View Post
    I have a Nikon 70-300mm telephoto zoom. At full extension it's still barely more than 20cm long. How can the distance between the sensor and whichever bit of the lens it's measured against possibly be 300mm?

    Or am I missing the point of focal length?
    Not entirely, although as you say, it would be measured to the sensor, which is another 40 odd millimetres behind the lens rear element.

    I suspect Sean will know the accurate answer, but possibly internal focusing applies some extra magnification to it.

    Good question, I'm sure someone will answer it better than I have

  3. #3
    William W's Avatar
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    Re: Why isn't my 300mm 30cm long?

    Quote Originally Posted by Xander314 View Post
    I have a Nikon 70-300mm telephoto zoom. At full extension it's still barely more than 20cm long. [U]How can the distance between the sensor and whichever bit of the lens it's measured against possibly be 300mm?

    Or am I missing the point of focal length?[/U]
    You are not missing the point, but just skimmed the sides.
    In simple terms, this lens is: a TELEPHOTO Zoom.
    It is not a LONG Focal Lens.

    Way back when pinholes had sex and lenses were created, the lenses were simple – a bit of tube and one chunk of glass at the far end. Slap on a camera and hey bingo a “Long Lens” was born and wow-gee-wiz we got “closer” to the subject (or more correctly “The Long Focal Lens” is the correct technical term).

    Those Long Lenses were, and are, still about 300mm long for a 300mm lens.

    However several smart blokes (women weren’t generally sent to school) thought about sticking other chunks of glass INSIDE the tube and the TELEPHOTO lens was invented. The fellow who usually gets the credit for inventing the TELEPHOTO lens for PHOTOGRAPHY is a Pom, Thomas Dallmeyer, and from memory that was in the 1890’s . . . but there were many others playing with it and there were other applications of the telephoto lens one most notable was the OPERA GLASSES.

    These extra lens elements placed inside the tube, bent the light and meant that the lens itself, could be made shorter physically, than the actual Optical Focal Length.

    There’s a truckload of technical stuff to it if you want to research it and the nuances and developed of the negative lenses and their relationship to the objective lens is interesting: if you have the head for it.

    One most notable recent achievement is the Canon “DO” lenses (Diffractive Optics) the first was the 400F/4 (about 2000 I think) and there is also a 75 to 300F/4.5~5.6. A TELEPHOTO and a TELEPHOTO ZOOM respectively.

    The DO design, makes each lens substantially shorter, again, than other telephoto or telephoto zoom in their respective classes. There is much discussion about the resultant difference in Image Quality, however I can’t comment as I have not used either lens.

    WW

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