Please Help me: Focus issue in C-mount-M4/3 adapter

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James Mayo

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Please Help me: Focus issue in C-mount-M4/3 adapter

PostThu Dec 12, 2013 5:54 pm

I have a:
Metabones: C-mount to m43 adapter (http://www.bhphotovideo.com/c/product/9 ... _four.html)

and

Computar 12.5 to 75mm lens (http://www.bhphotovideo.com/c/product/8 ... _5_to.html)

The Problem: when I mount my lens to my camera using metabones adapter, It doesn't focus when in full wide :(

i don't know if there is something wrong in my adapter or the compatibility.
But here's a video sample using computar lens in BMPCC:
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Uli Plank

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostThu Dec 12, 2013 6:12 pm

You may need to machine the lens, it may not get deep enough into the adapter.
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Jeff Manning

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostThu Dec 12, 2013 8:06 pm

I have the 12.5mm Computar prime and mine will not focus either. Even after getting a friend of mine to machine the back so it fits all the way into the adapter. Something you may want to try, if you're comfortable doing some tinkering, is removing the screw that limits how far the focus ring will turn. I'm not familiar with the construction of your zoom lens but if it's anything like my two Computar primes it's very easy to do and MIGHT solve your focus problem. It worked for my 25mm (without having to machine it), but not for my 12.5mm. Still trying to figure out what to do with that one...
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostFri Dec 13, 2013 2:44 am

Jeff Manning wrote:I have the 12.5mm Computar prime and mine will not focus either. Even after getting a friend of mine to machine the back so it fits all the way into the adapter. Something you may want to try, if you're comfortable doing some tinkering, is removing the screw that limits how far the focus ring will turn. I'm not familiar with the construction of your zoom lens but if it's anything like my two Computar primes it's very easy to do and MIGHT solve your focus problem. It worked for my 25mm (without having to machine it), but not for my 12.5mm. Still trying to figure out what to do with that one...


hi jeff! how did you remove the screw limit of the lens?
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Jeff Manning

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostFri Dec 13, 2013 6:01 am

On my lenses, there's a little texturized rubber ring that goes around the focus ring.
photo 1 (4).JPG
photo 1 (4).JPG (66.77 KiB) Viewed 5891 times

What you'll first want to do is grab a little screwdriver and remove the rubber ring
photo 2 (4).JPG
photo 2 (4).JPG (59.91 KiB) Viewed 5891 times

After that's off you'll see (on mine at least) 3 small flathead screws, and one larger phillips screw. The larger phillips is the one you'll want to take out.
photo 4 (2).JPG
photo 4 (2).JPG (62.73 KiB) Viewed 5891 times


After that, you should be able to turn the focus ring past the infinity mark. Like I said though, that extra little bit of focusing was enough for my 25mm but it made very little difference on my 12.5mm, even after I had machined the back of it so that it seats properly in the adapter. Let me know if it works out for you.
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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostFri Dec 13, 2013 8:22 am

The mount on this lens is too wide to fit into the recess of a C-mount to M43 adapter. I don't think you'd be able to machine it to fit. You'll have a much easier time if you just buy a different lens.

In any case, this is a 2/3" format lens. The 2/3" format has an image circle diameter of 11 mm. This lens won't cover the BMPCC's 14.3-mm diagonal, at least not without significant softness, chromatic aberration, and vignetting in the corners.

A 1" format lens is a much better match for the BMPCC. The 1" format has an image circle diameter of 16 mm, which more than covers the BMPCC's sensor. Something like the Navitar NMV-6X16 is the 1-inch equivalent of your 2/3" lens, and it fits any C-mount to M43 adapter.
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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostFri Dec 13, 2013 10:09 am

I have a 12.5mm 1:1.4 Cosmicar Television Lens here that works fine after machining down. There seem to be quite a few models under that name, from very old black, heavy, conical ones with a "zebra" focus ring (similar to old Zeiss Jena) all the way to silvery, lightweight, tube shaped ones like mine.

It covers the BMPCC with softer corners, but not too bad and reaches infinity after machining. And it's dirt cheap.

