Differences between Diglital and 35mm about lenses

Discussion in 'Digital SLR' started by Angelo, Jan 29, 2005.

  1. Angelo

    Angelo Guest

    Hi everybody! I am new to the world of photography, and, i know, my
    question sounds stupid. I have a Canon 300D, it's great. I have bought
    a EF 75-300mm lens, it's great and have great colors. The EF series is
    made for 35mm, isn't it? In my digital camera the focal lenghts
    change, isn't it? How? What's changed? My focal lenght change, so my
    lens is not a 75-300 compared to 35mm. Thank you very much guys! A.
    Angelo, Jan 29, 2005
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  2. Angelo

    Steve Wolfe Guest

    Hi everybody! I am new to the world of photography, and, i know, my
    Strictly speaking, the focal length (the length from the sensor/film to
    the focal point) doesn't change at all - it's still 75 to 300 millimeters.
    However, due to the smaller sensor, the *field of view* that you will get
    from a 300mm lens will be equivalent to what you'd get from a 480mm lens on
    a full-frame sensor (or film). (the "crop factor", or whatever you want to
    call it, is 1.6 for your camera.)

    Steve Wolfe, Jan 29, 2005
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  3. Angelo

    DoN. Nichols Guest

    Actually -- strictly speaking, the sensor should be *at* the
    focal point. However, what you mean is that the effective focal length
    from the diaphragm (at the node of the lens) of a fixed focal length
    corresponding to the current zoom setting to the focal point (the
    sensor) will not change. Remember -- all zoom lenses go through
    complicated optical tricks to make a lens of fixed physical length
    behave as though it were an infinite number of lenses between the two
    limits of focal length. (Sometimes, the location of the diaphragm will
    shift as you zoom, which is a component in the shift in maximum
    aperture as you go through the range in some zoom lenses.)

    Even most telephoto lenses do not have a real focal length the
    same as the equivalent focal length. They use optical tricks to make
    the overall length of the physical lens barrel shorter than it would
    otherwise need to be. At one time, the term "telephoto" was used to
    distinguish these lenses with optical tricks from simple lenses of long
    focal length.

    The rest which follows I can fully agree with.
    Note, BTW, that most lenses are sharper in the center than at
    the edges, so you may get a better image than you would with a full
    frame sensor and the equivalent focal length -- if you really care about
    the edges of the image. This depends, among other things, on which is
    higher resolution -- the sensor or the center area of the lens.

    DoN. Nichols, Jan 30, 2005
  4. Angelo

    John Francis Guest

    Actually, if we're really going to be speaking strictly,
    not all zooms maintain a fixed physical length.
    John Francis, Jan 31, 2005
  5. Angelo

    DoN. Nichols Guest

    DoN. Nichols, Jan 31, 2005
  6. Angelo

    jfitz Guest

    There are no stupid questions. There are many stupid answers on Usenet.
    This particular group is pretty good though.
    Can't beat it for the price.
    The image sensor in your digital camera is smaller than the image area of a
    35mm frame. This results in a different field of view (FOV) when comparing
    pictures taken with the same focal length lenses on your camera and a 35mm
    camera. The difference is a factor of 1.6. This means that a picture taken
    with a 50mm lens on your camera results in the same photo as a 80mm lens on
    a 35mm camera. Your 75-300 will give you the same results as using a
    120-480 on a 35mm camera.

    Technically, as other posts have explained, the focal length does not
    change. However the net result does due to the different size of your image
    jfitz, Feb 1, 2005
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