Getting at the front element of a Cosinon 28mm?

sci-fi@yahoo.com · Joined: 14 Oct 2013 Posts: 2 Location: United States

I've got this Petri 28mm f/2.8 M42 lens - it's a Cosinon lens, often seen as a Chinon and some other brands. It's a nice enough lens, but not worth much - certainly not worth paying to have CLA'd. However, there is a hint of fungus on the backside of the front element. I would like to get it off before it gets serious.

The name plate comes off the way most do, unscrewing via the filter thread. But underneath that is just flat metal - the barrel holding the front cell goes all the way into the lens body - there's no retaining ring or screws or anything. The rear of the lens is easily removed by removing screws, and the rear cell is easily unscrewed. So I got to the rear of the front lens cell through the diaphragm, but it's multi element. I'm lost as to how to remove the front cell or front element. I've tried just unscrewing the whole metal casing while holding the lens barrel, but nothing. I've tried moving the focus to the end of the range and twisting the other way, nothing.

I'm stumped. Nothing on the internet about disassembling this lens, either. I guess no one feels it's worth it, but it's actually a not-too-bad lens once stopped down.

Here is a picture of the lens:

and

TIA[/img]

mo · Expire: 2016-07-30

guardian · Joined: 18 Mar 2009 Posts: 1747

That's a very nice lens. I just went through my Petri lenses. My 28mm Petris are both f3.5, so different and I'm sorry I don't have a response on that specific lens.

I wonder are you unable to separate the helical? Have you looked gently and carefully beneath the rubber grip to see if there are any hidden screws there?

My gut feel says separation of the helical might be your key. But that's not anything you can rely on. Once you figure it out I hope you will post here your findings for benefit of others since, without question, that's a lens worth cleaning.

sci-fi@yahoo.com · Joined: 14 Oct 2013 Posts: 2 Location: United States

I worked on it some more last night. It turns out that it was a matter of just twisting the entire front cell counterclockwise - I had just not done it hard enough. I guess that I was nervous about breaking it. But with some serious elbow grease, the front came off. There are 3 elements in the front cell, in 3 groups. Two come out with the front cell, one tiny one is held in by a retaining ring just in front of the diaphragm. I was able to clean the fungus off (that rainbowing came off as well), so now the lens looks great, minus a few streaks still visible on inner elements that I couldn't get off (I can never get glass perfectly streak free with just lens cleaner and/or microfiber cloths).

This lens really is a bargain for anyone who needs a 28mm. I got it on the bay for almost nothing. It's an extremely solidly built lens, with 6 elements. It's pretty soft/hazy wide open, but as soon as you stop down 1 stop it gets sharp. I'll post some pics of the front taken off and some sample shots (pre-cleaning).

Sorry for the crappy IQ, these are just cell phone pics.

All of these are unsharpened, unedited except for my notes. The focus is a little behind the front of the shack, due to my bad eyesight Surprised

/ So look at the leaves in the tree over the right side of the roof for the focal plane. 10.1 Mp Canon Rebel Xti @ ISO 100. Tripod.

mo · Expire: 2016-07-30

Hi can you please resize these as they are too large to view,Thanks.