Lens, board, and shim: how critical is spacing?

[kunlun121]

After having my new LF wide angle lens separated from its Linhof board, I now wonder about the front and rear lens spacing. (See my earlier post here; that post mentions it's a technika board but it was in fact Linhof)

When I first unscrewed the rear element from the shutter a shim / spacer fell out. It cannot have been mounted correctly as it was sitting between the retaining ring and the rear lens's barrel. It didn't fit there as the diameter of the shim is too large. It looks like it should have been mounted between the board and the retaining ring. (The rear lens's barrel actuates against the retaining ring.)

As I happen to own a micrometer (a nice Japanese one from Mitutoyo) I found out the shim's thickness is around 60 micron (62 micron, i.e., 0.062mm is more accurate, or around 2.5mil for the imperially inclined). That's a pretty fine adjustment, considering that the Sinar lens board that I just mounted the lens on is around 800 micron (0.8 mm or around 31.5mil) thicker than the original Linhof board. Assuming that the shim was appropriate for the Linhof board, adding a shim here seems to be making things worse. More importantly, will this lens not perform properly if I don't have 800micron (or, better, 740 micron to account for the shim) machined away from the Sinar board's mounting area?

Making things worse: I don't know the provenance of this lens. So there is no way to know if it was originally mounted on the Linhof board that it came with. (I don't think it was, because it didn't have a notch to fit the pin on the shutter to lock the lens's orientation on the board.) Or even if it was originally mounted in the Prontor Professional shutter that it came with.

In other words, is there any way of knowing whether the lens was properly spaced on the Linhof board in this shutter, assuming that the shim was appropriate for that combo? What would be needed to verify the correct spacing of this lens?

Or should I simply not worry about this? This here post suggests that for standard and long lenses the spacing is not critical. For modern ultra-wide lenses and digital lenses one apparently should worry as the optics probably require much more precision. This lens is a Rodenstock Grandagon 155mm f/6.8 (non-N version) that I want to use on a 5x7 format. That makes it somewhere on the wide end of normal, but not super-optimized, ultra-wide optics by a long shot. Plus it's an old (1970s?) lens, as it is pre-N. (Don't know when the N version was introduced.) That sounds relatively unsophisticated optically.

[Mark Sampson]

I'm confused, a bit. Every LF lens I've seen has the retaining ring (or flange) attached to the shutter. And the rear lens group screws into the shutter by its own thread. So the shutter (not the optical groups) attach to the board.
Can you share some pictures? It's not uncommon for there to be a shim between a lens element group and the shutter, again not part of lens mounting. And of course some slim-depth lensboards may need spacers because they are not thick (deep) enough for the retaining ring to hold.

[kunlun121]

Hi Mark, my post must’ve been confusing. sorry about that. Obviously the retaining ring attaches to the shutter. The rear lens group, like you write, screws into the shutter.

What I meant by the rear group actuating against retaining ring is that, when screwed all the way in, the barrel of the rear group sits against the retaining ring, not against the end of the thread of the shutter. The distance between the front group and the rear group is therefore determined not by the length of the shutters barrel. I don’t know what’s normal in this respect.

My assumption is that the spacer is intended to add distance between the front and rear groups of the lens. If that is correct, the spacing must be off if you use a thicker lens board. That is my situation. If my understanding is not correct, I need some guidance on how to understand this.

If the spacer is there only to fill up empty space when using thinner lens boards, then forget I asked. In that case, I don’t need the spacer ring as there is plenty of thickness in the board. In both boards actually.

[Mark Sampson]

Have you measured the length of the lens/shutter combo unmounted, and mounted on a board? The retaining ring should not interfere with the seating of the rear lens group in the shutter. Putting lenses on boards should be a simple process, but it isn't always (as most of us have found out at one time or another).
It's a bit odd that your (very large) lens was mounted onto a Technika board- the Linhof Technikas are folding technical cameras with rather small lens boards, and the 155mm Grandagon was designed for 8x10.
Perhaps you should consult a camera repair service; most of them must be familiar with mounting view camera lenses properly. I'd recommend Richard Ritter if you're in the USA or Canada.

