Focal length of digital camera lenses

Discussion in 'Digital Cameras' started by Guest, Jan 21, 2007.

  1. Guest

    Guest Guest

    Is there some universal standard for specifying the focal length of a
    digital camera lens that will allow one to calculate the angle of view of
    that lens? 35mm cameras have a standard frame size of 24 x 36mm, so, if you
    know the focal length of the lens, you will know the exact angle of view.
    Since the size of the active portion of a digital camera sensor is generally
    unknown, how can one determine the viewing angle?


    Norm Strong
    Guest, Jan 21, 2007
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  2. Guest

    King Sardon Guest

    Google it.

    Or check the manual. If that doesn't give the angle of view, look up
    the "equivalent focal length" compared to the 35mm frame and work it
    out from there. If that isn't there either, then you need to determine
    it by measuring what you get.

    I am guessing that 99% of photographers don't know the angle of view
    of their lenses.

    King Sardon, Jan 21, 2007
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  3. Guest

    Cgiorgio Guest

    Sensor sizes are usually specified somewhere in the camera literature
    (manual appendix, data sheet). It might be the actual active area in mms
    width and height (DSLR's) or something like 1 / 1.8" for a compact. The
    angle of view is defined as the angle covering the diagonal of the picture,
    thus you have to calculate the diagonal if width and height are known or you
    can directly convert a fractional size to the diagonal. Unfortunately the
    fractional system is inherited from video equipment operating with video
    tubes, the size gives the outer diameter of the video tube, with the active
    significantly smaller (.633 times the outer diameter). To calculate the
    diagonal in mms multiply the fraction with 0.633 and 25.4. Of course the
    fractional measures are not accurate to the micron, but it should permit
    you to do approximate calculations.

    If you know who makes the sensor used in your camera you might be able to
    find a datasheet on the manufacturer's website.
    Cgiorgio, Jan 21, 2007
  4. Guest

    Matt Ion Guest

    I have a really cool little calculator for PalmOS that has presets for a number
    of different standard frame sizes for video, photo and CCTV cameras, that will
    tell you the FOV for any given focal length, as well as simple graphics
    illustrating the FOV and the resulting view of an object at a given distance.
    No idea if there's actually a Windows or MacOS version, but if you have a PalmOS
    device, I can tell you where to find it :)
    Matt Ion, Jan 21, 2007
  5. Guest

    Matt Ion Guest

    Oh yes, and the calculator uses the standard definitions specific to each
    industry... I use it mainly for CCTV, where CCD sizes are specified by the
    diagonal measurement of the sensor (1/4", 1/3", 1/2")... the CCTV selections are
    all listed in those terms.

    Very nice.
    Matt Ion, Jan 21, 2007
  6. Guest

    Paul Mitchum Guest

    Yes. It is called 'the metric system.' :)
    The size of the active portion of a digital camera sensor is generally
    *not* unknown. It's usually printed in the manual, along with angle of
    view and 35mm equivalents focal lengths.
    Paul Mitchum, Jan 21, 2007
  7. This is precisely why nearly all point-and-shoots are marked with
    "35mm-equivalent focal lengths". Despite many people railing against
    the practice. It's the only way in any kind of common use to specify
    what the range of views the camera will give you is.

    Directly marking the range of angles of view would be the other approach
    -- say "61 degrees - 10 degrees" or some such.
    David Dyer-Bennet, Jan 21, 2007
  8. Guest

    if Guest

    Not in mine. Panasonice describe it only as 1/2.5" which is 10mm. However
    if the sensor was 10mm in size you would expect it's "28mm equivalent" lens
    to be 6.5mm but in fact it is 4.6mm, so clearly Panasonic are
    misrepresenting the size of the sensor in the specifications.
    if, Jan 22, 2007
  9. Guest

    Paul Mitchum Guest

    Not to be a jerk about it or anything, but are you sure you're reading
    the figures correctly?
    Paul Mitchum, Jan 22, 2007
  10. Guest

    Bill Funk Guest

    I have a Palm OS device.
    Can I impose on you to provide the link?

    Jesse Jackson said Thursday
    it's all but certain he will
    endorse Barack Obama for
    president. Let the bidding begin.
    Barack Obama has already offered
    ten million dollars and a cabinet
    post if he will endorse Hillary
    Clinton instead.
    Bill Funk, Jan 22, 2007
  11. Guest

    Matt Ion Guest

    It's called pCAM... available from

    Matt Ion, Jan 22, 2007
  12. Guest

    Guest Guest

    This is probably the best I can hope for--and probably sufficient, if
    reliable. I need a camera that will show as much as a 28mm lens would on a
    35mm camera. I've noticed that digital cameras are a bit lacking in the
    wide angle department. Telephoto with anti-shake is very nice, but not a
    substitute for wide angle.

    I have a 24mm lens for my Canon film camera. The difference between this
    lens and the 28mm end of my 28-80mm zoom is dramatic--much more dramatic
    than the measily 4mm discrepancy would suggest. I use the 24mm more than
    either end of the zoom lens. 95% of my photos use either the 24mm or the
    50mm f/1.4. And while we're on that subject, why do you suppose that the
    extra 2 stops, relative to the f/2.8 zoom, come into play so often?

    Guest, Jan 22, 2007
  13. No. They are using an arbitrary but well defined, completely and insanely
    dizzy but industry standard, and confusing to folks with common sense method
    of specifying the size of the sensor.

