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10-23-2013, 06:45 PM - 1 Like   #1
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C Mount lens shims...

I've tried a bunch of C mount lenses on the Q. I use the two piece C to Q mount adapters for each of my C mount lenses. That allow me to adjust the adapter so that the aperture dial is facing up for each lens. One thing I have found with these lenses is the focal range is off. Infinity focus doesn't match up with the infinity mark on the lens. Infinity is focused at a shorter range.

I ended up buying some brass shims from McMaster Carr to allow me to dial in each lens. I bought 0.01" and 0.002" shims. The 0.01" shims are too thick but the .002"s are working out well. Some lenses take a couple of the .002s. I take a picture at infinity without a shim, add the shim and shoot again. Using the built in comparison feature on the Q I compare the two shots for sharpness. If the shimmed picture is sharper I add another shim and shoot again comparing the shot with 1 shim vs 2. Continue until the lens starts getting soft again and pull out the last shim.

This is especially important for lenses that are fixed focus such as the Pentax 4.8mm and Sigma 6.5mm.

This also seems to improve the lens performance as well. I have a Pentax 6mm f1.2 lens that is actually focused at infinity at around 0.8' on the dial. It has a lot of focus shift as you stop it down. When I shimmed this lens so that infinity matches up the focus shift is nearly gone when stopping it down. It also makes it a lot easier to zone/hyperfocal focus the lens.

Shims from McMaster Carr.


Shawn

10-23-2013, 09:20 PM   #2
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Interesting info. Many lenses are set to focus past infinity to deal with temperature changes. Do you think that is the case here? Perhaps the adapters are off? Good to know shims will fix the problem.

Only C mount I have is the Canon 50mm 0.95. Never tested it at infinity. Must do so now.
thanks for supplying info and orderng source.
thanks for sharing
barondla
10-24-2013, 11:32 AM   #3
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Brass shims? Talk about posh.
Newspaper works pretty well, just don't get it wet!

As far as I can tell, all my old C-mount glass don't need shims, but it's the newer cctv stuff that does. ymmv
10-24-2013, 01:32 PM   #4
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"Many lenses are set to focus past infinity to deal with temperature changes. Do you think that is the case here?"

I don't think so. Most are focusing at infinity near the other end of the focal scale, it isn't just a little off. The fixed focus C mount lenses are very soft unless shimmed.

"Perhaps the adapters are off?"

Probably, I've read most are a little to thin to assure infinity focus.

Shawn

10-24-2013, 01:37 PM   #5
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I was originally shimming the two piece adapters with paper between the halves. Worked pretty well but it is easier to be consistent, and keep the lens parallel relative to the sensor, with the brass shims.

"As far as I can tell, all my old C-mount glass don't need shims"

What C to Q adapters are you using?

I am using these:

Movie C Mount Lens to Pentax Q Mount Adapter | eBay

Maybe you are using a different adapter?

So far all of the lenses I have test (3.6mm, 4.8mm, 6mm, 6.5mm, 25mm f0.95) have benefited from shimming them. I have a bunch of others that I haven't really done anything with yet though.

Shawn
10-24-2013, 06:38 PM   #6
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Looks exactly the same as mine. The only lens I need to shim is my pair of pentax/cosmicar 8.5mm CCTV lenses. The two lenses I put in the Q lens database (a wollensak and an elgeet) seem to focus fine sans-shim. If they don't focus to infinity, it's very minimal and hard to distinguish. The 90mm is kind of hard to test, but I don't think it needs a shim either.

That said, a shim might help the scale accuracy on the long end. They are pretty much perfect on the close end though.

How is the f0.95?

Last edited by theeggplanthunter; 10-24-2013 at 06:52 PM.
10-24-2013, 07:07 PM   #7
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The 0.95 hasn't been shot much. It is tough to use on the Q. It is much larger and heavier than the Q, and has no tripod socket. So it has only been used hand held. Not the best way to test for IQ. Need to rig some kind of support.
thanks
barondla
11-03-2013, 12:22 AM   #8
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I have 2 c mount lenses and as far as I can tell they don't really have an infinity focus on the q the way that my K mount lenses have on my DSLR. I haven't been able to get a sharp infinity focus image at all. Seems like they are only good for work up close within 20 or so feet.

11-10-2013, 08:27 AM   #9
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QuoteOriginally posted by Shawn67 Quote
I ended up buying some brass shims from McMaster Carr to allow me to dial in each lens. I bought 0.01" and 0.002" shims. The 0.01" shims are too thick but the .002"s are working out well. Some lenses take a couple of the .002s.

Shims from McMaster Carr.


Shawn
thanks for the tip - I have a Som Berthiot Lytar D Mount that definitely needs some shims, and McMaster has 5/8" ID shims that should help - i used electrical tape, but it still wasn't right! I really appreciate the recommendation on where to find these shims!

erik
11-10-2013, 04:44 PM   #10
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" I haven't been able to get a sharp infinity focus image at all. Seems like they are only good for work up close within 20 or so feet. "

What happens if you unscrew the lens slightly. Can you focus to infinity then? If so the shims would help.

Shawn
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