Understanding Lens specs for a dummy

Discussion in 'Digital Photography' started by Dave, Nov 11, 2007.

  1. Dave

    Dave Guest

    I find it very difficult to understand lens specs.
    Such as: 500mm lens.
    In down to earth plain language, how does this translate into everyday
    useful information?
    Is this a zoom lens?
    How much zoom?

    For example:
    EF 500mm f/4L IS USM
    Super Telephoto Lens

    Item Code: 2532A002


    Focal Length & Maximum Aperture
    500mm 1:4.0

    Lens Construction
    17 elements in 13 groups

    Diagonal Angle of View

    Focus Adjustment
    Inner focusing system with USM

    Closest Focusing Distance 4.5m / 14.8 ft.

    Anyway, does anyone know of a website that explains lens specs in simple
    down to earth, non-esoteric language
    that is useful and understandable to a non-professional such as myself? Or
    does anyone have simple down to earth
    notes to that effect?

    I did a test of sorts with my lousy DMC-FZ50 P&S camera. With my camera on
    a tripod at a distance of 18.3 ft.
    (measured from the front of the lens) to the subject (a foot long ruler
    horizontally positioned), I took 3 photos:
    1) first with the lens set at 35mm equiv. 2) second with the lens set at
    210mm equiv. (6X). 3) Third with the lens set at 420mm equiv..
    I measured the width of the ruler in photoshop using pixels as the unit of
    1) first with the lens set at 35mm equiv. measured 206 pixels 2) second with
    the lens set at 210mm equiv. (6X). measured 1195 pixels 3) Third with the
    lens set at 420mm equiv. measured 2157 pixels.

    I don't get how the X value equates to the image size differences, since
    1195/206 = 5.8 and 2157/206 = 10.47
    Wouldn't the actual X values be 5.8X and 10.47X?
    Dave, Nov 11, 2007
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  2. Dave

    Paul Furman Guest

    500mm is a very long telephoto lens.

    f/4 is the max aperture, pretty fast for such a long lens.

    L is 'Luxury' built for abuse with the best specs.

    IS = Image Stabilized gyro built into the lens for hand held without a
    tripod. In-lens stabilizing, makes the viewfinder image stable versus
    sensor stabilized.

    USM means UltraSonic Motor in the lens (instead of a crank shaft from
    the motor in the body) for fast quiet autofocus.
    On the lens they write 1:4 or on paper, they write f/4
    Hard to get much meaning out of this without a detailed discussion
    Inherent in a 500mm lens
    This is useful info for doing closeups. Perhaps more useful is the
    magnification where 1:1 is a macro lens.
    The x value in a zoom lens refers simply to the relative range 35-420 is
    420/35=12x this is not like the terminology for binoculars or
    telescopes. Magnification in photo gear is usually defined with the
    focal length, 500mm in 35mm equivalent field of view (fill frame/film)
    or simply the angle of view: 5 degrees diagonal.

    Most DSLRs have about a 1.5x crop factor so that 500mm lens looks like a
    750mm lens. The angle of view is stated for a full frame camera.

    P&S compact cameras have much more depth of field than DSLRs so that's
    something which is going to look different at the same 35mm equivalent
    focal length.
    Paul Furman, Nov 11, 2007
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  3. Dave

    mianileng Guest

    The mm part of a lens spec is the focal length. It's the distance
    between the film or sensor and the lens when it is focused on a
    very distant object - technically at an infinite distance.

    The higher the mm number (long focal length), the bigger the
    subject appears in the photo. IOW, a long focal length can pull
    in distant objects so that they appear closer.

    500mm falls well into the telephoto category - those that make
    objects appear substantially closer than when viewed with the
    human eye.

    However, focal lengths may be given in actual or equivalent
    values. Equivalent focal lengths are given in terms of 35mm film
    cameras. Most digital cameras have sensors that are physically
    smaller than 35mm film, so that they need a shorter focal length
    for the same apparent image size. For example, for a particular
    camera, a lens with a focal length of 10mm may produce the same
    apparent image size as a 35mm film camera with a 60mm lens. This
    camera may be cited as having a 60mm (35mm equivalent) lens.

