Question about Sigma 18-125

Discussion in 'Digital Photography' started by Gautam Majumdar, Jan 8, 2005.

  1. I was reading reviews of this lens. Not many I could find. But one
    reviewer said that as this lens is for digital cameras only, its 18mm
    focal length setting is true 18mm for digital cameras. That is, it is same
    as 18mm on a film camera rather than being converted to 28mm as normally
    happens with other lenses.

    Is that true ? Has anybody in the group used it and found that to be
    correct ?
    Gautam Majumdar, Jan 8, 2005
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  2. Gautam Majumdar

    Roy Guest

    Hi there.

    The Focal Length of any lens is defined by the Laws of Optics. So an 18 mm
    is an 18mm, irrespective of the size of the Camera to which it is attached.

    On a Digital Camera that will produce a Wide Angle view of the subject. On
    a 35 mm it would produce an extreme Wide Angle View, and on a 120 Roll Film
    Camera it would produce a Fish Eye Type Image, (probably just a small
    circular image on the film).

    The confusion arises because Camera and Lens manufacturers compare the View
    effect of Lenses between Digital and 35mm. Saying that an 18mm on a Digital
    produces a similar view to a 24mm lens on 35mm Film Cameras.

    It is exactly the same situation as comparing 35mm Camera lenses to 120 roll
    film Camera lenses. A 50mm lens on a Roll film Camera is a Wide Angle Lens,
    on a 35mm Camera it is a Standard Lens, and on a Digital Camera it is a
    Short Tele Lens.

    It has all to do with the sizes of the Image and Angles - big 120 Negative,
    smaller 35mm Negative, even smaller CCD

    Lenses produced specifically for Digital Cameras do not need to produce as
    large a Circle of Light at the CCD, as lenses for 35mm Cameras, because the
    CCDs are smaller than the 35mm Frame. BUT the Focal Length will be the Focal

    God help us all if Camera Manufacturers start applying Hype and Distorting
    the Truth about Focal Length of Lenses.

    Roy, Jan 8, 2005
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  3. Sigma manufactures lenses that are made for digital SLRs - noted by the "DG"
    moniker. However, there is still the multiplication factor involved. It is
    not a "true" 18mm on a digital camera as it would be a true 18mm on a film
    camera. With this lens on my DSLR, it would be 28.8 mm (the mulitiplication
    factor for my DSLR is 1.6) and unless you have a DSLR with a full frame
    sensor then there is also a multiplication factor involved.
    Peter A. Stavrakoglou, Jan 8, 2005
  4. Gautam Majumdar

    Roy Guest

    Oh. Yes it is!

    An 18mm Lens is an 18mm Lens irrespective of whether it is fitted to a 35mm
    Film Camera, a Digital Camera or is fitted to nothing.

    When you talk about Multiplication Factor, you are discussing the Image
    Size, not the Focal Length of the Lens. The Image Size is what would change
    if you fitted this Lens to a 35mm Camera. It is still an 18mm Lens.

    As I said an 18mm on a Digital Camera is a wide Angle Lens. On a 35mm
    Camera it is an Very Wide Angle.

    On Peter's Camera it will give the same Image Size as a 28.8mm Lens would on
    a 35mm Camera. Hence his use of the multiplier of 1.6.
    On a Nikon D70, it would give the same Image Size as a 27mm Lens on a 35mm
    Camera, multiplier of 1.5.

    BUT --- It is still an 18mm Lens, and can only be described as 18mm.

    Those multipliers are only enabling us to mentally visualise the Image
    Sizes, because we know (or should do) what Image Sizes a 28mm and a 24mm, or
    whatever, on a 35mm Film Camera will produce.

    Could you imagine what would happen if Sigma started to describe this lens
    as a Nikon Fit 27mm, and Canon Fit 28.8mm. Every time a new size of CCD was
    produced, every existing lens would have yet another Focal Length
    description applied to it.

    Lens makers should stick to actual Focal Lengths, and we can use
    "Multipliers" in our Imagination until we get used to Digital Cameras with
    relatively small CCDs, and start to know what Image Size an 18mm lens will

    BUT -- That 18mm Lens is still a True 18mm Lens, and will always be a
    True 18mm Lens.

    Roy, Jan 8, 2005
  5. Of course you are correct, I knew that but perhaps I haven't explained it
    properly. I have heard people say that if you take an 18mm lens made for
    digital (such as a Sigma DG) then there is no multiplication factor - the
    image on a DSLR would be 18mm just like it would on a 35mm SLR. That is not
    true and that is the point I was making. We all know that if you shop in
    the wrong places you can get some really bad information from salespeople,
    like I've heard in Circuit City and Best Buy.
    Peter A. Stavrakoglou, Jan 8, 2005
  6. Gautam Majumdar

    John Bean Guest

    In any case, how could the lens "know" whether it was using a crop factor of
    1.5, 1.6, 1.7... Obviously silly.

