Using front element only, what can I expect?

Discussion in 'Large Format Cameras and Accessories' started by JMC1969, Jan 11, 2009.

  1. JMC1969

    JMC1969 Subscriber

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    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. archphoto

    archphoto Member

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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. jnanian

    jnanian Advertiser Advertiser

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    what he said ...
     
  4. Mark Fisher

    Mark Fisher Subscriber

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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 :smile:
     
  5. archphoto

    archphoto Member

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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. Mark Fisher

    Mark Fisher Subscriber

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    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. Lee L

    Lee L Member

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    Don't forget to recalculate the aperture according to the change in focal length.

    Lee
     
  8. JMC1969

    JMC1969 Subscriber

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    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. paul ewins

    paul ewins Member

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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. edtbjon

    edtbjon Member

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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...)

    //Björn
     
  11. JMC1969

    JMC1969 Subscriber

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    Paul, Thank you, this info will come in handy. As I was using this method, 2 thoughts came to mind. 1.) Would I not get a more accurate reading if I omitted the shutter and just shined the light behind the front element? 2.) Is taking this measurement actually needed to do the recalculation of aperture? If you are using only the front element to acquire this diameter reading and I am trying to recalculate in order to only use the front element, then they would be the same reading. As the lens is stated as a 210mm f-5.6, that reading would be 37.5mm. So to recalculate I would divide 430mm by the same 37.5mm to equal 11.4666666, f-11-16 split. Which is the figures that Björn has suggested. It would also make sense to me in the fact that I am doubling the focal length of the same lens, therefore I should double the F-stop (5.6x2=11 roughly). Is this true or just for this application?

    Björn, tack så mycket (just a guess), you answered the next question as to what that really means. F5.6 marker becomes F11 on the shutter. Now I guess I will need to go read that sticky about calculating bellows extension since they are basically all the way extended. Something tells me I am going to need a bright sunny day for this.
     
  12. Lee L

    Lee L Member

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    I'm not an expert, but the f:5.6 convertible Symmar lenses had rear components that were used alone rather than the front. For optical reasons, the rear and front sets were different focal lengths, possibly 375mm for the rear set on the 210. I believe those went from f:5.6 to something like f:12, but the front set may be longer focal length, and so have a smaller f-stop.

    You should probably either find lens specs or measure for yourself.

    Lee
     
  13. dpurdy

    dpurdy Member

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    Where is a polaroid when you need it? I have taken the rear element off my 210 Nikkor and it becomes a beautiful portrait lens till you stop it down. I seem to remember that focal length became much longer that way.
     
  14. JMC1969

    JMC1969 Subscriber

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    Dpurdy, As luck would have it, I got a polaroid back in the mail about 4 weeks ago and have not taken the time to use it yet. Coupled w/ the half box of 55 in the refrigerator and I took a couple of shots from the front porch.

    I had an extremely hard time focusing. There is enough bellows because I can tell when I have got too far in each direction, but for some reason I just couldn't pin point clarity today.

    #1- Exposed for asa25, meter reading was 32-45 split@1 sec, bellows factor was 1.25, I believe around 1/8th of a stop open/longer.

    I didn't make the adjustment and shot it at F32 @ 1sec.
    [​IMG][/IMG]

    #2- Exposed for asa25, meter reading was 90 @ 8sec on the dot. same bellows factor so I shot it at a fraction past 8sec.

    [​IMG][/IMG]
    Please don't judge too much on the image itself, I just wanted to do a quick test and I was trying to make it happen during my lunch break.
     
  15. archphoto

    archphoto Member

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    Dear Jerk,
    A b it like I expected, somewhat softer in the sharpness than I would expect from a full 4x5 lens, a bit like an Imagon, but not bad at all.
    You got your self an other lens !

    Have fun, experimenting works

    Peter
     
  16. JMC1969

    JMC1969 Subscriber

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    The second shot had some foreground interference that I knew was there, but I wanted to see if it would even be seen. The trellis in the garden across the street is around 25-30 yards/meters away. The tree branches that are going right through the middle of the image are about 4-5 yards/meters away from camera. I will definitely need to shoot some normal B&W and take some time to see what it will really do.
     
    Last edited by a moderator: Jan 12, 2009
  17. archphoto

    archphoto Member

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    Ofcoarse, but now you know that it works !!!!!
    The depth of field will be a bit smaller (longer lens now) but that should not be a problemm.

    I like this: making something out of something aparently useless.......

    Greetings,
    Peter