1.5 or 1.6 "magnification"; isn't it really just cropping.

Discussion in 'Digital Photography' started by Bay Area Dave, May 25, 2004.

  1. I'm not expert on digicams, but I do read a few mags each
    month...it's my understanding that the so-called focal
    length increase of 1.5-1.6 doesn't tell the whole story.
    First of all, doesn't a small sensor merely crop the image,
    so that it "seems" as though the focal length has been
    extended? Wouldn't we be better off with a full frame
    sensor and crop our own images? There would be more data
    coming from a bigger sensor, so cropping an image to match
    the area covered by a smaller sensor shouldn't result in
    data loss. Right?

    Another thing is that the perspective of an image taken with
    a certain focal length lens is the same whether used on a
    film camera, full framed digicam, OR on an APS sized sensor
    equipped digicam. The only difference is that the APS sized
    sensor crops the image. Perspective remains identical.


    dave
     
    Bay Area Dave, May 25, 2004
    #1
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  2. Bay Area Dave

    B.A.S. Guest

    Is a 35mm image just a "crop" of a 6x6 medium format image? Is a 6x6
    just a crop of a 4x5? Sure - in a sense.

    What really matters is the resolution obtained by that sensor. Why waste
    resources (ie, $$$'s) on unwieldy big cameras and big lenses to cover a
    large sensor area if an APS sized sensor has all the resolution you need
    (and eventually, it will)?

    There is the issue with perspective, but that's minor IMHO.

    YMMV,

    B.A.S.
     
    B.A.S., May 25, 2004
    #2
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  3. Bay Area Dave

    Guy Scharf Guest

    That is correct.
    I used to feel the same way. I'm no longer so sure.

    Sensor size is primarily an issue of technology and costs. A bigger
    sensor, with the same number of pixels/sq in as the small sensor would
    cost very much more, as evidenced by the cost of the few DSLRs with a
    sensor as large as a 35mm frame.

    There's no magic to the 35mm frame size. I have my grandfather's
    camera that uses 102 roll film. You can make a 4x6 contact print from
    a negative from that camera. 35mm developed over the years as the
    acceptable compromise between film, lens, camera, ergonomic and
    economic concerns. Smaller film formats such as APS and 126 have been
    tried but did not displace 35mm. Larger film sizes, notably 120, are
    regularly used. Cameras using 120 file are notably larger than 35mm
    cameras.

    Everyone talks about 35mm as "full frame" because that's what we are
    most used to. DSLRs are aimed at the same market as traditional 35mm
    cameras were aimed at, so the terminology is used. However, the
    technology is different. As long as the resolution can be as good as
    film (it's not yet IMHO), I don't care if the sensor is the same size
    as 35mm or not.

    The trend of computer and chip technology over the last 40 years has
    been "more and faster stuff in a smaller space." I expect that trend
    will continue for a while, resulting in increased resolution.

    Guy
     
    Guy Scharf, May 25, 2004
    #3
  4. Bay Area Dave

    dslr Guest

    Correct, Dave. It is just a crop, any magnification is limited to the
    increased enlargement needed to create a print of the same size as a
    full-frame camera's image.
    That enlargement does have an impact on depth of field but not, as you say,
    on perspective.
     
    dslr, May 25, 2004
    #4
  5. Yes on all counts. Welcome to the world of marketing digital cameras
    to the general public.

    There are arguments that can be made here, depending on your
    definition of 'perspective'. By the dictionary definition, perspective
    remains the same from a fixed vantage point, regardless of camera *or*
    focal length used. It is the process of foreshortening objects with greater
    distance.

    And if you *really* want to be technical, perspective never changes,
    period. Objects decrease in apparent size by the same percentage with
    distance, no matter what.

