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Thread: Hacking C-mounts and setting infinity...

  1. #1
    thearne3
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    Hacking C-mounts and setting infinity...

    Now that I'm getting used to filing lens mounts, the question arises: how do you know when you've reached 'infinity focus'? I have been testing by taking a picture of a building approximately 300 yards away - at f2, f2.8, etc, to f11. I then compare the shots at 100% (I don't use f1.3 because it is so poor) The smaller apertures (higher f stops) come into focus sooner. I then compare the shots and keep grinding until f2 is in focus. Not very precise!

    Are there any rules of thumb here? Is 300 yards a reasonable proxy for infinity? The longer the distance, the more lens resolution would seem to affect changes in perceived focus/sharpness. Looking at images, 300 yards may be too short a distance, but it's hard to tell!

    Thanks for any help...

  2. #2
    turbo
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    Re: Hacking C-mounts and setting infinity...

    I use a cell phone tower that is about 1/4 mile away. Nice spikey things sticking out to provide fine detail. For film cameras I use an 8-power loupe and a ground glass, for digital the finder or take a picture. All mounted on a steady tripod, of course.

    For the G1, pop on the manual focus magnifier on highest power and use the EVF/LCD, which ever is easier to see.

  3. #3
    thearne3
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    Re: Hacking C-mounts and setting infinity...

    Thanks for your answer...sorry for the delay in responding.

    OK got the distance - what about aperture? Use the widest usable, eg, f2.0? It would seem that DOF would play some part in this. Any advantage (other than less grinding) to stopping the process when clear focus occurs at f2.8? f4? etc?

    I see elsewhere that Godfrey sets infinity at 4' for a wide (12.5mm) lens and that works for near field subjects, given the fairly short hyperfocal length...I'm assuming that for 'full use' (all subject distances), infinity should be set for all usable apertures.

    -Tom

  4. #4
    turbo
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    Re: Hacking C-mounts and setting infinity...

    I guess it depends on what type of pics you take. If you mostly take landscape pictures you really won't be happy unless your lens focuses to infinity. DOF is great if you have near and far subjects in the same pic, but if 90% of your subject is at infinity, as in a landscape (assuming no important subject in the foreground) you want that 90% to be sharp. For example, the pics below were taken with an Alpa 50mm f 1.8 Macro-Switar. The first pic is at f1.8 focused at infinity. The second pic is at f 2.8 using the hyperfocal setting for f2.8, in theory everything from about 45 feet to infinity is in focus. But as you can see the infinity focus is much better with the lens focused at infinity.

    By the way these are cut from the full sized pictures without enlarging/shrinking (400x300) of the cell tower I talked about above. Using Google maps I determined the distance to the tower is almost exactly 1-mile. The leaves in the foreground are about 50 feet away, which is near the hyperfocal distance at f2.8. That is why the leaves appear sharper in the f2.8 pic.

    50mm f1.8

    http://forum.getdpi.com/forum/attach...1&d=1254184419

    50mm f2.8

    http://forum.getdpi.com/forum/attach...1&d=1254184419

  5. #5
    thearne3
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    Re: Hacking C-mounts and setting infinity...

    Thanks Turbo,

    The key message I'm getting is that I will want infinity in focus at the most usable wide-open aperture. "Infinity" can be what I want it to be...I'll start with 1/4mile and go from there.

    BTW, I looked up the DOF Online Calculator - If I'm setting it correctly, the hyperfocal distance for a 50mm lens on a 4/3s camera at f2.8 is 193ft, not 50ft (in focus from 96' to inf.) This would explain the significant blurring of the tower when focusing on the leaves at 50'. You need f5.6 or more, focused at 97' for both the leaves and tower to be in focus....if I'm understanding the calculator.

    Best,
    Tom

  6. #6
    turbo
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    Re: Hacking C-mounts and setting infinity...

    DOH! Not wearing my thinking cap. I just used the DOF scales on the lens, but that is calibrated for 24x36 format. Good thing I don't use DOF scales normally.

  7. #7
    Super Duper
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    Godfrey's Avatar
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    Re: Hacking C-mounts and setting infinity...

