Are front and rear lens cells 'matched'?

Hi Guys,

 

Can someone please advise whether it is generally possible to swap

out just the front (or rear) cells of two identical lenses? I've

noticed that short focal length lenses often have a shimming ring

fitted with one of the cells, but I've not come across a longer one

with shims. Is the cell spacing progressively more critical as f

reduces, which might make swaps only practicable on longer lenses?

 

I guess the simple answer would be "don't do it", but has anyone

tried and 'gotten away with it'?

 

Thanks,

David

I once got a replacement 180 Schnieder from ebay to replace one who's front element I'd damaged. The purchase was a disaster and the lens came with mismatched front and back elements -- I have no idea what the back element was for, but it surely wasn't for the 180. However, combining my old rear element with the "new" front one, got me a lens that was every bit as sharp as the original. That, and a substantial refund from the seller convinced me to let the transaction stand.

 

There were no shims to either lens, tho.

David,

I don't eexactly know, but i can imagine with my common sense, that the spacing between the lenses of both cells is as important - and sensitive - as the spaces between the lens groups of a single cell. The single lenses, the groups and the spaces also are part of the system.

Well, if you would screw the rear cell on a i.e. Symmar and you tighten it careful...

and the you tighten it with a little more power, you will see that it will be never more than 1°. If the screw thread would have a pitch of P=0,75mm, 1° would bring the cells ~0,0028mm apart! I think this is within the tolerance, because i am sure the screw thread is a finethread with a pitch of more less than P=0,75.

In short. Yes.

 

The focal length of the lens that is marked on the ring is not the actual focal length. The

actual focal is ± that marked focal length.

 

For proper results the front and back groups are matched so they are the same ± rather

then one being + the marked focal length and one -.

 

Mixing and matching groups at random may result in the proper combination. But it is

more likely to be less then optimal. As to spacing that to is dependent on the cells as well

as the individual shutter as there is some tolerance in shutter manufacturing.

 

If you are satisfied with the results then that's fine. But have you compared the results

head to head against a factory calibrated lens when you take both shots at the same time,

same subject, same processing, etc.?

Bob - That's interesting. The person who posted the question said that the two lenses in question were "identical." I assumed he meant, for example, that both were 135mm Rodenstock Sironar S lenses or 210mm APO Symmar lenses or whatever of the same model. Are you saying that two "identical" lenses (in that sense) could have different focal lengths, e.g. that my 135mm Sironar S might really be a 133 and someone else's might really be a 136? I understand that focal lengths are often rounded numbers, so that a 135 might really be a 136 for example, but I would have thought that whatever the actual number was, it would be consistent from lens to lens within the same model so that all Sironr S 135mm lenses might really be say 136mm. But I wouldn't have thought that one 135mm Sironar S lens of the same model could be 134mm and another otherwise identical 135mm Sironar S lens could be say 136mm. It probably doesn't make a huge difference to anything, it just seems surprising.

They vary ± something. Not something very large. But they do vary. They are

manufactured products and all manufactured prodects have tolerances.

Thanks to everyone for your responses.

 

Bob's answer agrees with my 'gut feeling' - I have an engineering background so the matter of tolerancing makes sense. LF lenses are probably not a 'volume' product, so more effort could be expended in matching the cells, even if it was done entirely by hand.

 

However, it has been pointed out many times in this forum that there *are* variations in 'performance' (however you decide to measure this) between ostensibly identical models, so maybe the sorting is not that precise - or that things would be even worse if no matching was performed at all!

 

On balance, I suppose you're taking a chance with swapping cells if you don't have a 'gold standard' for comparison. One problem with second-hand lenses is that you can't be sure someone's not swapped them for you......!!

"One problem with second-hand lenses is that you can't be sure someone's not swapped

them for you......!!"

 

True, but we had found that that was not limited to only used lenses. When we took over

the Rodenstock distribution after Berkey's bankruptcy we performed a 100% inspection of

their inventory.

 

We found lenses that Berkey had accepted for return that were then mismatched to make a

third "saleable unit". We found a 480 Sironar N with a huge fingerprint on the rear group.

 

All of the lenses that appeared to have been tampered with were returned to Berkey but

our experience was that they were shipping these lenses to dealers as "B" stock or were

using them as warranty exchanges with customers.

 

But then we had a similar experience when we took their Rollei inventory over in 1986 so

we had expected to find them.

Bob,

 

That's very interesting - I suppose that when dealing with relatively high-valued slower-moving specialized items, like LF lenses, even a distributor might be tempted to 'make do and mend'......

 

<P>Wouldn't it make sense if manufacturers were to engrave the lens serial # on both front <I>and</I> rear groups to discourage 'tampering'?