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    Default Long-focus lens and telephoto lens

    Do you know that there is a difference between the two? AOTBE, the ungainly long-focus lens is better but may not be very practical.

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    that's the opposite of retro-focus, right? long-focus lens is where the effective focal point is in front of the front element, while the retro-focus lens is where the effective focal point is behind the rear element.

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    All Other Things Being Equal. AOTBE.

    -dave

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    Quote Originally Posted by kwun View Post
    that's the opposite of retro-focus, right? long-focus lens is where the effective focal point is in front of the front element, while the retro-focus lens is where the effective focal point is behind the rear element.
    The first is a telephoto and the second is a retrofocus wide-angle lens. Telephoto and retrofocus designs share a common property that their principle plane, from which the focal length is measured, is not within the physical dimensions of the lens. For telephoto it is somewhere in front, for retrofocus it is somewhere behind. They are actually lenses with custom built "converters". Because so much work has been done on these two designs in recent years (the purist long focus and true wide-angle designs will not sell because they are not practical) the modern telephoto and the retrofocus are as good as, but not surpass, the long focus and true wide-angle designs.
    However, you gain a great deal in the modern telephoto and almost nothing in the modern retrofocus (the latter also known as an inverted telephoto) except that it is the only type of design that can be used on a slr or dslr camera.
    The telephoto design has shrunk in physical dimensions and is far easier to design to the same standards as the long focus design. Not so the retrofocus wide-angle lens. That is why slr and dslr wide-angle lenses are still a poor cousin to their true wide-angle design. It is also bulky, the very opposite of the telephoto. You save bulk in one and gain bulk in another.
    You can find out if your "long teles" are telephoto or long focus simply from their physical dimensions. A long focus lens is at least as long as its focal length minus your camera's lens register. Roughly, if a lens of 300mm is at least 256mm long plus your camera's register of 44mm (Canon EOS body) then it is a long focus lens. Otherwise it is a telephoto. Another giveaway is the diameter of the lens. A 300mm f/2.0 long focus lens will have a front element that is at least 150mm in diameter-a giant of a lens. At even longer focal length at fast lens speed you will need a truck to carry your long focus lenses. That is the reason why the modern telephoto is almost used exclusively today.

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    Ng 6 :-) It will be nice if you post example photos of "telephoto" and "long focus" lens for illustration.
    Last edited by Marky; 09-13-2007 at 11:06 AM.

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