Fixed Aperture Zoom Lens

Discussion in '35mm Cameras' started by Mike - EMAIL IGNORED, Nov 11, 2003.

  1. As I understand it, the minimum f-stop of a lens is equal
    to the focal length divided by the diameter of the lens.
    For a given diameter of the lens, the longer the focal
    length, the larger the minimum f-stop. This is why most
    zoom lenses have a range of minimum f-stops.

    However, there are some zoom lenses for which the minimum
    f-stop is constant. On the face of it, it would appear
    that to accomplish this, the minimum f-stop would be
    specified for the longest focal length, and for shorter
    focal lengths, some potential lower f-stop capability is

    I imagine that there is something wrong with this
    reasoning. Illumination would be much appreciated.

    Mike - EMAIL IGNORED, Nov 11, 2003
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    Tony Spadaro Guest

    But that is exactly how it is done. At shorter focal lengths the aperture is
    Tony Spadaro, Nov 12, 2003
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  3. The f-stop is the calculation of the focal length divided by the
    diaphram opening. For sake of simplicity lets condider a 50mm f/2
    lens. The maximaum opening of the aperture diaphram is 25mm (50mm / 2
    = 25mm). A variable aperture zoom can allow for the same physical
    size of maximum aperture diaphram opening in terms of absolute
    measurement, but the maximum f-stop will consequently decrease (move
    to a higher f-stop number) as the focal length increases. A fixed
    aperture zoom requires a more complex design whereby the physical
    measurement of the diaphram opening must increase as the focal length
    increases. For example, a theoretical 50-100mm f/2 zoom lens would
    require the maximum physical aperture diaphram opening to change from
    25mm at the minimum end of the focal length range to 50mm at the
    maximum end of the focal length range, and also to change at a
    constant rate throughout the entire zoom range. Now, if you take that
    same theoretical lens and make it a 50-100 f/2-4 variable aperture
    zoom, then you'd only need one maximum physical aperture diaphram
    opening of 25mm.

    street shooter, Nov 12, 2003
  4. If I understand this, you are agreeing that, by virtue of the
    more complex design, glass is being wasted at the longer
    focal lengths. Why is this good?

    Mike - EMAIL IGNORED, Nov 12, 2003

    brian Guest

    Fixed aperture zooms have been available for more than forty years,
    are straightforward to design, and do not require variable diameter
    diaphragms. Most early zooms were required to have fixed f/#'s
    because of metering considerations. The classic P-N-(P or N)-P
    configuration *does not* require a variable diaphragm opening to
    maintain a constant f/#. All that is required is to keep the
    diaphragm stationary with respect to a rear group which is not moved
    during zooming. This basic design has been reworked literally
    thousands of times for all manner of applications, ranging from huge
    TV broadcast zooms to 35mm zooms to camcorder zooms.

    To see what I'm talking about, consider a ray diagragm for the
    50-135mm f/3.5 Nikkor:

    brian, Nov 12, 2003

    Tony Spadaro Guest

    It is not being wasted. The lens design is optimized for the limited
    Tony Spadaro, Nov 12, 2003
  7. I think some of the replies you have received have been misleading.
    There is no "wastage" of usable aperture involved in fixed-aperture

    Whether a zoom lens has a fixed or variable aperture depends on the
    design. Most modern designs achieve smallness and lightness by a system
    known as "mechanical compensation", which moves various elements axially
    using complex mechanical cams. This has the effect that the aperture
    stays the same physical size, and hence the f-number varies as the focal
    length varies.

    Other designs (which AIUI included the earliest traditional designs) use
    a system of optical compensation, and in these designs the aperture is
    located in a different place; this varies in effective size as the lens
    is zoomed, in proportion to the change of focal length, and the effect
    is a constant f-number. These zooms tend to be larger and heavier. In
    the days of external metering and manual flash metering they were much
    preferred (for obvious reasons) but now the mechanical compensated types
    are cheaper to make and the optical type seem to be mainly used in
    top-level pro zooms where the size is more acceptable.

    If you want more detail, try "Applied Photographic Optics" (Sydney F
    Ray), though I must say I found the explanation in there rather lacking
    in detail.
    David Littlewood, Nov 12, 2003

    Don Stauffer Guest

    A better definition is that the f/# is the focal length divided by the
    aperture diameter, not neccesarily a 'lens' diameter. The actual
    limiting aperture is generally INSIDE the lens, and depending on its
    position may not be the actual physical size of an aperture. If the
    physical aperture is not at a given point in the lens, the effective
    value for the aperture may be a magnification (or minification) of the
    actual diaphram diameter, and this value may change as lens is zoomed.
    The actual value for the aperture is a complicated story.
    Don Stauffer, Nov 12, 2003

    brian Guest

    Hi David:
    Mechanical compensation in a zoom isn't directly related to whether it
    has a fixed relative aperture or not. The 50-135mm Nikkor I mentioned
    above is a good example of a simple mechanically compensated zoom
    which has a constant f/#. All the mechanical compensation does is
    maintain focus by moving a small compensator group without having to
    move the entire lens.

    Optically compensated zooms are pretty much completely obsolete, and
    were used back in the days when designers had a mental block against
    the use of cams. Part of this mental block was due to the belief that
    mechanically compensated systems would inevitably result in "holes" in
    the zoom range where certain focal lengths were unattainable due to
    the compensator crashing into adjacent elements. Proper design
    techniques have eliminated this problem, however.

    brian, Nov 13, 2003
  10. Hi Bryan,

    In which case it seems Ray is out of date - this does not surprise me, I
    did not find the explanation in there either clear or convincing.

    I guess the point is that some techniques place the aperture in a
    position where it gives a constant f-number, and some don't, which is
    sufficient to answer the OP.
    David Littlewood, Nov 13, 2003
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