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25mm C-mount choices: Computar, Fujinon or Pentax/Cosmicar?

Discussion in 'Adapted Lenses' started by the.growler, Jul 22, 2011.

  1. the.growler

    the.growler Mu-43 Regular

    Looking to tap the collective wisdom of the group for advice on a 25mm C-mount lens. I'm trying to resist the lure of vintage cine lenses and sticking with CCTV lenses to keep costs down. Any experience with the Computar, Fujinon or Pentax/Cosmicar 25mms? Any issues with mounting these lenses?

    Thanks in advance for your help!
     
  2. ekoe

    ekoe Mu-43 Regular

    73
    Sep 26, 2010
    You'll find the CCTV lenses for 1" video are better proportioned for m4/3 and easier to focus and handle than the legacy c-mount cine lenses. Especially for video.

    I love the Fujinon-TV c-mount lenses. They're excellent. I modify the Fujinon-TV 12.5mm/f1.4, 25mm/f1.4 and 35mm/f1.7 for m4/3. I highly recommend the Fujinon-TV 25/1.4 CF25B. It's my favorite normal lens on m4/3.

    Have a look at this comparison between the Fujinon-TV 25mm/f1.4 CF25B and the CZJ Tevidon: CZJ Tevidon versus Fujinon CCTV - The GetDPI Photography Forums

    The Fujinon-TV 25/1.4 CF25B does need to be machined. If interested, I can perform that modification for you. I also have the CF25B in stock. PM me if interested.
     
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  3. wojtekcz

    wojtekcz New to Mu-43

    1
    Aug 13, 2011
    I need to post a reply in order to PM
     
  4. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    Actually, if we're talking about the same lens, you don't need to machine the 25/1.4 CF25B--you only need to take off the collar beneath the aperture adjusting sleeve and replace it with a thinner spacer. The collar removes with three screws (easy), and I made a spacer out of an old bottle cap I trimmed up. The spacer simply fits between the mount and the adperture adjusting sleeve and keeps pressure on it so it doesn't slide down. With the adapter I can focus to infinity, no problem.

    Does this make sense?

    EDIT: ekoe, I know you're the expert on this--I've read many posts on modifying, and you're usually the one cited! So please correct me if I'm wrong.

    EDIT, AGAIN (!). Actually, I remember where I got the idea from--in the thread you cited above! In that thread there's a description of how to adapt the CF25B to infinity focus without machining. Much better explanation than mine, along with photos.
     
  5. tigertiger

    tigertiger Mu-43 Regular

    29
    Jul 30, 2011
    So, Fujinon and Fujian are not the same thing?
     
  6. starlabs

    starlabs Mu-43 Top Veteran

    856
    Sep 30, 2010
    Los Angeles
    I have the Pentax/Cosmicar 25mm and I'm happy with the build quality. Pretty sure it's metal (it's got a small little "heft" feel to it).

    Got it for $90 off eBay awhile back, came with the c-mount adapter too.
     
  7. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    Definitely not. Fujian is a Chinese lens--cheap but supposedly pretty good. Fujinan is made by Fuji, and is a high-end CCTV lens, and is not particularly cheap.
     
    • Like Like x 1
  8. ekoe

    ekoe Mu-43 Regular

    73
    Sep 26, 2010
    Hello Mellow,
    I remember RichA had a fix to replace the retaining ring with the collar of a plastic film container. I overlooked that solution in favor of keeping the original retaining ring and more robust grub screws. I can sort of see how RichA's fix or the bottle cap fix would work, but if the lens gets bumped or jarred couldn't the lens just drop off?
    To keep the original utility and integrity of the lens and because the mount must be rigid to hold collimation, I can't use epoxy or appropriated materials... but I respect that ingenuity and I do use plastic juice caps as rear c-mount caps and there was a milk carton cap I liked to use for the m4/3 mount for protection during shipping (but I can't find that cap anymore).
    What I do is turn the retaining ring on a precision metal lathe, removing approximately 2mm H x 3mm W. The height adjustment tends to vary by a few thous from lens to lens, and each CF25B must be individually adjusted into it's sweet spot for infinity focus. I call it 'hand working' a lens although much of it's done on my machine. Sometimes the best tool for making those fine adjustments is a smooth pillar file.
     
  9. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    ekoe,
    Actually, I don't think the 'plastic shim' technique affects the rigidity at all. The lens seats on a different surface, not the back end of the metal sleeve (the one that you shortened by machining). In fact, if I recall, the metal sleeve can't be too tight against the focus barrel or else it wouldn't be able to turn.

