M50f1.7 problem

Just reassambled my M50f1.7 and can't get it to focus properly ! It would seem I've replaced an element back to front, although I was being careful not to.

Can anyone help ? maybe an exploded view showing element curvatures !? Please
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It's probably the middle (single) lens. Flip that over and give it another try. Don't ask me how I know......
To be honest it is hard to see any difference in lens curvature looking at a side view of that lens.

This is from memory ...
Four elements in three groups.
Front group is two elements together - convex face to the front.
Center group - concave face to the rear.
Rear group - convex face to the rear.

The first quick check is whether it will focus to infinity - and with infinity somewhere near the index mark on the barrel.

Hope this helps
Geoff

I've reversed the front and rear elements (at least twice!) with little effect, but I don't remember removing the centre one at the front, though I do seem to remember cleaning 3 lots ! The centre section doesn't look as though it's been undone ! (no gouge

I'll give it a go Ta.

Geoff it's 6 elements in 5 groups, still good try I would not even start to guest out of my head. link

Huh, single middle lens?
The third one maybe?

Right Stu

If Stefan's cross-section is the right one (and my recollection is just plain wrong ) then it looks like most of the bits are of different diameters.
It is hard to see how the lens would assemble (and still feel right) unless it's the double element (fourth group).
Lets check - what do you mean by "won't focus properly"?
- when looking through the eyepiece across the whole range of adjustment?
- marks on the lens barrel don't line up the way that they should?
That's two different problems.
Geoff

Thanks Stefan (and others), bit late now, looks like I've marked the inner front one with all this messing about

It seemed to focus ok, but when I viewed the images later I noticed I hadn't got centre AND egde in focus at the same time !?

The only two that will reverse (for mechanical reasons) are the front and rear. Tried reversing each of those, maybe I should do both !?
Will try tomorrow, too dark to do infinity shots now

Ta

Hmm, looking at that cross section it must have been the rear one I reversed.
Guess my memory isn't what it used to be. Oldtimers disease......
Same symptoms, centre sharp edges way out.

Lenses are precision made, so taking them apart is potentially full of woes.

In particular, optical glass is very soft, so if you over-tighten the elements then you can actually distort the shape and ruin the quality of the lens. Also, how can you be sure the element is centred correctly?

If anyone told me they had dismantled and cleaned a lens, I certainly wouldn't buy it.

Yep, I know what you mean John.
However as this was only a fiver I figured it was worth the risk. I did put witness marks on all the threads before I started disassembly though.
I'm glad to say it came out very well, ( at the second attempt!)
In fact I can't tell it apart from my other two 1.7's, beginners luck no doubt.
Thankfully the 50mm primes are a fairly simple design. Wouldn't want to try a zoom

Go on, you want to really, go on be a devil...

I have a couple of M 50mm f2s that I've cleaned fungus out of. Both have something of a dust problem now (unavoidable) but the mould seems to be gone.

I really wouldn't fiddle with a zoom, all I did with the 50s was to unscrew the front element group and clean the mould off the elements with a cotton bud dipped in vinegar.

Update, thanks to careful attention with a microfibre cloth one lens now has next to no dust inside. The other will get the same treatment shortly.

From discussions on here John, I assumed this was standard practice !?

The lens was full of fungus and cost me a fiver, so nothing lost, I am however surprised at your comments, I assumed the lens would reseat itself naturally when replaced. Obviously not, maybe that's the problem, I'll have a look

Lenses are very carefully assembled, I presume on machines that have a fixed amount of torque when tightening. The precision is very high and even then decentering can be significant. The DIY option is not one that I'd have any confidence in, especially without the proper tools.