    What changes is the appearance they have in the final image, which
    renders perspective into a flat plane. While it doesn't change with focal
    length, the overall affect on the image does, so it can (arguably) be said
    to change with focal length. The other issues that come into play are field
    curvature and rendered field-of-view, where a wide-angle lens may distort
    the image more towards the edges, and will also render a wide field-of-view
    into a narrower angle on the print - the fisheye effect. Yes, you get
    greater curvature towards the edges, and yes, cropping the image will
    reduce the apparent effect of this. "Perspective", in and of itself, never
    changes, but the image is radically altered nonetheless. There just hasn't
    been a good term put forth yet to describe this.


    - Al.
     
    Al Denelsbeck, May 25, 2004
    #5
  6. Bay Area Dave

    Jim Townsend Guest

    Yes.. The best way to understand would be to take a 35mm
    film camera. Mount it on a tripod and take two identical
    frames. You'll wind up with two frames of film containing
    the identical image.

    Now cut an APS sized square from the middle of one of the
    frames. Then print both the full frame and the cropped square
    at 6" x 4"

    You have to enlarge the APS sized square more than the full
    35mm frame. As a result, the image made with the cropped piece
    of film will have a narrower field of view. It will appear to
    have been taken by a lens with a longer focal length.

    Other than that, nothing else about the image changes.
    Sure.. That's no problem.. All you have to do is buy a digital
    camera with a full 35mm sensor. They make them. They start
    at around $7000.00 US Here's one:

    http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=266188&is=REG

    Currently, large sensors are expensive to manufacture. Regardless
    of the size, the sensor adds the greatest expense to the cost of
    any digital camera. That's one reason why even simple P&S digital
    cameras are at least 3X the cost of a 35mm compact P&S
     
    Jim Townsend, May 25, 2004
    #6
  7. And where can I buy a camera with an APS sized sensor that performs the same
    as a Canon 1Ds? Even the sensor in the 1D mark II is bigger than APS.
     
    Philip Homburg, May 25, 2004
    #7
  8. Bay Area Dave wrote:

    You have the right idea, but ,let me add one additional thing to your
    thought process.

    All else being equal (same sensor quality and MP) the smaller sensor
    (1.6) would not only crop the image a little more, but it would also be like
    using a finer grain film since it can resolve the same total amount of
    information as the larger sensor.

    There does seem to be a tendency for larger sensors to have less problem
    with "noise" that is the stuff that looks a lot like grain in a high speed
    film, under low light and the seem to have a tendency to have fewer problems
    in some other areas, so over all the larger one may produce a better image.
     
    Joseph Meehan, May 25, 2004
    #8
  9. Al, I think this is misleading. The *relative* size of two objects at
    different distances from the observer *will* change if he moves closer
    or further away.

    Example: If you have 2 identical objects, one at 10 metres and the other
    at 20 metres, the latter will "appear" 0.5 times the size of the closer
    one. If the observer then moves back 20 metres, the further object will
    appear 0.75 times the size of the closer one.

    Also the apparent angle of a line between two objects not in a straight
    line with the observer will change on a change in the position of the
    observer.

    These effects are what is generally understood by "perspective", it
    changes with change of position, and *only* with change of position.
    Anything done by way of changes of lens focal length at the same
    position is merely a change of magnification.

    I realise you said the same thing in your first paragraph, but your
    second paragraph seemed to be contradicting the first, and to be
    incorrect.
     
    David Littlewood, May 25, 2004
    #9
  10. There seems to me be some lack of clarity in saying, on the one hand,
    "same sensor quality" and on the other saying the smaller one "can
    resolve the same amount of image as the larger sensor.

    It seems to me that the "quality" of a sensor (ignoring important issues
    such as colour and contrast) is related to pixel *density* rather than
    pixel *number*. To equate quality with total pixel number is to confuse
    quality with quantity. Nor does "higher density" always mean "higher
    quality" - see below.
    Yes, I have gained the same impression (which again reinforces the view
    that quality is not just pixel numbers). I suspect that the smaller
    pixels used on many digital cameras will never match the image quality
    of a similar number of pixels on a larger sensor with lower density, for
    quantum reasons. It is likely that "sensor quality", like optimum
    resolution for lenses, is fated for reasons of physics to rise to a peak
    at a given density and then decline.

