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hacking a lens to fit a C mount

Godfrey

Well-known member
Godfrey - looks good, although there seems to be a surprising lack of DOF for f/4... the bubbles in the espresso are sharp, but the spoon and truffles look to be OOF a bit.

Also, the vignetting along the edges of your shot looks quite straight-edged; I've got the 12.5/1.9 Cosmicar (the older, skinnier one) and its edge-falloff pattern is noticeably round.

-- Is the f/1.4 version "squarer"?

-- Or did you vignette the edges in post to blend in with the natural falloff in the corners? If so -- good trick! I'll have to try that myself.
Thanks for looking and commenting!

DOFmaster calculates that a 12.5mm lens on FourThirds format, focused to 9 inches and set to f/4, will render a total DoF of 1.5 inches. That seems about right, looking at this photo! ;-)

Yes, I add vignetting on these square compositions to blend in the hard corner vignettes a little more smoothly. I think it gives them a unique feel. The natural falloff to the edges of the lens coverage is another interesting adjustment this lens can make: at f/1.4, it has a natural center to edge falloff of about 1.5EV, at f/5.6 the falloff has disappeared, so there is a lot of different qualities to work with when I'm shooting and rendering.
 
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daggerlee

Guest
Thanks for the great work pellicle!

I'm about to order a C-mount adapter from hawkpeng, but which one should I get? The "C5 type TV Bolex C Mount" or the "Bolex W1 C"? Apparently the second one can be used with Pen F and Robot lenses, with care. Both listings use the same pictures, but the second one is more expensive. Thanks!
 

monza

Active member
They are identical except the second one has set screws so it can work the Hawk's Pen F flange, an original Pen F Extension tube flange, or Hawk's Robot flange.
 
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daggerlee

Guest
Thanks for the info monza, have ordered one of the basic ones. Thought about getting the one that can work with Pen F flange but am hoping that someone will soon put out a dedicated Pen F to M43...
 

monza

Active member
The Hawk adapter in combination with the Pen F flange, is essentially a dedicated adapter. Jinfinance also has a dedicated adapter.
 
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patbon23

Guest
Re: Focus to infinity with a tevidon 25mm

Hello,
On this page go to message number 8 - in the picture you will see 4 lenses, yours is the smallest one. If you don't have a lathe, print the picture and take it with you together with the lens and the adapter to a machine shop and explain that you need this part grinding down so it fits better in the adapter - this is why you aren't able to get infinity focus with the lens as it is.

I don't speak Italian, maybe you can understand some Spanish:

Hola,
En esta página sube hasta el mensaje número 8 - en la foto verás 4 objetivos, el tuyo es el más pequeño. Si no tienes torno, imprime la foto y llévatela junto con el objetivo y el adaptador a un tornero y explícale que necesitas rebajar esta parte para que encaje mejor en el adaptador - esta es la razón por la cual no consigues enfocar al infinito con el objetivo como está.
this is my C-mount lens, has been hacked but I can't however to use it at infinity
 

RichA

New member
Hi

I did a hack on a Computar lens to get it to fit an adaptor. I know at least Godfrey is interested to see what i did to the lens (it wasn't major)

http://cjeastwd.blogspot.com/2009/06/adapting-lens-to-fit-mount.html

I guess that this image will say it all for him :)



now, off to take some pictures with it (rather than of it)

:)
A lot of modern larger sensor format CCTV lenses do not fit. Older lenses for "TV" and 16mm movie cameras do tend to fit the RJ adapter, but the TV lenses often lack coverage because of the size of sensors back when. Of these lenses:
-12.5mm f1.4 Cosmicar needs modification and does not quite cover the field.
-25mm f1.4 Fujinon needs modificaction but covers the entire field of the G1, with vignetting that is gone by f11.
-6mm f1.2 Computar fits and its image circle = 1/3 of camera field
-18mm el cheapo Cosmicar fixed f1.6 aperture fits, but image circle doesn't cover the whole field.
-little Pentax 110 24mm needs adapting but does cover the field in the G1.
The 12.5mm and 25mm lenses are 1" CCTV lenses, the others cover smaller formats.
 

pentacon6

New member
Hi

I did a hack on a Computar lens to get it to fit an adaptor. I know at least Godfrey is interested to see what i did to the lens (it wasn't major)

http://cjeastwd.blogspot.com/2009/06/adapting-lens-to-fit-mount.html

I guess that this image will say it all for him :)



now, off to take some pictures with it (rather than of it)

:)
Hi, I am impressed by your work. Since I am not quite familiar with metalwork, may you post some pictures of your setup? I have some basic tools such as driller and a desktop polisher. I would like to know what kind of accessories I should have to trim it. Your demonstration really give me a more clear idea.

Many thanks:salute:
 

pellicle

New member
Hi

Hi, I am impressed by your work. Since I am not quite familiar with metalwork, may you post some pictures of your setup?
Many thanks:salute:
well, ok ... but I think you might be disappointed ... its just a table and my leatherman (which has a file on it). Totally nothing fancy.


as to my workmanship, heck I'm a bit shy about that, by metalwork standards its like the advice given by the cabinet maker to the carpenter

paint it and no one will notice
 
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patbon23

Guest
After two hours of work :eek:, my cosmicar 12.5mm works :thumbs:, as soon as possible I'll send the pictures
 
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patbon23

Guest
this is my "new" Cosmicar 12.5

Photo take F5.6 focus infinity :D
 
T

thearne3

Guest
Hi,

Just received my Computar 12.5mm f1.3 and before I start grinding, I have a question: should it rattle? When I shake it in the same axis as the light would pass through (lengthwise), there is a distinct sound of something moving inside. The sound is minimized when the aperture is wide open.

I have never had any lens behave this way. Is this unique to this model or a fault? The cosmetics of the lens 'like new'.

Any advice?

Thanks,
Tom
 
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