Re: helicoids, (long) was-[Leica] Re: Leica Users digest V17 #3

[Dennis Painter <dpainter@bigfoot.com>]

Alan Magayne-Roshak wrote:
> 
> ------------------------------
> LarryZ wrote:
> 
> <if Nikon and
> <Canon can sell high quality 50mm F1.8 or F2.0 lenses for approximately $100
> <then the ten times greater price of a similar Leica lens represents a
> <marketing decision rather than the actual cost basis.
> <LarryZ
> 
> I remember an article by Arthur Kramer in Camera 35 magazine from about
> 1958 wherein he explained that E. Leitz took the care(and expense) to make
> lens focusing helixes on various pitches to match the exact focal length of
> each individual lens.  I.E.,50.2mm would get one pitch, 49.8 another.
> Kramer wrote that Leitz was probably the only one going to this trouble.
> Who knows if they still do this?
> 
<snip>
> 
> Alan Magayne-Roshak

They still do it but according to Erwin Puts the manufacturing
techniques are even more sophisticated today.  

To reiterate, actually to paraphrase. 

It should be obvious that an element of a lens cannot be reproduced in a
manner which yields 100% identical pieces. There will be variation. The
degree of curvature of each element may vary slightly from the design
criteria, yet still be within the manufacturing tolerances for each lens
element. (even high tolerances)

What happens then is if you pick all the elements for a lens from the
batches manufactured for that lens you will have variation. This
variation will be shown in the fact that the lenses will not all have
identical focal lengths. To precisely focus a lens on the Leica the
focus mount must be built for the focal lenght. The cut of the helicoid
must match the focal length. Thus a lens of 51.9 mm focal length should
have different helicoid than one of 50.5 mm.  (the helicoid is what is
transferring the movement of the lens to the rangefinder mechanism)

With a SLR one helicoid will fit all 50mm lenses. The eye just does its
best to percieve the point of proper focus, there is no rangefinder
linkage here (obvious to all I am sure!), thus a potential cost saving
to the SLR manufactures.

What Erwin had posted was that Leica now inputs the actual curvature of
each element into a computer system and a program selects the elements
to be used to assemble each individual lens such that the acutal focal
lengths of the 50mm lenses only fall into 3 lengths. (no I don't know
what they are) Obviously this reduces the number of helicoids that Leica
has to cut. It may also result in better lens performance.  It should
also be obvious that measuring each individule lens element and
inputting its characteristics into a computer cost money. Picking and
assembling the computer selected elements costs more money than picking
the next element on a manufacturing line.

It all costs money, is it worth it? only the user can be the judge.  I
am not defending Leica's prices or saying they are too high, just
pointing out that there are costs that are not obvious at a casual
glance.

Dennis