Friday, 12 June 2009

adapting a lens to fit the mount

an old engineer friend of mine once said "if it jams force it, and it breaks it needed replacement anyway" (thanks Paul).

With this philosophy clearly in mind I had a go at fitting my lens onto a new adaptor I bought on eBay in an attempt to mount a Computar 12.5mm f1.3 TV lens onto my G1.

I've previously bought an Elgeet lens and RJ Cameras adaptor which worked fine, and looked like this.

Elgeet mounted on G1

But the Computar wouldn't fit in the recess (by a long shot) and RJ Camera said it couldn't be done. So I bought another adaptor from a fellow on eBay called hawkpeng2003, which I expected would be close ... what follows is my small journey to get it going...

The bottom of my lens looks like this:
C mount measurements

and comparing it to the RJ Camera mount:
c mount problem

it just clealy wasn't going to fit.

Well, the diagrams from Hawkpeng2003 on his site looked promising, particularly this one:

Here is my adaptor (and the lens in the background)

The dot you see on the lower left is the alignment dot. Note also the chamfer on the contcentric inner edges of the mount. The actual dimensions of the lens matched the dimensions of the mount in the diagram above from HawkPeng2003, but it did not take into account the square edges on the lens, and his adaptor has chamfered edges.

All of them other edges on the lens were within the sizes he quoted except one ... the second inner edge.

So armed with my trusty Leatherman I set to work to carefully file that second inner edge back. Here's the result:

Now, prior to working I used some masking tape around the mounting thread to form a protective 'tube' which I pinched off. This was just to keep filings and finger prints off the lens.

Now as you can see the lens and the mount will go together better:

and boy, its close as you can see ... but the chamfer matches (well perhaps its got some clearance too).

but it fits and finally I can screw it down properly so that the alignment mark comes very close to perfect.

the final check is that the alignment mark on the adaptor comes quite close enough to that of the lens to give me quite good infinity focus (even with the magnifier).

Here it is on the camera

G1 with 12.5mm f1.3 Computar

and it takes great images


Thanks HawkPeng

Have fun.


snatre said...

Hi Obakesan,
Very interesting post. Do you have any idea about the old X-ray lenses like Kowa 1.1/77mm. What mount are they? Is it possible to fit them onto micro 4/3 or to 4/3 for macro?

obakesan said...

Hi there

sadly I don't have any experience with those lenses. Please let me know if you do find out.

Phototransformations said...

Wondering how this lens is working on your G1. I'm considering a similar lens/adapter combo. Is there substantial vignetting? Is it fairly sharp?


obakesan said...


lens is working out ok ... vignetting and samples can be found here:

Anonymous said...

Thanks Obakesan, I got this lens and did the modification. However I also removed the focus lock screw underneath the rubber focus ring. This gives more focus ring rotation to achieve perfect infinity + closer focus possibility.

good work!

Anonymous said...

Just thought i'd say that the "final check" alignment mark on the lens is neither here nor there.
It doesn't have any particular position, seeing as its totally adjustable after loosening the 3 grub screws on the chrome collar and rotating the lens through 360 degrees... once the C-mount screw has seated home fully against the mount.
So unless you know the latter has been achieved, you cant take that mark to mean anything.

obakesan said...


good point. I can confirm that it does screw down to meet the surface firmly.

Lens Wide-Open Photography said...

I have one that focuses to infinity but minimum focusing distance is about 3ft., nowhere near .4m indicated on the focusing ring. Have you adjusted this as well?

obakesan said...


focus close is simply extending the lens further from the mount. Often the markers for distance are (quoting from "Pirates of the Caribbean") more like a guide line.

You can get closer focus by just unscrewing the lens a little on the mount (be careful it does not fall off, you will know where that point is) as both are just threads right?

Also you can get small 'extension' rings which allow you to mount the lens further from the mount for very little on ebay ... they are just simply threaded tubes. I have an example of these for FD lenses on this page

the C-mount ones are much more compact

Jeroen Wolf said...

Nice work! I have the Pentax 6mm 1.2 and have a C-mount which seems to miss the second 'outer ring' on your lens.

The rear barrel is 1mm too wide to fit the adapter. Is it safe to just 'shave off' the metal on the barrel? (btw: any idea why don't they make adapters that fit these lenses?)

obakesan said...


I believe so ... although the bottom will (like mine) look that wee bit ugly


Jeroen Wolf said...

Thanks. I'll ask someone with the proper gear to keep damages to a minimum!

Anonymous said...

