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05-18-2013, 04:49 PM   #1
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So do M series 28s not focus to infinity or what?

I have noticed that infinity focus seems a little tight on a number of my manual primes - enough of them, in fact, that it has me wondering about actual registration / flange distance on my K-x. (Even my DA L 18-55 is a hair short of true infinity at the short end.) My M 28/3.5 may be the worst offender here, it doesn't get a lot of AF confirm beep action for distant subjects at all, and several of my other 28s are noticeably sharper at 20-50 meters+. Now when reading reviews of this lens and its f/2.8 companion, you often find "it's nothing too exciting for distant subjects" or similar statements, including "corrected for intermediate distances". While I don't want to exclude the possibility of the latter, could it be that these lenses routinely do not reach infinity right from the factory?

Using a DOF calculator, I find hyperfocal distance for 35mm film even at f/5.6 to be less than 5 m, and in fact it would still be less than 8 m wide open. So possibly these lenses were adjusted for that instead? That wouldn't be good by modern-day pixel peeping standards on crop DSLRs. A slightly too big effective registration distance in the camera obviously wouldn't help either, seeing how little a 28 actually moves between 3 meters and infinity.

A little test shows that with my M 28/3.5 sample mounted on my K-x, focus indication at a measured distance of 2.3 m is 3 m, or very short of this at best. So it does look like calibration on this particular combination is in fact off. At 2.0 m measured, the 3 is about halfway between the red focus indicator's tip and the IR infinity indicator on the DOF scale, which I'd normally read to be about 2.5 m (taking a look at a 50, the 2m position should be right in the middle between 1.5m and 3m, and we're definitely closer to the 3m mark at this point). At the same time, an old Porst (Cosinon) Auto 50/1.7 M42 lens (using an adapter that retains infinity focus) is pretty much bang-on, a newer PK mount relative shows about 2.1m, and my M 50/1.7 even reads 1.9m or thereabouts. A battered Auto Revuenon (Chinon) MC 35/2.8 with some play in the lens barrel gives maybe 2.1-2.2m or so. None of them are very far out.

That leaves us with two questions:
1. Is this typical for M 28/3.5s or even M 28s in general (which I suspect to be the case), or did I just get a misadjusted one?
2. How would one go about adjusting infinity focus on this lens? Screws under the front ring?

05-18-2013, 06:11 PM   #2
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Sorry 52mm. My copy was so disappointing I don't use it. back in the days of film my favorite lens was 35mm. I wanted a 28mm which is about the same for a cropped sensor. I eventually found a really good use for my M28. I have put it onto my K10 to keep the dust out as I no longer use the camera.

I hope you can get your problem sorted and get some joy from the lens.
05-19-2013, 12:01 AM   #3
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QuoteOriginally posted by 52mm Quote
2. How would one go about adjusting infinity focus on this lens? Screws under the front ring?
If the construction is similar to the M50 then try this. Don't use a normal crosshead screwdriver - the screws are JIS head and if they are tight then you'll just strip the heads. Get a proper JIS driver or use a small flat blade one. To get the faceplate off, a suitable round object (eg a M42 rear cap) inside a disposable rubber glove usually works, if the filter threads are undamaged and clean. The usual caveats apply - don't undo anything you don't need to undo, keep track of what goes where, mark positions of anything that turns before you turn it, take your time etc...
05-19-2013, 09:30 AM   #4
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My Rikenon-P copy is so excellent, I shall never know how the M series performs and my SMC-A seems equally good. I hope your M28 can be tweaked so it performs great for you.

05-19-2013, 01:35 PM   #5
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It's not like it's a super big deal to me, as I do have several 28s (though I don't find any of them to be an awful lot better than the lowly kit lens, and some worse; generally, none of them render fine detail at a distance with as much "bite" as the aforementioned Chinon 35mm job). But being a scientific type, I'm always interested in why things are the way they are.

My M28/3.5 does do fairly well on a test chart @ ~1m, sharpening up faster than, say, a Vivitar K03 or the kit lens. But that's obviously of no use if it fails where detail retrieval is needed most often, i.e. at a distance. After a little garden test shoot, I'd say maximum focus distance is at about 10m.

As far as field curvature goes, I pretty much do not have to refocus when turning the camera so the focused object moves from image center to edge, i.e. the focal plane is most definitely curved. It ain't a macro lens for sure! (Expectedly, it didn't do too well on an extension tube.) However, I can see why this property may actually be useful for street shooting and the like. Sure beats having a focal plane curved in exactly the opposite way. Besides, many of my lenses seem to be like that, though obviously a larger viewing angle (shorter focal length) will magnify the effect.

I am still looking for the right size tool for the front ring. So far, nothing too suitable has turned up. I'll keep looking.
05-20-2013, 11:23 AM   #6
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I recently noticed that my m42 28mm f/3.5 wasn't focusing at infinity either. I have a split screen and couldn't get the image to "line up" at infinity. I used this link Takumar Super Multi Coated 35/3.5 disassembly - to help me with the disassembly and adjusted it to work correctly.

Follow kh1234567890's advice.

What I did to get the front ring off:
Cut thin pieces of electrical rubber insulating tape (not vinyl) and placed them on the front ring.
Used a plastic bottle cap that just fit inside filter ring. (This clears the glass in the center so you don't scratch it)
Hold the lens tight and push down hard while turning.

Good Luck!
05-20-2013, 12:05 PM   #7
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I just tried the -M 1:3.5 28mm and the -M 1:2.8 28mm and the Rikenon XR 1:3.5 28 mm on the K-01 with focus zoom, and the MX split screen with Magnifier M.
They are all focussing on the infinity stop on a tree branch about 1000 metres distant.
05-22-2013, 09:48 AM   #8
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Must be a sample / batch thing then. The S/N of my M 28/3.5 is 6626297.
The M 35/2.8 that I got in recently will focus to infinity just as much as any other of my primes. Maybe a hair short, nothing dramatic (like I said, that might be my K-x's fault).


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