lens assembly help needed

[Fast14riot]

I decided to tear apart my cheapie Sigma 28 f/2.8 mini-wide today because something condensed on the elements inside. I wasn't too concerned, I've pulled lenses apart before and frankly wasn't going to be to upset if I screwed up this lens.

I got things cleaned up and mostly reassembled, but am having difficulty with the foucing ring. The inner group has th coarse thread for focus, there is a reverse thread lock ring that the focus ring attaches to. The inner group and lockring to body is where I'm having trouble. Anyone have ever dealt with these? Anyone have service instructions? Or know where I can find them? This lens isn't worth having a pro work on, I'm not ready to throw it in the trash yet either.


-Xander

 

[Gerald C Koch]

Too late to help now but for future reference professional repairers make small scratches on internal parts to show the alignment of various components. You often see them when repairing old lenses.

 

[Fast14riot]

...yeah...


-X

 

[David Lyga]

it is so difficult to pinpoint just what you are trying to say, Fast, but I have taken apart many of this type. One thing to remember with 28s (and wider) is that the direction of the many elements is of paramount importance and you MUST make notes of what you are doing as you disassemble. Perhaps in your case you left the individual sets of element intact. Then you will have no problems with what I just said.

I get the fleeting impression that your problem is with the helicoid. You must re-enter the helicoid (where the individual coarse threads mate) at the correct point. There is a (brass?) ring that has fine threads on the outside for screwing onto the lens body and coarser threads on the inside which mate to the coarse threads on the front element group. It is difficult to explain precisely in your case as there is variation with the different off market 28s out there, but you must make sure that, when fully threaded, you are able to focus to infinity. Call me if you think that I might be able to explain more fully. No promises. 215.569.4949 - David Lyga

 

[Fast14riot]

David, yes its the helicoid and ring that attaches it to the body that's giving me fits. I'll have to get in touch with you tomorrow as I am working and the time difference between us would make I too late to call. In the mean time I can take some pictures of exactly where I am having trouble and email them to you so you can see what I am dealing with.


Thanx!

-Xander

 

[David Lyga]

Try any hour. I disconnect my phone when I am sleeping. You will NOT bother me. But, again, no promises. Hint: you just might get me at 5-6 AM EST. - David Lyga

 

[EdSawyer]

The helicoid will mate up at several locations. You have to try them all to make sure you get the right one.

 

[Fast14riot]

Yeah, that seems to be the issue I'm running into, I was hoping there was a reference measurement or something. I'll try that out and then possibly give David a call.


-X

 

[David Lyga]

Sometimes the 'mating' can be different from what it was when you initially took it apart because on some lenses there is an ability to set the focus at different points of the helical turn and match this new turn with the proper point on the focus ring.

Actually, you probably did not have to remove the helicoid; you maybe could have simply unscrewed the front and rear element sets. On some 28s that is so. But I usually do remove the helicoid because this gives me an opportunity to remove all the black gunk with lighter fluid and a brush, then re-lubricate with a tiny bit of Vaseline petroleum jelly. Most lenses have too darn much of this lubricant and ONLY NIkon was sparing in its application. That's why the focus on its lenses rarely stiffen with age. - David Lyga

 

[John Koehrer]

Unfortunately the helical must be correctly aligned.
If it's not correct, the lens may extend either too far allowing the helical to separate or; the flip side to this coin is the inability to focus closer than infinity.
The focusing ring being able to move 360 degrees as used in inexpensive lenses to allow the ring to be adjusted to the focus index on the barrel.

Vaseline may work(?) so does wheel bearing grease or possum fat, but a white grease is a correct solution.

 

[David Lyga]

No John, on most lenses you have a bit of leeway.

The screws that hold the focus ring in place allow the helical to be moved to a somewhat different place on the turn that begins the mate. This would 'make up' for the 'incorrect, exact alignment' that you were 'supposed' to have at the theoretically exact mating location. The narrow brass ring that matches the body of the lens to the helical has narrow threads on the outside. Those narrow threads allow for ONLY ONE point in the 360 degrees to mate to the lens' body. However, the inner threads, (they are coarser), allow for about three or four points on the 360 degree turn to mate to the helical. THIS is what we are talking about when we speak of the exact point on the turn to mate: mating the helical to that narrow brass (or aluminum) ring.

Yes, It is difficult to describe in words, but, in essence, these are the two ways to get that precise maximum focus that is infinity (ie, not before or after, but exactly infinity). In other words, if you put the lens back together again and the lens does not quite focus to infinity (or slightly exceeds infinity), you can adjust those screws that hold the focus ring in place to allow the focus to achieve infinity (ie, the true 'infinity' distance from the rear element to the film plane can thus be achieved). With SLRs this is handily achieved without too much fuss because of that VF prism which guides you to the correct determination.

Of course, that determination will be accurate ONLY if the VF focus is matched, precisely, to the film plane focus. That is where the angle of the mirror become paramount in importance; slight deviation in its height can forward false information, allowing one to think his/her lens is not sharp when the lack of apparent sharpness is the result of the lens not being in REAL focus (even though the VF says that it is!!!). That mirror rests upon either a prong or a set screw which is the sole determinant as to the mirror angle. It is very important that that angle be correct in order to match the VF (apparent) focus with the film plane (actual) focus. With set screws (Zenit, Fuji ST) all you have to do is adjust it. With prongs (K1000, Minolta SR-T) you must, ever so slightly, bend it. Cardinal rule: IF ACTUAL (FILM PLANE) FOCUS IS IN FRONT OF APPARENT (VF) FOCUS, THAT MIRROR HAS TO BE LOWERED SLIGHTLY. IF ACTUAL (FILM PLANE) FOCUS IS IN BACK OF APPARENT (VF) FOCUS, THAT MIRROR HAS TO BE RAISED SLIGHTLY.

Honestly, John, I think that Nikon alone used the 'white' grease, and appropriately sparingly at that. The others seemed to use either Vaseline or axle grease (!) and used too much so that, with time, those lenses attained a stiff focus when the grease dried out (horribly stiff in cold weather!!!), leaving much solid detritus behind to clog those narrow threads!. Less would have been more but 'dampening' and making the focus turn smoother (only when new!!!) was too trendy and trumped pragmatism. Time revealed this manufacturing error. - David Lyga

 

[Fast14riot]

David, sorry I haven't called you yet, I got stuck working on my days off this week. I will give you a call this coming week one day for sure.


-Xander

 

[David Lyga]

Fast:

don't be so fast to think that I will be able to solve your problems. I get paranoid when people begin to count on me. I am no sage.

I certainly will try to walk both of us through this miasma, but, you know that off market lens construction differs from each others products. I gave generic answers that might hold some water here but, again, no promises. I will try to help. - David Lyga

 

[Fast14riot]

I absolutely understand, I am very mechanically inclined but having someone who has done something similar to what I am attempting will be a help, none the less.

-Xander

 

[John Koehrer]

Yup, I read it as the helical had been separated.

How ever you're not going to find any photographic lens lubed with Vaseline. It liquifies in modest heat. Even possum fat won't do that.

 

[David Lyga]

Honestly, John, I always use petroleum jelly and it works fine, even over the years. (And, yes, the 'heavily sedated for your protection' part is riotous.) - David Lyga

Fast: yes I have done this to about 200 lenses (or more). - David Lyga