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  1. #1

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    Using front element only, what can I expect?

    I have a Caltar-S II 210mm, the rear element has a good sized chip out of it that has been masked off. I have been using it as is with good results, but O got to thinking today and wanted to see if my camera had enough bellows to focus at infinity with the rear element removed. It does and from my measurement it seems to be around 430mm in which this focus come in. I guess most anything is worth a try to see what you get, but thought maybe I could get some insight first. I have an idea that I will loose quite a bit of sharpness because I have removed a major part of the lens design and I don't believe it to actually be a convertible lens.

    Thanks,
    Jody

  2. #2

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    The proof is in the pudding or so......
    Why not take a test with a sheet of BW, low ASA, 100 or so, and see ?!
    Print, go hybrid or use a magnifying glass to see what comes out.........
    Experimenting with optics can be fun.

    Good luck,
    Peter

  3. #3

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    Quote Originally Posted by archphoto View Post
    The proof is in the pudding or so......
    Why not take a test with a sheet of BW, low ASA, 100 or so, and see ?!
    Print, go hybrid or use a magnifying glass to see what comes out.........
    Experimenting with optics can be fun.

    Good luck,
    Peter
    what he said ...

  4. #4
    Mark Fisher's Avatar
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    Given it a shot. If it is something close to a symmetrical lens (sounds like it is), I'd expect that the center will be reasonably sharp and fall off quickly....but I could be wrong (:

  5. #5

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    Mark, odly the Imagon uses just a rear lens group of 2 lenses: it comes out sharp but with a verry low contrast that you improve with the aperture disks in front of it....... I curios about the result of this.
    I expect to show anastigm to show up that is normaly corrected by the rear group.

  6. #6
    Mark Fisher's Avatar
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    Quote Originally Posted by archphoto View Post
    Mark, odly the Imagon uses just a rear lens group of 2 lenses: it comes out sharp but with a verry low contrast that you improve with the aperture disks in front of it....... I curios about the result of this.
    I expect to show anastigm to show up that is normaly corrected by the rear group.
    If you take a classic landscape lens (2 identical elements or doublets spaced eually from a central aperture), you end up with about the focal length described. I figured that it would behave similar to removing one of those elements.

  7. #7
    Lee L's Avatar
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    Don't forget to recalculate the aperture according to the change in focal length.

    Lee

  8. #8

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    Quote Originally Posted by Lee L View Post
    Don't forget to recalculate the aperture according to the change in focal length.

    Lee
    Thanks for the comments. Lee I'm glad you said something because it really didn't enter my mind. I don't think I know how to do that. I remember reading something that said the f-stop is the focal length divided by it's diameter. Well, this is a 210mm f-5.6 as stated on the lens, which means the aperture at 5.6 should be 37.5mm diameter. It's not, it's in a prontor press and wide open is 30mm @ 4.7. This shutter is supposed to be the same as a copal#1 and my exposures have been pretty much on the money in the past using it at 210mm. I'm a little lost at the moment. Could this mean that I have been using the wrong shutter for the elements? I never bothered to check when I got it. I just thought it was right because that was the way it came to me. Always something else to learn, but it sucks to learn you may have been doing everything wrong from the beginning.

  9. #9

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    Jody, the f-stop is the focal length divided by the apparent diameter of the aperture. This is measured by removing the rear cell, then shining a pinpoint source (in practice one of those single LED torch/flashlights is perfect) from the rear of the shutter and measuring the size of the aperture as it appears in the front element. The difference is that some lenses have a front element that magnifies while others don't. A quick check of an older Schneider Symmar show this to be the case so I imagine that your Caltar-S, based on the newer Symmar-S is probably the same.

  10. #10

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    With the old convertible Symmars you normally take away the front element and use only the rear. It's also possible to replace the front element with the rear.
    To start with you can take away two stops on the aperture scale, to get you in the ballpark. I.e. a setting for f/5.6 is really f/11. If you want anything decently sharp, you do need to use the "half-lens" at f/22-32. (I.e. a setting of f/11-16.)
    As some of the corrections from using both elements are gone, contrast will be lower, so a yellow (or stronger) filter is recommended.
    Someone should really get this info into some sort of FAQ, as this question seem to come up almost every month in one form or another. (I.e. which lenses does this work with, aperture scales, what to expect etc...)

    //Bj÷rn

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