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what kind of "spot" is inside this lens?
  • peaceonearthpeaceonearth

    I bought a variogon 18-90 on ebay in mint condition. It arrivided and I wouldnt bescribe it as mint but still in a quite good condition. I am worried as there is "something" inside. It's in the center, it looks round, with a sharp egde, reflective in the outer circle and kind of tarnished in the center. I have no experience in lenses with fungus but I would expect it look different. I contact seller and he replied: Nothing to worry about the spot. It's internal filter / aperture, that enhances sharpness and contrast. It's factory inserted by default. Did someone ever hear about this kind oft filter??

    image

    Picture from seller, I did not realize the spot but at the auction but it is there too.

    image

    Unfortunately I dont have a c-mount adapter yet for test. What do you think?

    thanks for your time

  • 3 Replies sorted by
  • balazerbalazer

    It is a neutral density spot filter. It's a semi-transparent graduated spot inside the lens next to the iris. As you close the iris, a bigger portion of the aperture is covered by the spot. So the brightness is reduced more quickly as you close the aperture than it would be without the spot. At the smallest aperture settings, the aperture is entirely covered by the center of the spot.

    In this lens's original configuration it probably had a motorized controller for the iris. The neutral density spot filter is basically a way of remotely controlling the exposure with greater variation than you could achieve by the iris alone, to be able to handle very bright and dark conditions.

    I've seen spot filters that were silvered spots applied directly to one surface of a refractive element, and some that were grey spots on a flat piece of glass. The first type cannot be removed. The second type can be removed, if you open the lens.

    I've found the silvered type spots to greatly reduce the contrast and saturation as you close the aperture, making the lens not very useful except at the widest few aperture settings. (Depending on how large the spot is. The smaller, the better.)

    TV lenses sometimes have other variations on this idea, all with the aim of coupling the iris control with greater attenuation: an opaque star in the center next to the iris; graduated neutral density filters that close in on the aperture inside of the opaque diaphragm leaves.

  • peaceonearthpeaceonearth

    thanks!

  • nomadnomad

    They seem to have made different versions, mine has no such graduated spot.

    This seems absolutely possible, since these lenses were not only used for cine, but in scientific and remote-controlled applications too.

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