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Dec 14, 2013 2:03 am

Uli Plank wrote:I have a 12.5mm 1:1.4 Cosmicar Television Lens here that works fine after machining down. There seem to be quite a few models under that name, from very old black, heavy, conical ones with a "zebra" focus ring (similar to old Zeiss Jena) all the way to silvery, lightweight, tube shaped ones like mine.
The old black zebra ones are f/1.9, not f/1.4.
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostWed Dec 18, 2013 4:39 pm

Jeff Manning wrote:On my lenses, there's a little texturized rubber ring that goes around the focus ring.
photo 1 (4).JPG

What you'll first want to do is grab a little screwdriver and remove the rubber ring
photo 2 (4).JPG

After that's off you'll see (on mine at least) 3 small flathead screws, and one larger phillips screw. The larger phillips is the one you'll want to take out.
photo 4 (2).JPG


After that, you should be able to turn the focus ring past the infinity mark. Like I said though, that extra little bit of focusing was enough for my 25mm but it made very little difference on my 12.5mm, even after I had machined the back of it so that it seats properly in the adapter. Let me know if it works out for you.



Hi!! I already tried this procedure, but unfortunnately, it didn't work to my lens. :(
Thanks for the help, I really appreciate it. Im losing hope. :/
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostWed Dec 18, 2013 4:42 pm

balazer wrote:The mount on this lens is too wide to fit into the recess of a C-mount to M43 adapter. I don't think you'd be able to machine it to fit. You'll have a much easier time if you just buy a different lens.

In any case, this is a 2/3" format lens. The 2/3" format has an image circle diameter of 11 mm. This lens won't cover the BMPCC's 14.3-mm diagonal, at least not without significant softness, chromatic aberration, and vignetting in the corners.

A 1" format lens is a much better match for the BMPCC. The 1" format has an image circle diameter of 16 mm, which more than covers the BMPCC's sensor. Something like the Navitar NMV-6X16 is the 1-inch equivalent of your 2/3" lens, and it fits any C-mount to M43 adapter.



actually, it is not too wide, it's too tight, the tail-end of the lens won't reach the end when you screw it to the mount. :(
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostFri Dec 20, 2013 2:39 am

helpppppppppppp!
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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostThu Dec 26, 2013 8:27 am

anyone? please help me.... :)
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Frank Glencairn

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostThu Dec 26, 2013 9:48 am

Had the same problem with my Canon 17-102. After trying out several adapters, I found one that let's you screw the lens all the way down, no mater how fat the rear end is. No machining needed.

http://www.ebay.de/itm/400584813227?ssP ... 1497.l2649
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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostThu Dec 26, 2013 10:13 pm

It still won't fit. Your lens is 39 mm wide 1 mm in front of the C-mount flange. The widest C-mount adapters are 37.7 mm within 1.8 mm of the flange surface. You'll need to remove some metal from the lens to make it fit. But still your lens doesn't project an image large enough to cover the camera's sensor.

http://computarganz.com/file.cfm?id=444

http://lh5.ggpht.com/_D3R0mpaThBE/Sas7M ... 6502_1.jpg
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Frank Glencairn

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostFri Dec 27, 2013 3:19 pm

The adapter I posted is infinite wide since it has no rim, i.e. totally flat, you could bolt a 2 feet wide lens on it.
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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Dec 28, 2013 2:05 am

Frank, you misunderstand.

It's true that some C-mount adapters have an extra rim that can interfere with the mounting of some lenses. In this image the rim has an inner diameter of 49 mm. But even without that extra rim, the C-mount flange is recessed within the throat of the camera's Micro Four Thirds Mount. C-mount has a flange focal distance of 17.52 mm, which is less than Micro Four Thirds' 19.25 mm flange focal distance. So the C-mount flange on every M43 adapter that comes even close to having the correct flange focal distance will be recessed by at least 1.73 mm inside the camera's throat, and practically, a millimeter more than that since the adapter's M43 flange will be a minimum of 1 mm thick. In this image, the C-mount flange is the flat part next to the C-mount threads, 37.2 mm in diameter for this particular adapter. Some are a little wider, but 37.7 mm is about as wide as it can possibly be and still fit inside the camera. Some lenses are simply too wide too close to the flange to fit within any C-mount to Micro Four Thirds adapter.

