Can you (or anyone) compensate for my stupidity?
I thought I was careful, I did take (not enough) pictures of the disassembly. I did not quite understand the double helicoil comments and before I knew it the 3 critical parts were separated and lying on the table.
I tried for 2 or 3 HOURS to reassemble these 3 parts and cannot do it. I am certain I understand how they should assemble, but canot get it right.
Any help you can give me places you on my list of really smart people, and thank you in advance.
Hi Dave,
First things first. Welcome to the club! I've been there, and lived to talk about it. Not with a 40mm Hexar lens, but I've done my time with "problem" children like Jena Biotars (not known as the easiest lens to work on), the 80mm Biometar (which, frankly, I thought was worse), and less problematic but still complex Oberkochen Sonnars and Distagons, (and many others I can't recall or don't, perhaps, want to recall!), so I have learned the hard way what to do and what not to do, and still err, occasionally on a new design. It can test the patience of a saint, but with perseverance, you
can get it back together.
One of the reasons, of course, this reasonably complex double thread arrangement is employed is because it enables the focus to be adjusted without actually rotating the optical elements, Ie. the position of the filter ring relative to the focus mark remains the same at all distance settings. It makes it that much more complex than a less sophisticated rotating optic lens to strip down, though.
Not having worked on that particular lens, and without the benefit of being able to examine it, I can only offer some general observations.
Firstly as I am sure you know the coarser threads have multiple starts but in most cases (not all, but typically) only one of those thread starts will mesh in and calibrate focus correctly. (I should add at this point that a suitable camera body on which to mount the lens for checking focus is nigh essential without specialist optical equipment, so I assume you have something to attach it to.)
To make matters more complicated needing to start the correct thread applies to all the threads involved. It's a genuine jigsaw puzzle as all the pieces have to start off correctly. If any one is off you will either get close but not quite there or will be so off beam it won't be on the right street let alone in the ballpark.
Please bear in mind that the presence of the slot in the helical I referred to originally is to enable the focus group to glide along both helicals without its relative position to each one changing. In plain English it won't be screwing out of one and onto the other, as much as moving along them both simultaneously. Now this is important because you may need to start each helical so that at or close to infinity they key together with the slot and key at the right time. Eg. You may have started one or both helicals at the correct thread start: but if they are out of sync to each other by half a turn or a turn (depending on the length of the threads etc.) it won't come in correctly.
I recall when I stripped down the 135mm Hexar (there was a lot of wobble in the barrel, I had to, in order to reach the appropriate fasteners) I had to uncouple the optical group from the helicals before I could remove them. That is where the danger zone starts because I had to note precisely where they were, and where they separated, so that as each one was separately started on its thread, they were linked together by the keyway at their correct height and threads to bring the infinity in on spec.
Where to start?
Well, the focusing thread tends to be a much longer one. It may be a helical (less starts than the main one typically) or it may well be one single start conventional thread, albeit, a finely cut one. If you really are at a total loss as to where to start, I would suggest looking to that one, and ensuring that you are able to get the focus scale at infinity on its thread with what looks like a decent amount of protrusion on the focus ring itself (Eg. with the Biotars I think, from memory there was enough thread that I could wind it a full turn, or nearly, past where it was meant to hit infinity, Ie it went around the clock a few times, looking OK each time it got there but only one installed height would be correct. You don't usually go more than a full turn in on the thread past the correct infinity position. If all else fails, thread the focus ring into its seat as far as it will go, back it off to the infinity mark and it's likely to be right, manufacturers don't cut much more thread than they need to... Now, depending on the lens design the rest of the parts may, or may not, come together with the focus ring installed. But at the minimum, try to satisfy yourself as to where it needs to be, and, just as importantly in many cases, where it needs to be started, because that is of course where you want it all to end up and, hence, is where you are trying to dial everything else into.
So, satisfy yourself that your focus ring is threaded up to hit the scale right or at least, where you need to start it to do so. Working back from there, your next step is probably to take a good look at the keyway, and the helicals, and take a best guess as to where you think you should start threading one according to how far "in" it is likely to go, to bottom out the optics at infinity. You might be super lucky and hit it first time. Expect to find the lens nowhere near where it needs to be and to have to try another start.
Of course with a double helical set up you get to double your fun by doing this with two ends at once. Do bear in mind clues (if you have the pics) such as where the makers name sat "around the clock" relative to the distance scale or camera body. Of course you are aiming for the optical group to end up positioned on its helicals the right way "up". This fundamental point is worth labouring because try not to think of it as threading the optics into the helicals as much as vice versa. Remember, as installed, the optics do not rotate as you focus the lens. It's the helicals that "turn". The keyway prevents the optics from "unscrewing" themselves along the helical threads as you turn the focus ring. Anywhere the makers details on the front sit at the correct orientation is a good basic reference point, you are then mating each helical together with the optical group, via the keyway, to get the optics sitting in the right orientation at the correct distance from the mount.
I'm sorry I can't be more specific on this particular lens. Given that it was in front of me, it might take me a couple of hours (or more) but I'd get it assembled again eventually. There's sometimes an element of luck involved and sometimes you can over think it. With one of the Zeiss Jenas, I was starting the helicals where I thought they'd want to start for an hour or so and it was almost lining up but was never right no matter which start I used out of three or four. Eventually, getting nowhere, I started it off several more away from where it "looked" like it ought to be started and after two or three tries it just fell into alignment. It's probably 40% brainpower, 50% perseverance, and 10% luck (or some other combination of those traits). Keep me posted.
Cheers
Brett