These are links to C-MOUNT 1/2 and 2/3 fast wide angle lenses that probably would fit GH2 and would fill the ETC 2.6x sensor area (some lenses needs modification in body or rear barrel):

the rule to find the 35mm equivalent focal lenght is:
C-Mount focal lenght / 8.3 x 43,2 = full frame 35mm equivalent

Observation: I do not know if some of them are fish eye

Kowa 4-10mm

Kowa 3.5mm

Kowa 4.5mm

Kowa 6mm

Tokina 6-15mm

Pentax 4.2mm

Pentax 4.8mm

Pentax 6mm

Pentax 6mm (other)

Fujinon 6mm

Fujinon 3.8-13mm

Tamron 4-12mm

Ikegami 4-12mm

Arecont 4-10mm

Shrigg said...

Very useful post! The newest Hawks adapter is quite a bit slimmer although my computar 12.5 f/1.3 still needs chamfering for infinity focus. Same with my Pentax 6mm f/1.2, although much more chamfering is needed since the lens does not step down like the computar.

Jeroen Wolf said...

I don't understand. I had someone grind down the rear barrel of my Pentax 6mm, grind some of the adapter when it wasn't enough and I still don't get infinity... drives me nuts.
I want to grind down more of the adapter but have a question before I do so: can the lens sink in too far? If I grind down another 2mm, could it be that that would be too much and affect the functionality of the lens?

obakesan said...

Joren ... do you know if the base of the lens is actually making it to touch the bottom of the adaptor? Need to be sure that the flat surfaces are meeting. Try a thin piece of paper and see if when you screw the lens on, that the sufraces are squeezing it together.

Jeroen Wolf said...

Here's a link to the adapted lens+adapted adapter. The inner ring of the adapter is not completely ground down but the surface of the rear barrel seems on par with the edge of the adapter...

But maybe I have to grond the adapter until the inner ring is completely gone! (that's why I asked: can the lens sink too deep?)

thanks for your assistance!

obakesan said...

I'm sorry, but that picture doesn't tell me anything about how the adaptor and the lens are interacting, what you've ground off the lens and how.

Jeroen Wolf said...

I posted 6 more pictures that should give you all the info you need.

thanks again,


obakesan said...

Hi Jeroen, this is an interesting experience in communication. Looking at your first picture I can see I was not clear enough. I meant the meerest tiddle bit of paper (think 2mm x 2mm) to just sit in there and as you tighten up the lens you will "feel" it if the two surfaces (the adapter and the lens back) touch. that way you know if it is screwing down to the right amount.

I see from your other pictures that you are using the RJ Camera adaptor (as in my first image in this blog). This one will just cause heaps of hassle for you and you will not be able to be modify the adaptor without it becoming dangerously thin and risk braking.

I have annotated your image here:

do demonstrate a couple of things.

1) do not grind down the back part of the lens near the thread, that will cause you problems in getting infinity properly in focus. Instead you need to grind down the outsides of the cylinder to allow it to fit against the surface of the adaptor.

2) you really need to think about the lens you have when picking an adaptor OR be willing to do heaps of modification.

From what I hear the Pentax is a tough one to mount, but that it can be machined down (again, I stress on the outside of the cylinder not the base) to fit and there is quite some amount of brass which can be removed to have it fit.


Jeroen Wolf said...

Thanks again. I tried your piece of paper but I can't feel if it's a proper fit...
However I ordered an adapter like yours (your link doesn't work anymore but I found an identical adapter)
When I have it I will try again and start grinding if I don't get infinity. I'll keep you posted!


obakesan said...

good luck with that, probably it'll fit without further adaptation :-)

remember, grind the diameter of the rear barrel of the lens, not the length of the cyclinder.

Anonymous said...

Good and inspiring but alas, so-oo 0.5 mm away from what I need to do myself with my just-purchased Pentax 6mm TV c-mount lens on my GH2.
I really hope that many people have done this before me!

obakesan said...

I guess its worth keeping in mind that not *EVERY* C-mount can be adapted to work (and in particular not on every adpator). So before spending bucks on a lens best to check out its dimensions :-)

Jeroen Wolf said...

Well, it took a while but I wanted to tell you it worked. The adapter I bought that you had did not do the trick but I bought a Dremel (small drill-like machine) and shaved some more off the outer ring. After 2 tries I got it! So I wanted to say thanks for your patience and sound advice.

Jeroen Wolf

Unknown said...

Just saw this. I have a similar problem with the Kern Switar 50mm f1.4 on the NEX-C count. It's just about 1 to 2 mm shy of getting to infinity, if I file the edge of the lens. But I can't bring myself to doing this to a beautiful lens.