Credit goes to Hawk's Factory for the image. He makes very high quality adapters.
DSC_6502_1.jpg
C-mount adapter dimensions
DSC_6502_1.jpg (142.83 KiB) Viewed 5609 times
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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Dec 28, 2013 2:21 am

If you still can't visualize it, here's an image of a lens that is very wide very close to the mount flange: 55 mm wide less than 1 mm from the flange. It won't mount properly on any Micro Four Thirds adapter.
IMG_20131227_181356.JPG
C-mount lens
IMG_20131227_181356.JPG (356.05 KiB) Viewed 5609 times


But on an actual C-mount camera, it's no problem. Note that the camera's C-mount flange surface sticks out past every other part of the camera.
IMG_20131227_181430.JPG
C-mount camera
IMG_20131227_181430.JPG (225.07 KiB) Viewed 5609 times
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Dec 28, 2013 7:02 am

My problem is my lens won't reach the end because of these two metals (red marks) in my adapter.
Is it possible if I flatten/remove these two it will resolve my concern?

my adapter fits on my camera, but the lens won't fit on the adapter because of these two mounted metals on the side. I can't screw the lens to the end because of these.
Attachments
Image1217.jpg
Metabones C-mount adapter
Image1217.jpg (447.8 KiB) Viewed 5601 times
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Frank Glencairn

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Dec 28, 2013 11:20 pm

Maybe it's a bit hard to see, but with this adapter, the c-mount lens reaches beyond the actual MFT flange into the housing.

I get proper focus on both ends with this combination.

IMG_20131229_001338.jpg
IMG_20131229_001338.jpg (115.51 KiB) Viewed 5574 times
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSun Dec 29, 2013 3:53 am

Frank Glencairn wrote:Maybe it's a bit hard to see, but with this adapter, the c-mount lens reaches beyond the actual MFT flange into the housing.

I get proper focus on both ends with this combination.




is your adapter same as mine? did you machine the lens or the adapter?
in the image i posted above, do you think i can remove the mounted metal on the side of the adapter?
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Frank Glencairn

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSun Dec 29, 2013 12:50 pm

jmyo wrote:is your adapter same as mine? did you machine the lens or the adapter?
in the image i posted above, do you think i can remove the mounted metal on the side of the adapter?


No it's this one: http://www.ebay.de/itm/400584813227?ssP ... 1497.l2649

No machining involved, you can use it as it is.
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Jan 04, 2014 8:23 am

UPDATE:
I ALREADY REMOVED THE TABS ON THE SIDE OF THE ADAPTER (THE RED MARKS IN THE PICTURE I POSTED ABOVE)
AND MY COMPUTAR LENS FIT EXACTLY ON THE ADAPTER, BUT THE THE PROBLEM IS... IT IS STILL BLURRY WHEN I'M IN WIDE SHOT WITH A FULLY OPEN APERTURE. BUT WHEN I ADJUST THE OPENING, OBVIOUSLY IT DARKENS BUT THE IMAGE SHARPENS SOMEHOW...

WHAT'S WRONG WITH THIS SITUATION?
THANKS.
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James Mayo

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Re: Please Help me: Focus issue in C-mount-M4/3 adapter

PostSat Mar 22, 2014 6:16 am

Geoff Baxter wrote:That is quite normal, as you reduce the aperture, you also increase the depth of focus, so it will tend to sharpen the image. The issue here appears to be the distance the lens is front the focal plane. There are two possibilities, you need to screw the lens further into the adapter, or unscrew it slightly to achieve back focus.

Most likely you will not be able to screw the lens in any further, but you may be able to unscrew it to achieve focus. if that works, then you will need to have a shim made to fit to the rear of the lens.



this is my first time that i am going to have a shim made to fit for my lens...
but, i don't know where? does a machine shop for automobiles can do it? thanks...
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