    OTOH, it could be that having such a large investment in 35mm lenses I
    am just desperate to convince myself that full-frame sensors at
    reasonable prices will arrive in the not-too-distant future. Otherwise I
    will be condemned to be deprived of decent wide angle in all my digital
    work....
     
    David Littlewood, May 25, 2004
    #10
  11. Perspective is controlled by *location* -- where you're standing
    relative to the subjects. The focal length has nothing to do with it.
    The classic demonstration of this is the series of photos commonly
    provided as counter mats by various lens manufacturers showing a
    series of pictures taken with each of their lenses all from the same
    position and pointed the same direction. The size relationships
    between the objects are the same in all of the pictures (that's what
    "perspective" means in this context); they look like they're just
    crops out of the center of the widest picture in the set (except, for
    the nitpickers, that there's no increase in grain or anything).
     
    David Dyer-Bennet, May 25, 2004
    #11
  12. 6x6 film camera is a good example.
    Besides perspective, depth of field and lens speed are changed on digital
    SLR.
     
    [email protected], May 26, 2004
    #12
  13. Bay Area Dave

    Mark Johnson Guest

    Wouldn't it be fair to say, though, that all the various noise
    components could conceivably be reduced in small ccds, with each new
    generation, but that the best optics and photos tend to come from more
    'glass', rather than smaller diameter lenses, regardless?
     
    Mark Johnson, May 26, 2004
    #13
  14. I would say so. Of course this all leads back to the same conclusion:
    It is best to judge the results than the equipment used to create them.
     
    Joseph Meehan, May 26, 2004
    #14
  15. No.

    There is no reason why optics smaller than current 35mm ones should not
    perform as well, and indeed lots of practical indication that they
    perform better (until, presumably, one gets so small that interference
    effects kick in).

    Sensors, OTOH, are already using technology which has dimensions close
    to the wavelength of the light.
     
    David Littlewood, May 26, 2004
    #15
  16. Bay Area Dave

    B.A.S. Guest

    Well said, Guy. I've been coming to the same conclusions.

    B.A.S.
     
    B.A.S., May 26, 2004
    #16
  17. []
    No.

    - there are ultimately quantum limits to the noise, which will favour
    larger chips. How large depends on how much noise you can tolerate, and is
    therefore subjective.

    - more glass is bad, because it causes more light loss, more internal
    reflections and increases manufacturing complexity. Lenses for formats
    smaller than "35mm" can be just as good, providing diffraction does not
    become an issue. So again, there is an engineering and subjective
    trade-off between the performance and the lens complexity.

    Cheers,
    David
     
    David J Taylor, May 26, 2004
    #17
  18.  
    Al Denelsbeck, May 27, 2004
    #18
  19. Georgette Preddy, May 27, 2004
    #19
  20. Yes, but resolution tests are done on that crop, so the whole story is
    told.
    You just figured out the Canon 10D vs 1Ds. The 1Ds actually has
    (slightly) lower resolution than the 10D, shown here...

    http://www.outbackphoto.com/artofraw/raw_05/essay.html
    Right, Sigma makes it quite easy to understand with their "sports
    finder." The sports finder is a great feature, a big advantage over
    all the other D&SLRs, but truth be told its a money saver. All they
    did was darken the cropped-out part of the SA-9's (film SLR) full
    frame view finder. When you put any lens on the SDs, you see exactly
    what is happening, and how it gets cropped...

    http://www.dpreview.com/reviews/sigmasd10/page3.asp

    It's a really great feature because you can compose/anticipate using
    cues beyond the imaged frame, why all cropping DLSRs don't do this
    baffles me.
     
    Georgette Preddy, May 27, 2004
    #20
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