    Zooms are a different matter. A zoom lens is one whose focal
    length can be varied so that objects can be made to appear closer
    or further away.

    A typical compact camera may have a zoom range from 35-105mm
    equivalent (the actual focal length range may be 6-18mm). The
    longest focal length is three times the shortest, and is specced
    as having a 3x zoom range. A 3x zoom lens may also be 28-84mm or
    40-120mm or any other value with a 3:1 ratio.
    mianileng, Nov 11, 2007
  4. The focal length is 500mm. The 'normal' lens is generally considered around
    50mm(*), so that 500mm lens has a magnification or tele-factor of about 10.
    In short: a very powerful tele-lens which typically you won't be able to

    *: important note: the 50mm are for a full-frame SLR. For a DX sensor it
    would be smaller by a factor of 1.5 or 1.6, i.e. around 32 to 33mm and this
    tele becomes a 15x or 16x tele.
    No it isn't. If it were a zoom lens, then both, the minimal and maximal
    focal lengths, would be listed, e.g. 200-500mm would be a zoom lens with a
    zomm factor of 2.5x where even the shortest lenght would still give you a
    magnification of 4x.
    The f/4 means that the maximum apperture of this lens is 500/4mm, i.e.
    In laymen terms: the maximum apperture tells you how much light the lens
    will capture and thus indirectly how fast the shutter speed can be.
    All lenses with the same apperture can use the same shutter speed given the
    same light conditions regardless of their different focal lengths. This is
    what makes the apperture information so important.
    IS is Canon's designation for lenses with active vibration reduction.
    Got that part covered already
    The lens has a total number of 17 glass elements, some of them glued/merged
    together, such that only 13 individual pieces are actually mounted inside of
    the barrel.
    The diagonal on the picture will correspond to a 5 degree angle of view in
    nature. Again this number is probably for a full-frame and has to be reduced
    by 1.5 or 1.6 for a DX sensor.
    'Inner focus' means the lens does not extend (i.e. does not change length)
    when focussing.
    No idea what the USM designation is, I don't use Canon.
    You cannot but an object into sharp focus unless it is at least 4.5m away.

    For Nikkor lenses there is a good overview of the cryptic designation on
    http://bythom.com/lensacronyms.htm. I suppose something similiar may exist
    for Canon, too.

    Jürgen Exner, Nov 11, 2007
  5. Well, sorry, no. The number on the lens is _always_ the actual focal lenght.
    There is nothing like an equivalent focal lenght for a lens. Putting it the
    way you did it just confusing.
    Well, poor terminology. Not the lens is equivalent (much less the 35mm) but
    it's observable behaviour. Better to say
    On this camera this lens has an angle of view/magnification factor/behaviour
    that is equivalent to a 60mm lens on a full-frame camera.

    Jürgen Exner, Nov 11, 2007
  6. The 500 mm figure is the focal length. The image size of a distant
    object is directly proportional to the focal length. On a 35mm camera
    or most digicams 500mm would be thought of as a telephoto, these
    days. The name used to mean something else, but now means a long lens
    meant for "blowing up" size on a distant object. Note that on a large
    camera, such as an 8 x 10 film or view camera that would only be a
    mild telephoto.

    f/# is the maximum aperture diameter divided into the focal length.
    So the actual diameter of a 500 mm f/4 would be 125mm.

    17 elements means there are 17 seperate pieces of glass, most cemented
    together but with air gaps between two of threee sections (each
    section is several elements glued together.
    Don Stauffer in Minnesota, Nov 11, 2007
  7. Dave

    mianileng Guest

    I don't know what point you're trying to make here. I tried to
    explain the difference between actual and "35mm equivalent"
    focal lengths as the latter is often - perhaps *more* often -
    cited in camera specs, especially with P&S cameras. The
    terminology is not mine. It is well established in the trade a
    well as in the field.
    mianileng, Nov 11, 2007
  8. Dave

    mianileng Guest

    I somehow missed seeing the rest of the OP's post after the
    part I cited. Otherwise I would have concentrated more on
    explaining the figures and terms in the rest of the message,
    as has been done by others.

