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Lens orientation markings

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#1 AllanDystrup

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Posted 27 February 2021 - 01:49 AM

So, you find a marking ">" on the rim of a lens element.

How would you interpret this marking?

 

Is it an arrow pointing to the surface that should be facing the sky?

Or is it a "ray tracing" picture where the opening should be oriented towards the sky?

 

What is the convention (if any) here?

 

     -- Allan


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#2 Piggyback

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Posted 27 February 2021 - 02:31 AM

Who applies these markings? In case of my Revue I did it myself when taking the lens apart for cleaning. So it's a personal thing. On the other hand I have seen lenses with parallel running vertical markings, signature scribes etc. Hard to say if some or any factory opticians scribbled them. That is the good thing about cemented lenses. You just need to remember the up and downsize position of the assembly.


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#3 Avgvstvs

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Posted 27 February 2021 - 03:42 AM

Not sure there are any conventions

Manufacturers would know how to assemble their optical trains

Adding markings would just add to cost and possible edge of field effects



#4 catboat

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Posted 27 February 2021 - 08:02 AM

So, you find a marking ">" on the rim of a lens element.

How would you interpret this marking?

 

Is it an arrow pointing to the surface that should be facing the sky?

Or is it a "ray tracing" picture where the opening should be oriented towards the sky?

 

What is the convention (if any) here?

 

     -- Allan

Allan,

I've asked myself that question and in precisely the same terms —and wondered how best to mark orientation myself.  Sky pointer or  incoming light cone?  From the few arrowhead markers I've seen, mostly here on CN classics and not necessarily original optician's marks, the arrowhead points to the sky.

 

My favorite marking system was presented in this forum not long ago (by clamchip, I think).  Matching dots.  For example, on a doublet

                   sky   |• :|  gap  |: §|   eye  

where § stands for three dots.  (Apologies for the poor graphics!).

 

Edward


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#5 starman876

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Posted 27 February 2021 - 08:19 AM

is that one mark > on both lenses?  If it is you align the > so it form that mark again.



#6 PawPaw

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Posted 27 February 2021 - 08:30 AM

Here is one that is easy to interpret......directly marked by the manufacturer and the only lens I have come across with actual writing on the side. 

 

I have seen many with no markings, the dot dot, straight lines across both elements and the aforementioned arrow across both elements.  I have learned the hard way not to trust handwritten orientation marks.  

 

Cheers

 

Don

 

 

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#7 Tenacious

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Posted 27 February 2021 - 10:38 AM

I've read more than once on CN that the "V" should form an arrow pointing to the sky, though I like the logic of the opposite way.  This last convention should be easy enough to check in the case of say, an achromat. 

 

 

 

 

Just for reference, there is a great illustration floating around (I'm sure everyone has seen it) showing 1 correct and 7 incorrect ways to re-assemble an achromat.  I don't know who the originator is, but some of us owe that person a debt of gratitude. 

 

 

Found it!     Sometimes it is hard to distinguish the difference between cases 1 and 2 by simply examining the elements (the curves are nearly the same).  Case 1 produces a far better star image!

 

AchromaticLens.jpg


Edited by Tenacious, 27 February 2021 - 11:19 AM.

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#8 DAVIDG

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Posted 27 February 2021 - 12:03 PM

 The standard convention is that arrow points toward the sky. Having said that and having working on many many vintage and antique scope I doesn't trust any marks I find. I have seen many be wrong. The problem is that someone in the past has dissassembled the lens, and marked it incorrectly then everyone since follows these marking. I personally know of a couple of large antique lens  that were assembled incorrectly for DECADES and no one questioned the image quality. A flipped element or the whole lens placed backward won't cause the image to be a fuzzy mess that won't come to focus so people believe they got right. 

  Also the "standard" orientation of the crown  element facing the sky is not universal  for example lens made by Brashear had the flint element facing the sky.   There are also many non standard designs. The fool proof method is to bench test the lens and see what orientations produces the best correction.

 

                    - Dave 


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#9 LukaszLu

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Posted 27 February 2021 - 08:41 PM

A few days ago I finished the fight to get a decent image from the Kenko TA-910 (60/910) lens. I ended the fight with a failure, and on the way I tried all positions of the lenses in relation to each other, checking all possible combinations. To be honest, the lens seemed to be completely immune to these changes - the image did not differ much, no matter in which directions the lenses were directed - both the Crown and Flint or the whole doublet.



#10 starman876

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Posted 28 February 2021 - 04:47 PM

A few days ago I finished the fight to get a decent image from the Kenko TA-910 (60/910) lens. I ended the fight with a failure, and on the way I tried all positions of the lenses in relation to each other, checking all possible combinations. To be honest, the lens seemed to be completely immune to these changes - the image did not differ much, no matter in which directions the lenses were directed - both the Crown and Flint or the whole doublet.

a magical lens.  Pure magic it made it out the factory loading dock.



#11 LukaszLu

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Posted 28 February 2021 - 04:58 PM

a magical lens.  Pure magic it made it out the factory loading dock.

those Japanese... ;-)




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