Optical difference between film SLRs and DSLRs?

Discussion in 'Digital Cameras' started by 223rem, Nov 17, 2005.

  1. 223rem

    223rem Guest

    Is it only the size of the sensor? In other words, if I use
    the same lens (say 50mm), focused the same way, on a film
    SLR and on a DSLR, both cameras viewing the same scene from the
    same vantage point, then the image taken by the DSLR will simply
    be a cropped version of the image taken by the film SLR? That is,
    if I cut off the borders of film photograph I will obtain the
    image taken by the DSLR?

    223rem, Nov 17, 2005
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  2. 223rem

    John Bean Guest

    Exactly so. If only everybody could get it into their heads
    that it's a *crop*, not a change of focal length.
    John Bean, Nov 17, 2005
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  3. Not if you use a full frame DSLR such as a Canon 1DsMkII or 5D.


    "I have been a witness, and these pictures are
    my testimony. The events I have recorded should
    not be forgotten and must not be repeated."

    -James Nachtwey-
    John A. Stovall, Nov 17, 2005
  4. 223rem

    Charlie Ih Guest

    Physically the focal length has not changed and the fact is that the
    image is cropped". However, in digital photography, we no longer
    have a "standard" 36 x 24 format. To make things more easy to understand,
    we now use the 35 mm equivalent focal length w.r.t. image angular coverage.
    The sensor size (diagonal) of many of the DSLR is 1.5x (or 1.6x) smaller than
    that of a film SLR, therefore the 35 mm equivalent focal length of
    DSLR is 1.5x (or 1.6x) longer.

    BTW, the diagonal of 36 x 24 mm is 43 mm and that of 36 x 36 mm is
    50.9 (or 51 or 52). The accepted standard normal lens of 50 mm is by
    "accident". Many very old SLR had 43 mm and/or 52 mm "Normal" lens.
    Charlie Ih, Nov 17, 2005
  5. 223rem

    223rem Guest

    I thought that the normal lens is defined by the fact that, if you
    look through the viewfinder of a SLR fitted with such a lens,
    the apparent size of objects is same as seen with the naked eye.
    Indeed, if you look at a scene with one eye through a viewfinder at
    50 mm and the other eye "naked", you have no trouble fusing the two images.
    223rem, Nov 17, 2005
  6. 223rem

    John Bean Guest

    No, that depends on the magnification of the finder as well
    as the focal length of the lens. The "standard" is usually
    defined as the diagonal of the image frame, which is about
    43mm for a 24x36mm frame.
    John Bean, Nov 17, 2005
  7. 223rem

    223rem Guest

    But surely it is not a coincidence that around 50 mm, with a 'normal'
    viewfinder, images tend to be as I described.
    223rem, Nov 17, 2005
  8. 223rem

    Skip M Guest

    Well, yes, on most digital SLRs. But there are two exceptions, both
    relatively expensive, the Canon 5D ($3300) and the Canon 1Ds mkII ($7000).
    Both use sensors that are effectively the same dimensions as a frame of 35mm
    film, referred to as "full frame." The Kodak DC/n (Nikon lens mount) and
    DC/c (Canon lens mount) used a "full frame sensor, too, but they're out of
    Skip M, Nov 17, 2005
  9. It probably has mostly to do with practical viewfinder design. A
    viewfinder with a much larger image would be unwieldy, and a much
    smaller image would be useless. The practical range of viewfinder
    sizes is determined by human anatomy. Whether or not the human
    anatomy is a coincidence is subject to debate.

    Given the restrictions on viewfinder size, the fact that a 50mm lens
    gives the sort of image you describe, is a matter of geometry. If we
    lived in a non-Euclidean universe, the situation would be different.
    Måns Rullgård, Nov 18, 2005
  10. 223rem

    Neil Ellwood Guest

    The original definition was a lens that equalled the diagonal of the film
    frame. With 35mm this would have been about 45mm although most cameras
    used 50mm as the normal lens.
    Neil Ellwood, Nov 18, 2005
  11. And then there was the Nikon Fujix E2 series that had a small sensor but
    an extra lens in the body to make sure that the field of view remained
    the same as on 35mm film.
    Philip Homburg, Nov 18, 2005
  12. 223rem

    Skip M Guest

    Wow, that's an interesting Rube Goldbergish approach to the problem, now
    isn't it? I've never heard of that one, I'm going to have to do some
    Skip M, Nov 18, 2005
  13. If it happens that the view through the finder with a normal lens
    mounted is exactly 1, so you can fuse the viewfinder image with your
    other eye, that's probably because the camera manufacturer deliberately
    selected a finder magnification that does this. It's unlikely to happen
    by accident. What camera did you observe this with?

    But this has not been true of any SLR that I have used; the "normal
    lens" viewfinder image is not unity magnification.

    Dave Martindale, Nov 19, 2005
  14. It's a way to use interchangeable SLR lenses with a sensor that's much
    smaller than full-frame. However, it's difficult to make the additional
    optics work well with all the possible lenses you might want to mount,
    since they may have very different ray paths from lens to film/sensor.

    You also lose light and contrast in the extra optics.

    Dave Martindale, Nov 19, 2005
  15. The Olympus OM-1 with an, IIRC, 50mm lens has this property, or at
    least close enough for my eyes not to notice.
    Måns Rullgård, Nov 19, 2005
  16. 223rem

    223rem Guest

    20D, and also on a Nikon FM10
    223rem, Nov 22, 2005
  17. It's never been true of any SLR that I've used either, and I suspect the
    people who keep saying that are either (a) just repeating what others have
    said, or (b) not judging the viewfinder magnification very well.

    The notion that "normal lens" meant "same size image in an SLR viewfinder as
    with the naked eye" has been around for so many years that I tested it
    myself, well over a decade ago, by adjusting a zoom until the viewfinder
    image really did match the naked-eye image, and then checked the position of
    the zoom ring. In every camera I tried, the f.l. that matched was somewhere
    between 70 and 100 mm, as I recall. It definitely never was 50mm or close to

    Neil Harrington, Nov 23, 2005
  18. 223rem

    Father Kodak Guest

    And the early Nikon F had a 58 mm f1.4 lens.
    Father Kodak, Dec 20, 2005
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