    As far as I can tell, the metal sleeve just is a filler . . . something to keep the focus barrel from sliding off the back of the lens. The problem with putting the lens on a C-mount adapter is that it gets pinned between the focus barrel and the adapter, which doesn't allow the lens to seat properly.

    Since the metal sleeve is just a filler, replacing it with a plastic shim doesn't affect the lens rigidity or anything like that.

    In my first attempt at making a plastic shim I ground it too much, and it was too small. The result was that the focus barrel was able to slide a little bit back and forth--but the lens was locked down tight and rigid. The trick is to get the length just right so it fills the gap perfectly without pinching the focus barrel.

    Sorry, but I'm describing all this from memory . . . I've since replaced the metal sleeve back on the lens (unmodified) because I'm thinking of selling it.

    Hope this makes sense!

    (Undoubtedly the machining is a more elegant solution . . . but it does leave the lens permanently modified. The advantage of the shim is that you can put it back the way it was.)
     
  10. apicius9

    apicius9 Mu-43 Veteran

    348
    Feb 1, 2010
    Honolulu, Hawaii
    Maybe it's my lack of imagination, but does anybody have a picture of such an improvised modification on the 25mm Fujinon? Before I send my lens half way around the world to ekoe, I want to at least get a better understanding of the other options. Oh, @ Mellow - if you sell your lens, I'll take your plastic sleeve, that may be the least amount of work ;)

    Thanks,

    Stefan
     
  11. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    Stefan,
    OK, I'll try to put it back together tonight so I can take pictures. It's not that hard (though you do need a very, very tiny screwdriver!).

    One more thing: I'm pretty sure ekoe's modification is more than the mount. You need to confirm with him, but I think he also aligns the distance scale properly so you can zone focus, and aligns the lens so the f-stop and distance scale appear on the top, where they belong. With some lenses I think he even adjusts the amount of vignetting (though I have no idea how one would do that!). Point is, there may be more than one reason to have someone who knows what they're doing (i.e., NOT ME!) work on your lens.
     
  12. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    Modifying a Fujinon 25mm f/1.4 lens with a plastic shim

    OK, here are some pictures to show what I mean.

    This is the unmodified lens. The lens actually seats into the adapter along the shiny silver surface, not the end of the black sleeve. But the problem is that the black sleeve jams against the outer parts of the adapter (farther from the threads) before the lens can seat. This causes the lens to be positioned too far from the sensor, so you can't get infinity focus.
    [​IMG]

    Here I've removed the black sleeve by loosening the three set screws with a very tiny screwdriver. If I were to screw the lens into the adapter now it would fit snugly, but the aperture barrel would slide down (NOTE: I mistakenly called this the 'focusing barrel' in the last post . . . it's actually the sleeve that adjusts the f-stop).
    fujinon-8319665.

    Here I've replaced the black sleeve with my high-tech plastic shim, made from a pill bottle cap I sawed off and sanded to the right thickness:
    fujinon-8319666.

    Finally, here is the lens mounted onto the adapter. The shim is sized so that doesn't allow much fore-and-aft play in the aperture adjustment barrel, but still lets it turn freely. Obviously this would have looked better had I chosen a black bottle cap, but I used what was lying around!
    fujinon-8319667.
     
    • Like Like x 1
  13. ekoe

    ekoe Mu-43 Regular

    73
    Sep 26, 2010
    I see what you mean, Mellow.
    The c-mount thread is not retained by the rear ring, but rather by 3 screws located under the aperture ring. It's the aperture ring rather than the focus ring that the rear ring retains, and it also provides the detent click-stops for the aperture in conjunction with a small spring and bearing. Without the rear ring you'd have no click-stops, but also the aperture ring can slip off the base of the lens. So the film canister or bottle cap is actually holding the aperture ring in place. I imagine you could also wrap a zip-tie around the base to hold the aperture ring in place.
    What concerns me with this approach is the exposure of the lens, particularly outer components of the aperture, to dirt and moisture that could migrate inside the lens.
    I like that you're keeping the rear ring intact, but have you kept the detent spring and bearing as well?
    The modification I do to the rear ring does not alter the original functionality of the lens in any way. That's a key principle I follow for all my modifications because I also use c-mount lenses on motion picture cameras. So while the modification is 'permanent', it does not rob the lens of any functionality.
    Another aspect of the retaining ring, is that it serves to align the lens for upright marks. With the rigid mounting of the c-mount thread by 3 screws beneath the aperture ring, it's difficult to align the marks where you want simply by rotating the c-mount thread within the 3 positions provided by the screws. This is where the fine adjustment of the rear retaining ring height is integral. You can vary the position of the lens barrel marks by altering the thickness of the retaining ring (and the outer face of the adapter if needed), causing the ring to come to rest against the m4/3 adapter at the desired position for upright lens barrel marks.
    This is the method I've arrived at after much consideration of how to properly modify and collimate the lens to work seamlessly on m4/3, while keeping the original appearance and preserving any legacy compatibility.
     