[B.S.Kumar]

Someone more technically inclined will correct me if I am wrong.

A Technika board can also be a Linhof Technika board, unless you mean the larger Kardan style boards.

The spacing between the front and rear cells are critical, and are usually adjusted by placing one or more shims between the front cell and the shutter. Some lens barrels have a ridge on the front cell, and the shim fits there snugly. Many lens barrels do not have these ridges, and the shims are simply dropped into the front of the shutter. The inner diameter of the shim is just slightly larger than the outer diameter of the tube that accepts the front cell, ensuring that it doesn't foul the cell when it is threaded into the shutter.

The lens is designed to work correctly when both cells are completely threaded into the shutter.

I have seen shims used at the back of the shutter between the retaining ring and the lens board. They are usually made of card stock, but occasionally brass also. These were placed to ensure that the shutter was tightly attached on the lens board, and also to ensure that the retaining ring and lens board did not bind together.

Kumar

[kunlun121]

Thanks Mark & Kumar, very helpful both your comments.

First off - I had made a mistake. Somehow I wrote Technika while the board was a Linhof. So that's that.

Second - the retaining ring does interfere with the seating of the rear group in the shutter. It did on the Linhof board and it does on the Sinar board. So it can't be screwed into the shutter completely. The only way of remedying this I can think of is to have 1mm machined out of the Sinar board in the mounting area. Will look into having that done.

Third - the shim. It had occurred to me that the most logical place for it to sit was between the shutter and the front group. Thanks for confirming that.

As I wrote in this here post, my mechanic said he thought the front group had been glued into the shutter. Or it has been screwed in so tight that it appears to be glued. He tried loosening the front group, but failed to do so. The amount of force that was safe to apply to the shutter was not enough for it to come off. I'll get in touch to ask him to spend more time on it. If I have a go at it myself I'll likely inflict damage.

[B.S.Kumar]

A "Linhof" board can be a 2x3 Linhof Technika board like this:



a Linhof Technika board for a 4x5 camera:



or a Linhof Kardan board like this:



This is a recessed board, but you get the idea.

If your lens is currently mounted in a board like the second one, you can buy an adapter and mount it directly.

If the retaining ring interferes with the seating of the rear group, I don't see how it can work as designed. This implies that the rear cell is further away from the shutter than necessary. To compensate for the spacing, the front cell would have to be screwed in more than necessary. But this is impossible, since the cell cannot be screwed in more than the threads will allow.

Some photos would help us help you.

Kumar

[LabRat]

On my older Tek III, there was a factory thick precision spacer (around 1/8" thick) between shutter and board on my 150mm lens... I figure it was probably added so the RF + distance scales would match line up with this spacer... (But I don't use the RF with this camera, and added other non-standard lenses to it over time, so don't know if there would be an error without it...)

Steve K

[B.S.Kumar]

kunlun121, I am re-reading your first post.

You are speaking of the spacing with reference to the lens board, but are asking about the spacing of the lens cells with reference to the shutter.

If your concern is regarding the lens board, it does not matter. Recessed and extension boards are widely used, and simply change the distance between the lens and the image plane. They have no effect on the optical performance of the lens.

Sinar made much of the lenses they supplied, with a major selling point being that they were professionally mounted.

QUOTE from their brochure:
High-performance lens systems must be mounted at exactly right angles to the lens board and precisely centred. SINAR therefore supplies the world's top lenses ready mounted and thus guarantees a precision standard to match the whole camera system.

It would take an extremely inept person to mount a lens at an angle other than a right angle if he/she was using an original Sinar lens board.

If you are concerned about the lens spacing, only photos of your lens can solve the mystery.

Kumar

[Greg]

I once bought a 115mm f/5.6 Ultragon Lens in a Compur shutter. At f/22 the corners were terrible in covering the 4x5 format so returned the lens. I had a suspicion that the lens may have been remounted into another shutter. Maybe the spacing of the front and rear cells not correct?