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Thread: Changing Focal Length with constant Aperture

  1. #1

    Changing Focal Length with constant Aperture

    Consider a focal length specification as listed below

    16x zoom: 5.0 (W) 80.0 (T) mm
    (35mm film equivalent: 28 (W) 448 (T) mm)

    I want to take two photos in f/3.5 aperture. I want to take these photos in 6.3 mm focal length and 7.5mm focal length.

    Suppose, I have a camera with specifications listed in

    What are the set up that I need to make to get the photos in required focal lengths?

    What are the factors that determine the focal length given a constant aperture value?

    Note: At present I am using 4x zoom

  2. #2
    davidedric's Avatar
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    Re: Changing Focal Length with constant Aperture

    Not sure I have properly understood your question, but what you would normally do is select aperture priority (Av), set the aperture to 3.5, and zoom to the focal length that you want. However, following your link f3.5 is only available at the shortest focal length, so I think that if you use Av you will get the widest aperture available at that focal length.
    Last edited by davidedric; 2nd June 2013 at 08:30 PM.

  3. #3

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    J stands for John

    Re: Changing Focal Length with constant Aperture

    I am sure it is a good camera but the description is typical advertiser's guff 720 might have been HD some time ago.
    One of the problems is that in the desire to meet a presumed consumer demand for compactness it is not normal for makers to make constant zooms ... exception is Panasonic's FZ200, and the FZ20 before it, which have a constant f/2.8 throughout the zoom .... so most zooms loose aperture as you zoom so you simply cannot do what you are trying to do with the camera mentioned ... but if you select a smaller aperture the use of A mode is the answer for you as Dave wrote. With my f/4-f/5.8 zoom I would have to choose f/5.8 to work at a constant aperture thoughout the zoom ... in practice I pick f/6.3, don't ask me why

    By taking a series of pictures and checking the EXIF it would be possible to find a larger aperture that would work at the focal lengths you mention but I don't know any camera that you can find those focal lengths until after the exposure ... the camera knows what it is doing but it doesn't tell. My camera [lens] does tell me the max aperture when in part zoom but I don't know what the focal length is with accuracy and it is marked in equivalant focal lengths rather than actual.

    "What are the factors that determine the focal length given a constant aperture value?"
    A question the designers face of course though I think the question should be reversed to "How do you maintain a constant aperture when the focal length changes"
    Last edited by jcuknz; 2nd June 2013 at 11:02 PM.

  4. #4
    rpcrowe's Avatar
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    The oxymoron of the terms "Constant Aperture" and "Variable Aperture"

    The f/stop is determined mathematically by the ratio of the lens's focal length to the diameter of the aperture.

    We talk about a constant aperture in a lens such as the 70-200mm f/4L IS and a variable aperture in a lens such as the 70-300mm f/4-5.6 IS.

    The "constant aperture" of the 70-200mm f/4L IS is actually a "variable aperture" in that the size of the aperture increases as the lens is zoomed to a longer focal length. This keeps the ratio between aperture and focal length the same - resulting in the same f/stop throughout the zoom range. It is more complicated and usually results in a more expensive lens. It is also "at this time" not possible to have a constant f/stop of f/2.8 or wider in a zoom lens with a focal range of over 3x.

    And actually, the "variable aperture" of the 70-300mm f/4-5.6 IS is a really "constant aperture" which stays the same throughout the zoom. This results in a variable ratio between the focal length and the aperture as the lens is zoomed to a longer focal length; resulting in an f/stop which gets smaller (f/4 to f/5.6 - larger numbers mean smaller f/stop) as the focal length is increased.

    BTW: there have been motion picture lenses, like the 12-120mm f/2.5 Angenieux which maintained a constant f/stop throughout a 10x zoom range, availble for at least fifty-years! This lens also produced excellent image quality which shows you that it was easier to formulate a lens for the 16mm format than for the 35mm still camera format. The first still camera zoom lens that I used was the terrible 43-86mm Nikkor Zoom. The first time I used it, I thought it was just wonderful to be able to vary my focal length without changing lenses. But, when I saw my proofs, I never used the lens again...
    Last edited by rpcrowe; 3rd June 2013 at 01:07 AM.

  5. #5
    Moderator Manfred M's Avatar
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    Manfred Mueller

    Re: Changing Focal Length with constant Aperture

    I suspect that you are being limited by your camera.

    Point and shoot cameras generally have very limited control over the settings and have a high level of automation, which may be difficult if not impossible to get around. Shooting the way you want to would be trivial with a DSLR, but quite impossible with the point & shoot and cross-over cameras I own.

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