How to "shim" lens ?

[srtiwari]

Am just getting into using some legacy lens on sony/fuji bodies. None of the adapters seem to focus exactly to infinity (I am told) at the end of their rotation- some go too far, others not far enough. My own experience is limited to a Metabones Leica M to NEX adapter that would not allow focusing past about 20 ft.
I've heard of "shimming" here on RFF, but don't know where to start. I understand doing this properly should fix those focus issues.
Can someone tell me how, and what with ? Is this just for the mechanically skilled, or can the rest of us novices also do this ? If possible, I'd love to try this, and avoid multiple adapter purchases, and returns !

 

[wolves3012]

I can explain the basic idea but I'm afraid the details vary according to lens. The lens is made up of an optical module (or several) and a mount, the physical bit that holds the optics together and fixes it to the body. On many lenses, there is a shim or shims between the two. It's like a washer with a large hole. The purpose of the shim is to position the optics module precisely in relation to the mount.

Using a thinner shim moves the optics nearer the camera and the reverse is obviously true. If a lens focuses beyond infinity you need a thicker shim and vice versa. It is possible to calculate the change needed but generally the difference is small so usual practice involves trial and error. Shims can be made from paper, aluminium foil and pretty much anything else you care to use, depending on what thickness you need. Existing shims can be reduced by careful sanding if they need to be thinner.

The main issue with re-shimming is knowing how to remove the optics and where to find the shim(s). On most russian lenses, for instance, the entire optical block simply unscrews from the front and the shim(s) will be directly behind it. Lenses vary considerably in design, however, so it can be a complex process needing more skill or special tools than many people possess. It's probably also a bad idea to tinker with exotic or expensive lenses!

There is another caveat but it won't really apply to using older lenses on digital bodies. Changing the shim on an RF lens doesn't (usually) change the RF coincidence or the lens distance scale. If you're actually focussing "live" on a digital, this doesn't matter but if you're adapting one RF lens to a different RF body it can matter a lot.

By the way, this probably ought to have gone in "optics and lenses" forum.

 

[srtiwari]

I can explain the basic idea but I'm afraid the details vary according to lens. The lens is made up of an optical module (or several) and a mount, the physical bit that holds the optics together and fixes it to the body. On many lenses, there is a shim or shims between the two. It's like a washer with a large hole. The purpose of the shim is to position the optics module precisely in relation to the mount.

Using a thinner shim moves the optics nearer the camera and the reverse is obviously true. If a lens focuses beyond infinity you need a thicker shim and vice versa. It is possible to calculate the change needed but generally the difference is small so usual practice involves trial and error. Shims can be made from paper, aluminium foil and pretty much anything else you care to use, depending on what thickness you need. Existing shims can be reduced by careful sanding if they need to be thinner.

The main issue with re-shimming is knowing how to remove the optics and where to find the shim(s). On most russian lenses, for instance, the entire optical block simply unscrews from the front and the shim(s) will be directly behind it. Lenses vary considerably in design, however, so it can be a complex process needing more skill or special tools than many people possess. It's probably also a bad idea to tinker with exotic or expensive lenses!

There is another caveat but it won't really apply to using older lenses on digital bodies. Changing the shim on an RF lens doesn't (usually) change the RF coincidence or the lens distance scale. If you're actually focussing "live" on a digital, this doesn't matter but if you're adapting one RF lens to a different RF body it can matter a lot.

By the way, this probably ought to have gone in "optics and lenses" forum.

Thank you for that enlightening explanation. I would not even consider tampering with one of my expensive lenses. I had wondered whether buying a (relative to lens) inexpensive adapter ( Leica M to NEX etc.) and shimming IT would fix the problem. Since the metabones adapter I had bought would not reach infinity, I would guess (from your explanation) that it would entail sanding off some thickness of the said adapter. I actually opted to send the adapter back, and may try one of the others that claim to " allow focusing to infinity".
But, again, thanks for an interesting explanation.
BTW, I had wondered about putting it in the other forum, but didn't know if my query involved enough "hard core science", rather than just a casual DIY project. 😉

 

[wolves3012]

.... I would guess (from your explanation) that it would entail sanding off some thickness of the said adapter.

No, it wouldn't. Both sides of the adapter are bayonets - one male & one female. Removing material off either side would render the bayonet loose on that side (camera-to-adapter or lens-to-adapter). The only solution would be to re-shim the lens, which would then render it "off" on it's original intended body (Leica M) - you may or may not be willing to accept that.

If the adapter doesn't allow infinity-focus and the lens is known to be ok, then the adapter is too deep (thick) but you can't simply sand some of it off. Solution: correct thickness adapter 😡

 

[sevo]

No, it wouldn't. Both sides of the adapter are bayonets - one male & one female. Removing material off either side would render the bayonet loose on that side

Grinding down the bayonet would be a bad idea indeed - besides creating looseness, it might not even succeed at making the adapter optically shorter, as the depth of many bayonets is not determined by their outward flanges.

I suppose he meant unscrewing one bayonet, sanding the exposed tube edge down to proper dimensions and reattaching the bayonet. On complex adapters with optics or (aperture or AF) couplers the disassembly might be complex or even beyond amateur means, but the most common simple adapter variant is just a bit of tube with male and female bayonet screwed to it, so that the dimensions can be changed that way.

Extending will be harder - you'd need shims of the right size, strength and shape. Unless you have a buddy with a laser metal cutter, be prepared to spend a few weeks learning how to cut brass sheet with a jigsaw to the required precision.

 

[srtiwari]

No, it wouldn't. Both sides of the adapter are bayonets - one male & one female. Removing material off either side would render the bayonet loose on that side (camera-to-adapter or lens-to-adapter). The only solution would be to re-shim the lens, which would then render it "off" on it's original intended body (Leica M) - you may or may not be willing to accept that.

If the adapter doesn't allow infinity-focus and the lens is known to be ok, then the adapter is too deep (thick) but you can't simply sand some of it off. Solution: correct thickness adapter 😡

Of course, I had forgotten, assuming that one side was flat (or at least the bayonet was deep enough) not to make a difference. I think returning my Metabones Leica M/Fuji X adapter was the best decision ! Thanks again.

 

[mdarnton]

FWIW, most lenses I've seen don't actually have a shim to set infinity. The usual strategy is to build the infinity stop into the focus ring, and make its connection with the helical movable, so that once the helical has the lens focused, then the focus ring is moved independently to indicate the proper distance, and finally the two, helical and focus scale, are locked in that relationship. Manufacturers have various clever, non-universal ways to accomplish this.

The way to fix your adapter, in theory, is to remove one bayonet or the other from the adapter (not your lens!!!) and grind on the face of the tube that's the body of the adapter, that the bayonet attaches to, thinning the tube between the separate bayonet fittings. If there's any shim involved, it's likely to be hidden under one of the bayonets on the adapter. There may also be some other hidden way to make the adjustment that only the manufacturer knows about.

If you have to ask about it, you really don't want to do this. On a new item it's the manufacturer's problem, not yours, anyway, right?