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Thread: Year end: Topic-1

  1. #1
    Super Duper
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    Smile Year end: Topic-1

    One contributor (Mr.P) PMed me about a "custom made" 12.5mm f/1.4 lens I have mentioned in some of my posts.

    It is derived from a c-mount Fujinon-TV CF12.5A 12.5mm f/1.4 lens.

    [Note: There are many other 12.5mm Fujinon c-mount lenses! None of them are part of this thread!]

    1. It is a straight fit on a standard c-mount adapter.
    2. Orientation is easily adjusted.
    3. Though it does not cover the whole of a m4/3rds frame, it covers more than any other (computar, cosmicar, etc) 12.5mm f/1.4 lenses.
    4. 52mm filter thread (can not use any hoods though the lens is prone to flare!).

    My first test shot:


    Closest focus distance (possible slight crop, stopped down a bit).

    Low light shot (wide open, cropped):





    Shortly after a bit of test runs like those, I modified my original sample (will post some details later, it took me > a month of slow and deliberate work and the first one was not all that satisfactory (CA)- I intend to modify another sometime in the future) so that it will cover the whole frame.

    Wide open sample:



    f/2.8





    Still, up-close, this is an useful lens:

    http://www.flickr.com/photos/vivek-i...7617698967864/

    Slide show:

    http://www.flickr.com/photos/vivek-i...98967864/show/

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    Re: Year end: Topic-1

    I am surprised mama goose didn't object to you getting so close .

    I really liked the B&W with the lens.

    I'll be very curious to hear about your mods to the lens.

    Doug

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    Re: Year end: Topic-1

    It was easy with the G1 and a low profile lens. Thanks, Doug.

    Basically, the modification involves taking the front half of the lens unit and placing it closer to the rear unit to increase the image circle.

  4. #4
    thearne3
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    Re: Year end: Topic-1

    Hi Vivek,

    Just curious: wouldn't moving the 'front end' closer the 'rear end' also affect other aspects of the lens, such as focal length and/or focus range? I have an extra Computar 12.5mm f1.3, with a broken mounting base (too much filing!), which is now completely taken apart. I could duplicate what you describe - and fairly easily bring the two ends up to 1/8" closer. How much are you changing? What are we giving up for a larger light circle?

    Best,
    Tom

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    Re: Year end: Topic-1

    Tom, Serious questions that would get me into trouble if I am careless.

    The computars and cosmicars, after having experimented with, I came to the conclusion that they can not be tinkered with to get larger image circles while keeping the focal length pretty much the same. There isn't much wiggle room towards the diaphram (you could move the units apart but that causes lesser angle coverage). Besides (perhaps most important) is the very short helicals these lenses have.

    The Fujinon's helicoid is superb (FWIW, it is brass). Not much is lost.

    Wide open, there is a glow. Difficult to focus (low contrast). But, this is the case with an unmodified lens as well.

    The increase in coverage needed is something like ~2mm in the corners.

    I think I moved about ~5mm (less is possible if one makes a dedicated casing for the front unit. I just do not have the resources to make a one).

    I am still out hunting for some old snaps of the lenses. If I can't locate them, I will make new pics.

  6. #6
    thearne3
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    Re: Year end: Topic-1

    Ok. The Computar lens has two lens structures, each of which has two fixed lens (the inner lens of each structure faces the diaphragm, the outer lens of each structure is exposed). The diaphragm is held within an aluminum(?) sleeve, which has turnings on the outside as well as the inside. The outside turnings engage with the focus ring casing. The inner turnings engage with the incoming light/larger lens structure. I assume the 'helicoid' is this turned sleeve that regulates the distance between the rear/sensor side/small lens structure and the front/incoming light/larger lens structure - is that right?

    Not sure if I've been clear, but how is the Fujinon lens different?

    In any case, I applaud your willingness to experiment - can't wait to see what you end up with! I like the Computar 12.5, but the sensor coverage is its biggest drawback. Haven't seen the Fujinon lens - any suggested sources for future reference?

    Again, thanks for your post - this is part of what makes m4/3 so much fun!

    Best,
    Tom

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    Re: Year end: Topic-1

    Hi Vivek. Would you consider posting a description of your lens surgery ? I bought the same model you have, and am viewing it now.
    The rear element can be removed with a lens spanner.

    Now I am stuck - I'm not really sure what you moved in the lens assembly ..

    Quote Originally Posted by Vivek View Post
    Tom, Serious questions that would get me into trouble if I am careless.

    The computars and cosmicars, after having experimented with, I came to the conclusion that they can not be tinkered with to get larger image circles while keeping the focal length pretty much the same. There isn't much wiggle room towards the diaphram (you could move the units apart but that causes lesser angle coverage). Besides (perhaps most important) is the very short helicals these lenses have.

    The Fujinon's helicoid is superb (FWIW, it is brass). Not much is lost.

    Wide open, there is a glow. Difficult to focus (low contrast). But, this is the case with an unmodified lens as well.

    The increase in coverage needed is something like ~2mm in the corners.

    I think I moved about ~5mm (less is possible if one makes a dedicated casing for the front unit. I just do not have the resources to make a one).

    I am still out hunting for some old snaps of the lenses. If I can't locate them, I will make new pics.

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