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07-10-2009, 03:43 AM   #1
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Diy lenses anyone?

I just reached the point of semi-functionality with an old c-mount lens I've been tweaking to mount on my Pentax. Has anyone else on here built or modified thier own lenses?

I'll show some samples when I get back from a weekend camping trip.

07-10-2009, 07:18 AM   #2
Voe
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I tried few times to modify a c-mount lens without sucess.
Can you post some pics? You got me interested
07-10-2009, 07:53 AM   #3
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Check out the Impossible Lens Club on this forum... and Manual Focus Lenses :: Index
07-10-2009, 09:52 AM   #4
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I'd rather Manual Focus Forum

or even: Alternative Gear & Lenses - FM Forums

Infinity + C-mount and other cine lenses, Leica M etc. on a digital camera: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/43249-pentax-110-lenses-g1.html

F/0.75 lenses will become f/2 super-macro lenses on DSLR: exhibit fast lenses | muellerworld
and then it looks like this: Need for Speed

Hello Steve,
hope you have a nice trip. I love your expression "semi-functionality". The above links show they end up with microscopic macro-pics, mmmh.
Still we are nearing the c-mount-at-infinity-on-digital-world from different paths, but it seems there is no mirror involved in any of them.
Best, Georg (the other)

07-10-2009, 01:53 PM   #5
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I've got to make this quick so please forgive me for any typos.

take 1 computar 4.8mm tv lens (c mount)
remove the two centermost lens elements
replace the rear element with one from a cosmicar 8.5mm
hack together a mount using an old body cap as shown in photo below
shoot at f8

I'm pretty sure I flipped the third element as well.

I'll play with it this weekend and post some pics when I get home.

Edit: I'm using it on my K10.
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PENTAX K10D  Photo 
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PENTAX K10D  Photo 

Last edited by Steve Beswick; 10-24-2011 at 08:42 PM.
07-12-2009, 05:28 AM   #6
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Interestinfg effect!
As you have discovered the main problems with C mount lenses is that the flange focal distance is much too short and the image circle is much too small (most C mount lenses are designed for sensor upto 2/3" or 8.8 X 6.6mm). How much trial and error did it take before you managed to increase the flange focal distance and acheive these images?

This thread has inspired me to gho back to a load of things I have been meaning to do for a while. With the intention of something simple that anyone can do I though it was about time to try a pinhole lens. There is loads of stuff about making one on the web, most of which start with a body cap. I didn't have a body cap spare but I had a spare T mount adapter lying around. I am also lucky enought to have a lathe and so I machined a plastic disk to mount the pinhole onto. This is totaly over engineered and the same result could be acheived much more easily with cardboard and tape. Anyhow here is the result and a picture of the finished pinhole lens on the front of the GX20. The exposure time for this was 10 sec so a tripod is a must. A bit of fiddling will be necessary to get the pinhole just right, the smaller the hole the sharper the image but only untill diffraction takes over.

Next on the list: A zone plate (diffractive) lens Zone plate - Wikipedia, the free encyclopedia . Anyone tried one of these bofore?

EDIT: A few back of the envelope calculations reveal:
The pinhole should be as close to the sensor as possible,
The optimum pinhole size (where geometric and diffractive blur are equal and the overal blur spot is minimized) is about 0.25mm
The minimum blur spot size (for a distant object) is about 0.25mm which on the GX20 or K20D is around 50 pixels.
Anyone with a good 1:1 macro should be able to measure the pinhole size by pixel peeping a macro photo of it.
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Last edited by MattGunn; 07-12-2009 at 06:12 AM.
07-14-2009, 02:12 AM   #7
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QuoteOriginally posted by MattGunn Quote
Interestinfg effect!
As you have discovered the main problems with C mount lenses is that the flange focal distance is much too short and the image circle is much too small (most C mount lenses are designed for sensor upto 2/3" or 8.8 X 6.6mm). How much trial and error did it take before you managed to increase the flange focal distance and acheive these images?
There are a few things I probably knew but didn't figure out until I was almost done. Fixing the registration distance also:
- enlarges the image circle
- stretches the focal length
- darkens the image

the last two are not really desirable but the first one is.

there was a considerable amount of trial and error, with two distinct moments of realization. The first was when i discovered that I should try removal of some of the elements. The second occurred when I swapped the rear most element for one from another, completely different lens. this alone had the single largest affect on the registration distance.

I thought I'd throw in another image, this one from my camping trip. Enjoy!
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