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    DIY focus collimator
    #1
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    Just "finished" my DIY focus collimator device. Been wanting to build one for a while to check the back focus adjustment on our ENG cameras at work, and to use to set up lenses after I've fixed them and check mount swap and adapter length, etc. When I bought my GH2 a while ago, it came with a bunch of misc. lenses, so for this project I choose a decent lens in an FD 100f2.8 breach lock that I couldn't convert to EF. Everything I've read says that you should use a long lens for this type of creation, so I thought the 100mm would be good. Last week I decided enough was enough and I was going to build this thing. Found a suitable (non-functional) Canon T50 on ebay for $12 shipped and when it arrived I wasted no time tearing it apart.

    This first two things that must be done are to get the mirror out of the way, and get the shutter out of the way:





    After removing the prism, the mirror is held up by aluminum tape. The shutter was brute force removed and cut off where I could get on it.

    Then I needed to create a pattern and print it, I used transparency film and printed with a laser printer, there are other more simple ways to get this done too.



    I'll need to add a light behind this, but you could also use paper and put a light in the viewfinder space and front light it. I may move back to this and front light, depends on the kind of trouble I have fitting a light behind the transparency material


    Looks like this:



    Shooting down through the lens (at infinity):



    On my rig:





    And the as seen from the GH2 taking camera:



    and focus mag:



    What you can't see in these pictures is that if everything is at the right distance, you can clearly see the texture of the transparency material and the individual dots of toner melted together. That's how I know I'm at the correct distance with this lens and this adapter (LensTurbo). (edit: I guess you can see the texture a little bit)

    That's about it, go forth and modify! More info can be found on the web with explanations of why this works. I can repeat this here if needed.


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    #2
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    Cool, you basically built your own version of the NULL Lens & NULL Target that I helped bring to market. I'd suggest working on that target design some. A Siemens Star will only ever tell you so much, but you could do line pairs or dot patterning that will give you a direct read on acuity. That's especially useful when you are dealing with a camera & lens system that needs shimming instead of adjustable flange depth. Cool!
    Mitch Gross
    Cinema Product Manager
    Panasonic System Solutions Company


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    Yeah, there is a lot I'd like to do with the target, but the resolution of the printer really starts to become an issue. That was why I needed to put the 4 way division circle in the middle, the lines just became a jumbled mess in the print. Really need a photographic process to make a really high resolution target (take a picture on film). If you have suggestions on target patterns, I can try to make them and experiment a bit.


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    #4
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    Do you think this is on the right track:


    On the Lensrentals site there is a blog post about the circle and dot helping to show de-centered or tipped elements, when you de-focus the image the circle will go toward an oval in the direction of the problem.
    https://www.lensrentals.com/blog/201...ts-a-makeover/


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    If you do line pairs then you need them in both vertical and horizontal to check astigmatism and spherical aberration. Corner targets are also good to look at falloff, parallelism and field flatness. Diagonal lines are also useful for acuity.
    Mitch Gross
    Cinema Product Manager
    Panasonic System Solutions Company


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    #6
    Senior Member Michael Carter's Avatar
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    If you live in a fairly large city, there's likely still graphic service places that will print film at 1200 dpi.

    Use a vector file (like illustrator) and you'll be resolution independent (no pixelation). I used to have 35mm slides printed from photoshop files, even up to 8x10 portfolio transparencies, and the quality was fantastic. Someone must still do that??


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    I'm using a CAD program and it is for all intents resolution-less. I may try running one through a different laser, but I'm pretty sure I'm already getting 1200x1200 on these prints. The film does resolve a little better than paper too.

    Here's another try, trying to make it look "nice" while providing some features.



    The diagonal lines need some work, they get a little thin, and I improved the star with better detail so a smaller dead spot. Hard to see but the upper right angle lines are twice as thick as the lower right angle lines.


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    Pretty good, but I think you should add something to the corners of the frame, way outside that circle. It's important to see what's going on outside the center. If there is an issue with parallelism, the center could be in perfect focus but one corner or a whole side could go soft.
    Mitch Gross
    Cinema Product Manager
    Panasonic System Solutions Company


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    I'll need to cut more of the shutter curtain out of the way. That said, most lenses may not fill the entire image area, the above picture before I used focus mag. shows very little of the image area. I'll have to try a wider lens on the target and see if it still performs well, have a few 50mm kicking around.


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    Think I have the final configuration... Added an Electro Luminescent panel for a back light with AA battery inverter, seems to be plenty bright on the receiving end at f2.8 and ISO 160.







    The backlight allows me to put the film back/pressure plate back in place to close everything up.

    Here is what I see with the 100mm on the target and 24mm + Lens Turbo on the camera:



    Now with a 50f1.8 on the target camera:



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