    Quote Originally Posted by thearne3 View Post
    Now that I'm getting used to filing lens mounts, the question arises: how do you know when you've reached 'infinity focus'? I have been testing by taking a picture of a building approximately 300 yards away - at f2, f2.8, etc, to f11. I then compare the shots at 100% (I don't use f1.3 because it is so poor) The smaller apertures (higher f stops) come into focus sooner. I then compare the shots and keep grinding until f2 is in focus. Not very precise!

    Are there any rules of thumb here? Is 300 yards a reasonable proxy for infinity? The longer the distance, the more lens resolution would seem to affect changes in perceived focus/sharpness. Looking at images, 300 yards may be too short a distance, but it's hard to tell!

    Thanks for any help...
    How far to set the reference infinity target depends on the focal length, format and maximum aperture of the lens. I would pick a target situated at least double the hyperfocal distance at the largest lens opening as a reference target. Use DoFMaster to determine that hyperfocal distance.

    E.g.: for my shortest lens, a 12.5mm f/1.4 lens on FourThirds format, hypterfocal distance at f/1.4 is 24.2 ft, so pick a reference target at least 50 feet away and test with exposures at f/1.4, f/2 and f/2.8 ... testing at the three largest apertures can you see the difference between infinity focus and lens performance.

    For my other extreme, a 135mm f/3.5 lens on FourThirds, hyperfocal is at about 1200 feet so you need a reference target at least 2400 feet away (and a clear day!) to test infinity focus with similar accuracy.

    Here's a chart I created for my lenses ...


    Obviously, to do this testing properly, you need a sturdy tripod, a remote release, and proper exposure as well.

    hope that helps.

  8. #8
    thearne3
    Guest

    Re: Hacking C-mounts and setting infinity...

    Seek and ye shall find....

    Godfrey, you are the best! I knew that there must be some practical approach to setting a minimum distance for infinity without confusing with overall lens sharpness, etc - a 2x multiple of the hyper-focal distance makes sense. Your table is just what I need. I am, luckily, working on a 12.5mm f1.3, so I can set up a target in my own yard at 50'.

    BTW, do you know anything about the 'legacy' Cosmicar 12.5mm f1.4 lenses? They are silver and not cylindrical like the new ones. They appear to have a much smaller mount shoulder. My Computar has a three leaf aperture. Do the Cosmicars also?

    Thanks again.
    Tom

  9. #9
    Super Duper
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    Re: Hacking C-mounts and setting infinity...

    Quote Originally Posted by thearne3 View Post
    Seek and ye shall find....
    Glad to help! :-)

    BTW, do you know anything about the 'legacy' Cosmicar 12.5mm f1.4 lenses? They are silver and not cylindrical like the new ones. They appear to have a much smaller mount shoulder. My Computar has a three leaf aperture. Do the Cosmicars also?
    I have no experience with the older models like that.

    My 12.5/1.4 has a six-bladed iris with no click stops and a "closed" position, presumably to protect the TV sensor that it was originally designed for? or provide a true black out position for 'fade to black" shooting? Bokeh at f/1.4 when focused in the near range is a bit funky, I find it nicer if I stop down to f/2.8-4 depending on the complexity of the scene. This is typical of fast, short lenses in my experience.

  10. #10
    thearne3
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    Re: Hacking C-mounts and setting infinity...

    I am not finished with the infinity setting, but since you mentioned funky bokeh, I thought I'd show some here. The three blades make a clear triangular shape, most noticeable in the upper right corner...I think this was f4.


  11. #11
    Super Duper
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    Re: Hacking C-mounts and setting infinity...

    This is the Cosmicar 12.5mm's rendering at my 'infinity' setting (4' focus distance) and wide open, f/1.4:


    The slightly hollow ovoid OOF blurring is a bit funky on complex backgrounds like this. However, you can set that the lens becomes much nicer and the OOF blurring much more subtle when stopped down ... this about f/5.6 or f/8, I believe:


    It's a fun lens to work with as I've got it set up, with an interesting imaging signature. Of course, it doesn't replace something that's well corrected and covers the whole format properly. I plan to add the G Lumix 7-14/4 at some point.

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