    Here's what the 1/nn" sizes mean.

    David J. Littleboy
    Tokyo, Japan
    David J. Littleboy, Jan 23, 2007
  14. Guest

    King Sardon Guest

    Most point-and-shooters don't care what is written on the barrel of
    the lens; they compose so it looks right and shoot. But for the user
    who does care, the focal length numbers can be used to calculate DOF
    and magnification. And they can be used to plan shots. They are
    useful, and the interested user will soon be familiar with them.
    But the owner doesn't have to shoot all those cameras, only his own.
    And many if not most are not familiar with lenses designed for 35mm
    anyway... and the ones who know those focal lengths are a steadily
    diminishing number.

    King Sardon, Jan 23, 2007
  15. Yes, in the P&S a 28mm-equiv. is rare and a 24mm-equiv *very* rare.
    There's one Kodak model which goes so far as to have two lenses, a
    moderate zoom and a fairly wide lens. Which is strange since I didn't
    think the hard part was the lens itself, but the lens-sensor
    interaction. I don't know; market forces? Or some technical issue that
    would cost them whole dollars?

    In the DSLR world, there are a raft of special ultra-wide lenses made
    for the less-than-full-frame DSLR sensors. I've got the Tokina 12-24mm
    f/4, which is quite good (the worst aspect is the f/4, which they made
    no secret about :)). I previously had a 17mm lens for my film cameras,
    and several manufacturers made a 14mm (quite expensive though), so I'm
    not back to where I was with film (by one stinking mm), and not close to
    where I *could* have been with film (but then I *could* buy one of a
    couple of 10mm lenses for my DSLR, which at 1.5x brings me to 15mm,
    which puts me *close* to 14mm anyway).

    Also nobody that I know of has made a circular fisheye for 1.5x or 1.6x
    Yeah, I got a 24mm when I found myself using the 28mm end of a 28-90mm
    zoom much more than I expected (around 1983). And I found I used the
    24mm quite a lot, and around 1990 picked up a 20mm, and a couple of
    years later a 17mm. I was never interested in wideangles my first 10 or
    15 years in photography; go figure!
    David Dyer-Bennet, Jan 23, 2007
  16. Listen to him! And to the article at dpreview. He is precisely correct.

    While the nomenclature seems insane to us, it derives sanely from the
    nomenclature of the industry it arose from. Read the article at
    dpreview; but very *very* briefly, the video camera industry was used to
    thinking of the diameter of a vidicon tube, and the actual working
    surface was smaller than the tube diameter. That whole shebang got
    extended to describe CCD sensors for video cameras, and then was carried
    onwards to describe CCD sensors even if they were put into still
    cameras. But it didn't evolve in an attempt to confuse or defaud
    anybody; each step was sane and reasonable by itself.
    David Dyer-Bennet, Jan 23, 2007
  17. Guest

    Paul J Gans Guest

    Well, it *is* a physical property of the lens.
    I'm not sure it is all that intuitive. Many people, myself
    included, are not all that good at estimating angles.

    On the other hand, that information and the focal length would
    allow the sensor size to be determined.

    So perhaps all three numbers should be in the specs?
    Paul J Gans, Jan 23, 2007
  18. Guest

    ASAAR Guest

    Then the dimensions stated in camera manuals should be for the
    boxes that the cameras are shipped in. Just as (in)sane. :)

    For technicians that might have had to occasionally replace tubes,
    knowing the diameters of the tubes they'd be handling might well be
    more useful than knowing the dimensions of the invisible innards of
    the tubes. Photographers are more interested in knowing the actual
    diameter of the sensor's active surface than the size of some
    theoretical surrounding "tubes" that never existed for the sensors.
    ASAAR, Jan 23, 2007
  19. Do you think you know what, precisely, is "35mm" in some dimension? Or
    what "120" (and "220") actually mean? And what the actual dimensions of
    a "6x6" negative are? Our nomenclature has always been pretty obscure,
    I think; not directly physical for sure.
    David Dyer-Bennet, Jan 23, 2007
  20. It was brand new when I bought the Tokina, I discovered it existed at
    the camera store that day. I had a bunch of sample photos off the net
    from the Tokina, Tamron, older Sigma 12-24mm, and Nikon 12-24mm, and had
    decided the Tokina was used by photographers I liked (I put it that way
    because I wasn't really *directly* judging all that much about the lens
    by studying web-resolution photos).

    And I do use the 12mm end; more than I used my separate 17mm on film.

    Glad to hear Sigma has made another good lens, and that *somebody* has a
    good 10mm DX zoom.
    I borrowed the Sigma 12-24mm for 24 hours once and had a lot of fun with
    it -- but it was on a Fuji S2, so I didn't personally test the
    full-frame part. But yeah, that's what they claim, that's what I hear.

    If I wanted to more wider, I guess I'd get the Sigma 10-20; 2mm is *big*
    when it's the 2mm between 12 and 10. Rather than experimenting with
    lenses for rangefinder cameras in adapters, anyway.
    I've come to regard 24mm as the wide end of the "normal" range. If I
    could get a small, light 24-120mm (35mm equivalent) f/2 lens with superb
    optical quality, I could dispense with the vast majority of my lens
    collection pretty happily.
    David Dyer-Bennet, Jan 23, 2007
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