35mm (film) lenses on digital camera, do I lose lens speed as well?

Discussion in 'Digital SLR' started by Steve, Feb 18, 2006.

  1. Steve

    Steve Guest

    Okay, I have to multiply focal length by 1.5 when I put a lens for film
    on my digital camera. Does this mean I have to do the same with F-numbers
    like a teleconverter?


    Steve, Feb 18, 2006
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  2. Actually, it is more of a FOV divider.
    Ed Ruf (REPLY to E-MAIL IN SIG!), Feb 18, 2006
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  3. Steve

    Alan Browne Guest

    No. The multiplier is meant to mean "_effective_" focal length vs. the
    35mm standard 36x24mm frame size.

    The sensor is smaller on your DSLR (~ 24x16mm). The image circle formed
    by the lens is still the same size, but the sensor is that much smaller
    than the std. 35mm frame. So it is like you're "zoomed in" by 1.5x the
    FL of the lens.

    Since the true FL has not really changed and the diameter of the
    aperture has not changed (for a given setting), the aperure number
    (ratio of FL over aperture diameter) has not changed either.
    So, exposure is not affected*.

    The DOF _has_ changed as that is purely related to print size. So since
    you would need to blow up the smaller sensor image to a greater factor
    than with the larger film image to get to a given print size, the DOF is
    shallower than the markings on the standard 35mm format lens suggest.


    *As the different camera/sensor manufacturers do not seem to be
    perfectly lined up with respect to ISO on digital cameras, there is a
    variance between cameras in results. If you meter with a seperate meter
    then different cameras will give different saturation for the same
    scene. This difference is about 2/3 of a stop among the various
    brands/models. Which could fairly be said for the wide variety of film
    types in any case.
    Alan Browne, Feb 18, 2006
  4. So, if I put a 200 mm f/4 lens with a 2x teleconverter (effectively a
    400 mm f/8) on my Nikon digital SLR, I'm getting the equivalent of a
    600 mm f/8? That's useful to know...
    Alexander Arnakis, Feb 18, 2006
  5. Equivalent what?
    Ed Ruf (REPLY to E-MAIL IN SIG!), Feb 18, 2006
  6. Steve

    Nick Beard Guest

    Correct, in effect.
    Nick Beard, Feb 19, 2006
  7. Steve

    Steve Guest

    Whoa, I followed you right up to the DOF changing. Does the DOF become
    shallower because the circle of confusion would be blown up along with the
    sensor image to get a given print size?
    Steve, Feb 19, 2006
  8. Steve

    Alan Browne Guest


    The CoC is constant in size v. the constant in size aperture, but now
    smaller sensor; DOF is related to intended print size. The lens
    manufacturer makes an assumption about print size (depending on what you
    read and when and where it was written the "standard" CoC print size is
    8x10 (from sheet film cameras)) when putting the DOF scale on the lens.
    (and it could be that lenses for cropped sensor cameras (Nikon, Canon,
    Pentax, Sigma, etc.) take this into account, but I don't know for sure.

    To get an 8x12 from a 24x36mm negative, the blowup is 8.47 times.

    To get an 8x12 from a 16x24mm sensor, the blowup is 12.7 times.

    So the CoC's are "spread" out that much more so you get shallower
    "critical focus" DOF on the print.

    Of course to get the same (almost) image from a 1.5 cropped sensor at
    the same focal length you will have to step back 1.5 times further
    resulting in deeper DOF and somewhat restoring balance to the DOF
    universe. Or at the same distance use a shorter FL. But this will have
    a different outcome as the hf distance is related to the square of the
    FL so it's not linear like the focal plane distance from the film plane.

    I'm too lazy to bother with the math, I'll leave it for you...

    the main point to remember is that the DOF markings on a full frame lens
    are wrong when used with a cropped sensor.

    Alan Browne, Feb 19, 2006

  9. It becomes more shallow if you use the same actual focal length, but
    deeper if you adjust focal length to preserve field of view (FOV).

    The following web-pages demonstrates what happes to FOV, DOF and
    perspective when you use a cropped image circle:

    - http://hannemyr.com/photo/crop.html
    - http://hannemyr.com/photo/crop2.html
    Gisle Hannemyr, Feb 21, 2006
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