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Thread: 150mm f5.6 Symmar-S Image Circle

  1. #1

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    150mm f5.6 Symmar-S Image Circle

    This lens shows image circle at 220mm @ f/22. On my 5x8 camera (yes, you read that correctly, it's 5x8)... there is what seems to be even illumination in the corners even at f/5.6. The diagonal for this format is 238mm. I thought I understood that an image circle is usually larger at higher f-numbers. What am I missing? How is this lens covering my format?

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    Re: 150mm f5.6 Symmar-S Image Circle

    Is it even illumination to they eye, or on film also?

    Eyes can be subjective and film is quite a bit more objective.

  3. #3

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    Re: 150mm f5.6 Symmar-S Image Circle

    Tom, posters on this forum use two different coverage definitions. Circle illuminated, which is what you see on your camera's GG. Circle of good enough definition, which can be discerned clearly only on film. This is what lens makers use.

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    Re: 150mm f5.6 Symmar-S Image Circle

    Are you focused at infinity, or closer?

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    Re: 150mm f5.6 Symmar-S Image Circle

    Quote Originally Posted by BrianShaw View Post
    Is it even illumination to they eye, or on film also?

    Eyes can be subjective and film is quite a bit more objective.
    Well, it is clearly illuminated to the edges on the ground glass. If I can see even illumination then it should appear even on film also? Again, trying to understand what I'm missing. And I fully realize I may have to just do a test with film to see...

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    Re: 150mm f5.6 Symmar-S Image Circle

    closer than infinity.

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    Re: 150mm f5.6 Symmar-S Image Circle

    Now this is very confusing. It's either illuminated or not... When I look through the cut corners I can clearly see light from the lens at all apertures. Again, just trying to figure out if I have coverage before I use up film...

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    Re: 150mm f5.6 Symmar-S Image Circle

    Quote Originally Posted by TomN View Post
    closer than infinity.
    Keep in mind that the image circle expands as you extend the bellows to focus closer. Imagine projecting a cone of light on to a flat screen, and then think about what happens to the illuminated circle as you move the screen away from the light source.

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    Re: 150mm f5.6 Symmar-S Image Circle

    Quote Originally Posted by TomN View Post
    Now this is very confusing. It's either illuminated or not... When I look through the cut corners I can clearly see light from the lens at all apertures. Again, just trying to figure out if I have coverage before I use up film...
    No, you have illumination but itís outside the area the lens critically covers. Itís just light outside the rated coverage. Shoot a couple of sheets of film at infinity and check sharpness at the edges and corners. Make sure you have something there like a brick wall or fine tree branches or telephone wires, etc.

  10. #10
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    Re: 150mm f5.6 Symmar-S Image Circle

    I used a Symmar-S 210mm (IC 294) on 8x10 for a few years. There are some tricks like back tilt that relax a format's coverage criteria. Other times, there was nothing to resolve in the upper right or left corner (sky).
    Click image for larger version. 

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