    In any case, the zoom ring on my FZ30 is graduated in 35mm
    equivalent values, and I believe it's the same on the OP's FZ50.
    "35mm equiv. 35-420" is also printed on the side. The actual
    focal length is printed on the front of the lens where it is
    much less conspicuous, and much less relevant for most users.

    There are so many different sensor sizes, with different
    results obtained with the same focal lengths, or, conversely,
    different focal lengths are required to get the same apparent
    image size. Therefore, it is often more convenient, and less
    confusing, to cite "35mm equivalent" focal lengths.
    mianileng, Nov 11, 2007
  9. Dave

    Ron Guest

    We need to get away from referring Lens length to film cameras. You re
    referencing something they have never used. Be it a 35mm, 6x7cm or 4x5"
    camera its irrelevant most new photographer have never used any of these
    camera so they don't have a clue what you are talking about.

    Explain it in terms of Magnification like 15x magnification of a distance
    object when used with a Cannon camera.
    Ron, Nov 11, 2007
  10. I do not know if this terminology is established or not. My point is, that
    it is missleading at best.

    A single lens can have many different "35mm equivalent" focal lenghts,
    depending upon which camera it is used with. For arguments sake let's take
    e.g. a Sigma 200mm.
    On a full-frame camera it behaves like a 200mm lens, on a Nikon DX like a
    300mm lens, and on a Cannon DX like a 320mm lens. Therefore it is just
    meaningless to assign _a_single_ "35mm equivalent" number to this lens.

    Same with the 500mm UMS IS the OP cited except that mounting it on anything
    but Canon is a bit more involved.

    Jürgen Exner, Nov 11, 2007
  11. Dave

    mianileng Guest

    How big an object appears to be in the image depends on the
    ratio between lens-to-subject distance and sensor-to-lens

    The focal lengths given are the distances between lens and
    sensor for subjects at an infinite distance. When the subject
    is closer than infinity, the lens has to move away from the
    sensor to maintain proper focus. The movement will be more for
    longer focal lengths. This changes the magnification factor.
    Your 18.3 ft distance is much less than even an approximation
    of infinity.

    Again, even when the "actual" focal length is cited (7.4-88.8mm
    in your FZ50), this is still an equivalent number. A practical
    camera lens is made up of many separate lenses of different
    focal lengths, and their combination gives a single equivalent
    focal length, with an equivalent single, virtual lens
    positioned somewhere on the axis.

    A lens with multiple elements can be more compact than a single
    lens of the same focal length, though this is not the only
    reason for using multiple elements.

    The Canon lens you're interested in is 15.2" long. If this
    were a single lens, it would have to be almost 20" in front of
    the sensor when it's focused at infinity. To focus at 18.3 ft,
    the lens would have to move outwards by about 2", making it
    necessary for the whole structure to be nearly 2 ft long.

    If such a single lens were actually used, then the focal length
    and the sensor-to-lens distance will be the actual physical
    values (but the optical quality will be poor).
    mianileng, Nov 11, 2007
  12. [I missed the second part of your post below those awfully long horizontal
    lines at first]
    That would be the front focal lenght. Distance for photographic lenses is
    measured at the rear principal plane which is typically somewhere near the
    end of the lens. See http://en.wikipedia.org/wiki/Focal_length for details.
    Forget about "real" focal lenght or "equiv" focal lenght, it is not relevant
    in this context.
    I would say those values are amazingly close to what you would expect.
    35mm being your base unit, then 210mm is a magnification of 6 times (as you
    noticed already) and 420 a magnification of 12 times.
    Well, yes, but:
    - there is always some inaccuracy in us more mortals doing simple
    experiments like that
    - you were using the front focal point
    Given that you were measuring from the front of the lens instead of from the
    pincipal plane your numbers match up _very_ nicely.

    Jürgen Exner, Nov 11, 2007
  13. I totally agree. The notion of "35mm equivalent" is antiquated and
    Well, I'm afraid unfortunately it's not that easy because magnification is
    not an inherent property of the lens alone. So saying 15x on Canon is wrong
    because it can be correct only for either the full-frame or the DX, but not
    for both.

    Listing all the combinations of a particular lens with all possible bodies
    is just not feasible, either. Who would update the specs and information for
    a lens every single time a new body is released?

