Disassembling FA28-70 f4

Hi all,
I have an SMC-FA 28-70 f4 which has some contamination on one of the rear elements. It doesn't look like fungus (ie not streaky), but more a misty kind of thing.

I have sourced a replacement which is fine, and since this one is almost certainly beyond economical repair I was going to have a stab at a repair job (at least before chucking it in the bin!!)

Does anyone have any suggestions, maybe even an exploded diagram, that would give me a couple of clues on how to get in (and back together!).

Thanks
Matt

Hi Matt,
I will have a look at mine tonight and try to post something. In the meantime, can you give a clue as to where the problem is? Is it definately on the rear group and if so on the back of it or between the elements?

Kim

Thanks Kim!
If you look from the rear, it appears to be on the second element (i.e. not the element that's touchable. If you set the zoom to 28mm, you can see the element about 1.5cm inside, held in by some kind of collar.

It appears that the rear is the only way in. I would guess that by removing the four small screws around the edge should allow removal of the black cover, and then unscrew the elements.

Let me know what you find!
Matt

Actually, it's three screws to remove the rear black cover. The fourth (nearest the aperture finger) holds a stop in place. Don't remove this! (It's a real pain to put back!)
Having removed the cover, I can't for the life of me see what comes next!!
Matt

Hi Matt,
I am loath to take apart a fully servicable lens when I don't have to so this is my best guess rather than a proven fact.

I was going to say leave the four screws around the face of the silver rim in place. These allow you to remove the ring which in K mount lenses give you access to the aperture/stop down mech. If you do take these off, be wary of the ball bearings underneath which give the detents. In the case of this lens it shouldn't be necessary in order to remove the rear group. If you have removed the black plate, it should now be possible to unscrew the rear group. This may be loctited and stiff to start. The group should have 3 holes around the rim. These are desined to take special tool with 3 spikes to "grip" the group rather than a lens spanner. You may be able make an appropriate tool. For lenses with 2 holes, I use a modified pair of thin nosed pliers with the tips ground down. Another alternative may be to use a "lens bung" Any sort of round bung with a grippy surface roughly the right size will normally work. Be careful if you try this method here. The rear element protrudes and you may have to carve away the center of the bung to prevent damage. The proper bungs are made of very soft rubber so are easier to use. I would go for the holes first.

Hope this makes sense. : Remember to use the D or make diagrams as you go so you get it back in the right order.

Kim

Not strictly relevant Matt, but I've got a Tamron SP flat-field 2x converter (the good one) that I picked up quite cheap because of a misty rear element.
It was a simple job to unscrew it and a gentle wipe with a lens tissue removed the mistiness completely.
I don't know what caused it in the first place but it's remained clear ever since - so you never know

I'm rather intrigued to know how accurate you think the replacement of the elements or groups would be. (And perhaps how accurate it needs to be). What about the amount of torque applied when re-tightening?

I only tried opening a lens once - a SMC Pentax-A 50mm f1.7 with a suddenly faulty diaphragm ring. Do watch those ball bearings, they roll a long way....

It seems most likely that this is the element separation problem for which these lenses are infamous.

As far as I know, there is nothing to be done. It is unlikely that you will be able to clean off the old adhesive fully and then glue the elements back in perfect alignment. (If you do manage to, you could start a business.)

These are reputedly excellent lenses. It is a shame this happens to them. Funnily enough, the 28-70 f2.8 also has a fatal flaw, but mechanical rather than optical.

In Pentax, this zoom range is to be avoided.

G

John,
Generally speaking, reseating elements or groups in a good quality lens is not a problem providing you use good practice and make sure there is no dirt or dust traped when you do it. Most elements fit against a machined flange and the purpose of the locking collar is purely to hold it against that flange. As such, the torque is not that important as long as you don't apply too much and crack the glass. Somewhat OT, but with the FSU lenses, there is more of a problem. The groups are spaced using alloy rings which are fairly soft. Not only are they not well set up in the first place but overtightening can cause compression of the rings resulting in collimation errors. Not too much of a problem on SLRs as this can be accounted for because they are WYSIWYG but it can be a serious problem on rangefinders.

George,
From Matt's description, this doesn't sound like a balsam fault. This usually starts at the edge and will only affect part of the area. In the odd cases, it can start in the middle but this is unusual. There are several firms who will re-balsam a lens group although it is not a "cheap" job.
As to the 28-70's, I can vouch for the quality of both lenses. I wouldn't get the F4 version unseen for the reasons you mention but as long as you get one without the problem, they are very undervalued and in a pound per quality unit contest, they are probably one of the best value lenses you can get.
As to the mechanical properties of the 2.8, I think the problem is somewhat overstated. It is a big lens and certainly not cheap. With the amount of glass that needs to be moved by a smallish motor, it would have been better if Pentax had made it IF and internal zoom. This would also have avoided the rotating front element. The price and spec push it very much toward the pro market. In comparison with some other lenses, it probably is not strong enough to stand up to the rough and tumble of prolonged pro use. The problem mostly stems from use of the power zoom. I tend to avoid the power zoom as I find it too coarse, so with the care of normal amatuer use and more so if you don't use the power zoom, it is a perfectly viable lens which has an incredible performance and is a delight to use. I have compared some results alongside the Sigma EX equivalent and the Pentax is a far better lens with none of the compatibility issues. I wouldn't reccommend anyone buying a new one due to the cost and I would avoid an ex-pro one but other than that I would say that if anyone has the chance to get one at reasonable cost, they should grab it.

Kim

Thanks for the hints etc
I'll put a link to a couple of images I've taken of the lens - you can see the "contamination" as well as the mechanics

I actually had two of these 28-70's go in exactly the same way. The third (thanks to ebay) seems just fine and has rescued my wife's MZ10

Matt

Here's the link:
http://www.pbase.com/mattmatic/smcpfa2870f4

Now I'm deciding what to remove next...

Looking at the photo, it is difficult to tell what is causing the problem. The next stage is to remove the rear group. The view you show is with the rear group forward. Zoom it back and remove the ring directly around the lens elements. You can see the 3 holes I was referring to. This should unscrew and then the rear group should fall out.

Kim

Thanks Kim. I can't immediately see a ring around the rear elements - either that or it's too far inside. I'll check it tomorrow, and maybe post some more photos.
Matt

Hi Matt,
I have downloaded one of your photos and highlighted the ring and the 3 holes. It is on my website here. http://pentax-manuals.com/kmpphotos/0611_1k.jpg Let me know when you have it and I will delete it.

Kim

Thanks Kim
Saved for tomorrow's attempts
Matt