    • Like Like x 1
  14. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    ekoe,
    I'm sure your method is better--mine was quick and dirty, but worked. You're right, I lose the indent stops for aperture, so it just turns smoothly--but I've had other C-mounts where they're designed like this (old cine lenses, in particular) so this doesn't bother me at all. I really don't know if I've retained the 'detent spring' or bearing because I have no idea what they are! I did the entire modification with the lens upside down, as shown in the pictures, so I don't think anything fell out (or jumped UP).

    Guess I'll put it back together now and see if it still works, LOL.
     
  15. ekoe

    ekoe Mu-43 Regular

    73
    Sep 26, 2010
    Thanks for those photos. I agree the lens is really nice without the click-stop aperture too. I offer the stepless aperture as an option on my mods for subtle adjustment of aperture while recording video. I replace the original grease on the aperture ring with a small amount of thick helical grease to obtain fluid action. It's typically still photographers who would miss the detents, but many of the best cine lenses have aperture detents as well, including half-stops.
    I think I have a spare spring and bearing around here somewhere. Let me know if you need them and I'll locate them. They're practically invisible and very easy to lose.
     
  16. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    Bearings and springs

    I think I see the spring--it's really tiny, and attached to the removable piece (the one you modify and the one I removed). I assume there should be a tiny ball bearing that goes between the two that causes the barrel to 'click' into place--is that right? If so, you're right--I've lost it.

    Oh well, I guess this lens just won't have click-indents.

    Thanks for all your responses . . . you've been very helpful. I have another question you might know the answer to: is it possible to modify a lens to reduce vignetting? I'm asking because I have a 12.5mm computar that's really quite nice but vignettes heavily, even some in 6:6. I seem to recall reading a post from someone (you?) who claimed to be able to semi-correct that.

    If it's not you, do you know how it's done? Is it something a careful amateur could do?
     
  17. apicius9

    apicius9 Mu-43 Veteran

    348
    Feb 1, 2010
    Honolulu, Hawaii
    Excellent, thanks for the pictures. That seems like a quick and easy solution, but I think I am leaning more toward one that goes beyond that. I had a lot of different 25mm c-mounts and sold most of them in the meantime. I want to keep a few nice ones, and from all I have seen, the Fujinon is worth the modification. A nice contrast to my ancient uncoated Cooke Kinic and probably a bit nicer in the colors than the Computar (I hope). Still debating with myself whether to keep the Angenieux 25/0.95. Now, if the Angenieux could be modified to have less vignetting, that would be interesting but I don't think I want to provide mine as a test object ;)

    Stefan
     
  18. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    Angenieux 25/0.95 . . . . I'm jealous!

    (I have the Kinic too and it's just SOOOOOOOOOO much fun, but not for every photo.)
     
  19. RobWatson

    RobWatson Mu-43 Hall of Famer

    Navitar!

    The Navitar 25mm F0.95 is OK and fits a Fotodiox adapter and has no interference and gets infinity focus (plus a bit as the adapter clearly has to wrong back focus distance).

    Quite soft and swirly wide open but fairly sharp at F2-F4 and if I recall my old measurements from a few years ago maximum sharpness at F2.8.

    Note that many of the photos in my gallery are asymmetrically vignetted due to the cameras IBIS being broken. No vignetting in video!

    Cost wise this lens is no bargain.
     
    • Like Like x 1
  20. ekoe

    ekoe Mu-43 Regular

    73
    Sep 26, 2010
    Hello Stefan!
    @Mellow... The modification I do to reduce vignetting is for the Fujinon-TV 12.5mm/f1.4. It started with the efforts of GetDPI forum members 'Vivek' and 'zcream', then I picked it up and developed it a bit further.
    The unmodified Fujinon 12.5/1.4 provides the best sensor coverage among the fast 12mm c-mount lenses, and with the modification it provides a pinch of vignette. More about it here: Flickr: Discussing ekoehler7 Fujinon-TV 12.5mm/1.4 full coverage modification in C-Mount on Micro 4/3
    and,
    Fujinon 12.5mm/f1.4 modification - Mu-43 Gallery
    It's not something that can be done with every lens. There's a few variations of the Computar. I offer modification and adjustment of the Computar 12mm/f1.3 (approx. 2" tall, black housing, standard CCTV in appearance w/ rubber focus grip), but I don't believe it can be modified for fuller coverage with the elements provided. There are however older, larger Computar 12mm's in silver and zebra-stripe housings I have not considered. Which variation of Computar do you have?