    As you say, 18mm is 18mm...
    John Bean, Jan 8, 2005
  7. Thanks very much. That is what I thought. But the reviewer specifically
    mentioned that it is a true 18mm equivalent on DSLR, so I asked for

    By the way, are you happy with the performance of this lens ? I am
    thinking of buying it as the standard lens to remain on my 300D most of
    the time.
    Gautam Majumdar, Jan 8, 2005
  8. Gautam Majumdar

    MTBike1970 Guest

    The lens doesn't "know" what the multiplication factor is...that's
    determined by the size of the imaging sensor in the camera.
    A smaller sensor will give you a narrower field of view than a larger
    This mental experiment should help... take a picture with a 35 mm film
    camera and a 35 mm lens, then print an 8 x 10. If you crop the center of
    the negative then enlarge the center to print an 8 x 10, it will look like
    you took the second picture with a 50 mm or longer lens. The lens focal
    length hasn't changed, the field of view has changed. That's what happens
    when CCD or CMOS sensors are smaller than a 35 mm negative.

    The OP question wasn't answered completely... The 18 mm lens on a Nikon D70
    dSLR is still 18 mm, but it will give you the same field of view as a 27 mm
    lens on a 35mm film camera. This lens only works properly on dSLRs with
    sensors that are no larger than the APS size, like the D70. On film
    cameras, or dSLRs with full frame sensors, the image will be severely
    vignetted. This is because the physical size of the image projected by the
    lens on the sensor is smaller than lenses designed for 35 mm or full frame
    sensors. It's the same thing as putting a 35 mm lens on a medium format may work and produce an image on the film, but it'll be a small
    circular image in the center of the negative.

    MTBike1970, Jan 8, 2005
  9. Gautam Majumdar

    MTBike1970 Guest

    There's a full page ad for this lens in the December issue of Popular 57. The copy specifically states that this lens should
    not be used with cameras having sensors larger than APS-C equivalent or with
    35mm film cameras.
    This lens is 18 mm at the widest, but it will give you an image with the
    same field of view of as a 27 mm lens on a film SLR.
    MTBike1970, Jan 8, 2005
  10. Gautam Majumdar

    Bill Tuthill Guest

    Bill Tuthill, Jan 8, 2005
  11. Gautam Majumdar

    Tony Guest

    They are referring to the size of the image circle rather than the focal
    length. In the case of this lens it would simply not adequately cover the
    24x36mm of a 35mm negative but will cover the smaller size (APS and smaller)
    of most digital sensors in bodies based on 35mm prototypes. There are a few
    cameras with full 24x36 sensors (or close to that) which would not be good
    choices for this lens. If you own Canon I would suggest you avoid Sigma
    products as they will not license the mount and therefore have later
    compatibility problems - which they claim they will fix but won't.
    In general Sigma is for temporary.

    home of The Camera-ist's Manifesto
    The Improved Links Pages are at
    A sample chapter from "Haight-Ashbury" is at
    Tony, Jan 8, 2005
  12. Gautam Majumdar

    Steve Wolfe Guest

    Sigma manufactures lenses that are made for digital SLRs - noted by the
    I believe it's "DC", not "DG". Are you sure that the crop-factor is still
    present? Sigma isn't entirely clear, but does say that " Image circle is
    designed to match the size of the image sensor of digital SLR cameras ".
    While it isn't perfectly clear, it sounds like they had the APS-C sensor
    size in mind when designing it, which would eliminate the crop factor - or
    at least reduce it from something like 1.5/1.6 to 1.05 or so, depending on
    whether you've got a D70 or a DigiReb.

    Steve Wolfe, Jan 9, 2005
  13. Gautam Majumdar, Jan 9, 2005
  14. It is "DG" and there is still the crop factor.
    Peter A. Stavrakoglou, Jan 9, 2005
  15. Gautam Majumdar

    Mark B. Guest

    No, it doesn't eliminate the crop factor. What is meant by it being
    designed for digital is that it is designed to project the image circle on a
    APS sized sensor. To do that, the lens is smaller & lighter, but the crop
    factor remains. If you were to use it on a 1.3x or full-size sensor camera,
    you would get vignetting.

    Mark B., Jan 9, 2005
  16. Gautam Majumdar

    John Bean Guest

    John Bean, Jan 9, 2005
  17. It appears that we are both correct, according to the Sigma lens catalogue I
    am holding in my hands on first reading. It reads "DG (Digital) Lenses" and
    also "DC (Digital) Lenses". However, you win this one because further
    reading of the catalogue under the "DG (Digital) Lenses" section states that
    these lenses "are equally suited for digital and analogue cameras". If you
    could see it you would note how confusing the catalog is on this matter.
    Peter A. Stavrakoglou, Jan 9, 2005
  18. Sigma's website tells that the angle of view is 69.3º - 11.5º which
    corresponds to ~30 - 210 mm lens for a film camera. So the 1.6
    multiplication factor is there.
    Gautam Majumdar, Jan 9, 2005
  19. Gautam Majumdar

    Mark B. Guest

    Is there an echo in here?
    Mark B., Jan 9, 2005
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