    Actually the focal length of a lens is the only inherent property of a lens
    you can go by. And if people would just learn the "normal" focal distance of
    their camera (50mm for full-frame, 33mm for Nikon DX, 32mm for Canon DX,
    ....) then all the rest like magnifaction factor etc. of lens-body
    combinations becomes trivial to calculate.

    Jürgen Exner, Nov 11, 2007
  14. Dave

    Marty Fremen Guest

    Magnification WRT to what though? You have to define what 1x magnification
    is. With things like binoculars it's easy because you're talking about the
    view through the eyepiece, which you compare to the naked eye view, but
    this is irrelevant with a camera (and indeed meaningless if you don't have
    an optical viewfinder).

    The only objective and sensor-independent way of describing the field of
    view you get with a lens is the actual angle of view. But whilst this is
    fine for medium to wideangle optics, people have difficulty visualising
    things like a 1° or 5° angle of view. Nevertheless if manufacturers would
    switch to it people would start to get a feel for the figures I think.

    The magnifying ability of a lens is not quite the same thing as field of
    view (though in practice the two tend to be closely related). Ultimately it
    comes down to its angular resolving power which determines how big you can
    make the image when viewing it. For instance I was just comaring lens tests
    for a camera with a 35-350mm lens and one with a 35-390mm lens. Now you
    would think the latter would give you 10% more magnification, but its lens
    was slightly softer with 10% less resolution, so in the end they came out
    the same in terms of the degree of magnification that was achievable in the
    final image. Indeed the 350mm could end up the better bet, since the
    shorter focal length would give a slightly wider field of view at maximum
    Marty Fremen, Nov 11, 2007
  15. Dave

    mianileng Guest

    I agree that the present situation can be confusing to someone
    who's not familiar with the pre-digital era.

    But "15x magnification" relative to what ? The term "15x
    magnification" by itself, even when applied to a particular
    camera, is meaningless.

    Even the actual focal length or magnification ratio has to be
    related to *something* in order to have an idea of how it will
    perform. If you tell a complete novice that model X comes with
    an 18-54mm kit lens, he will still have no idea what to expect
    in terms of image size even if he understands that the zoom
    range is 3x. After using that lens for some time, he will know
    what to expect from a 70-135mm lens on the same camera.

    The point is that everything is relative. But there's no
    universal standard to relate to, except the 35mm film camera,
    however unsatisfactory that might be.
    mianileng, Nov 11, 2007
  16. Dave

    Dave Guest

    Thanks Paul.

    Dave, Nov 13, 2007
  17. Dave

    Dave Guest

    Dave, Nov 13, 2007
  18. Dave

    Dave Guest

    I somehow missed seeing the rest of the OP's post after the
    part I cited. Otherwise I would have concentrated more on
    explaining the figures and terms in the rest of the message,
    as has been done by others.

    In any case, the zoom ring on my FZ30 is graduated in 35mm
    equivalent values, and I believe it's the same on the OP's FZ50.
    "35mm equiv. 35-420" is also printed on the side. The actual
    focal length is printed on the front of the lens where it is
    much less conspicuous, and much less relevant for most users.
    Yes, what you are saying about the numbers on the camera itself is exactly
    35mm equiv. 35-420" is also printed on the side (rotating adjustment ring)
    7.4-88.8 ASPH on the front of the lens.
    Thanks for taking the time and trouble!

    There are so many different sensor sizes, with different
    results obtained with the same focal lengths, or, conversely,
    different focal lengths are required to get the same apparent
    image size. Therefore, it is often more convenient, and less
    confusing, to cite "35mm equivalent" focal lengths.
    Dave, Nov 13, 2007
  19. Dave

    Dave Guest

    So, in actuality would the 200-500mm lens would really be a 4x-10x zoom
    range lens?
    Dave, Nov 13, 2007
  20. Hmmmm, well, I guess. That 200-500mm lens has a zoom factor of 2.5x
    (500/200) and on a _on_a_full_frame_(!!!) camera a magnification factor of
    4x to 10x. If that's what you mean with zoom range, then yes.
    Normally people would call the 200-500mm the zoom range of this lens,
    because the zoom range is independant of the camera while the magnification
    factor is not.

    Jürgen Exner, Nov 13, 2007
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