Re: Lens Standards

[Dan Cardish <dcardish@microtec.net>]

At 12:02 PM 09-03-97 -0000, Erwin Puts wrote:

[snip]

>Some examples. I measured the curvature of field of the old and new 
>Summicron 90mm. The new one had: 0,02 and the old one: 0,07. So from the 
>results the new one is "better". But in practice you do not see these 
>differences.

If you do not see any diffence between the old and the new, then of what
purpose is reporting the results of field curvatures?  They appear to be
mere numbers that have no relationship to OBSERVABLE properties of the lens.


> I also measured the decentring in an old Minolta lens and a 
>new Leica lens. The Minolta had a value of 0,00 (!) and the Leica 0,04. 
>Is the Minolta better?

If measuring the centering of lens elements means anything, then you WOULD
have to conclude that the Minolta lens is better than the leica lens.
Otherwise, why bother to measure decentring at all?  On the other hand, if
the Leica lens still produces better pictures, then there is a problem.  It
could also be that if you did the test on 100 Monolta lenses versus 100
Leica lenses, perhaps only one of the Minolta lenses would have better
centering, and this is the lucky one that you tested.   Maybe the Leica
lenses have better consistency in manufacturing (I have no idea if they do
or not).   Then from a statistical standpoint you could say that the leica
lenses are better.  But if there is any possibility of significant variation
in a lens parameter between samples, then reporting scores for only one lens
is meaningless.

It all comes down to what several respondents have alluded to; if you buy a
lens to score highly on lens tests, then comparing lens test results should
be important to you.  But if certain lens measurements can only be compared
under laboratory conditions and cannot be reproduced in *real life*
conditions, then of what use are they to the typical Leica camera user?  I
have never questioned the quality or ability of the POP people in their
tests.  If they conclude that a Minolta lens has better contrast than a
similar Leica lens, I'm sure that it did on their optical bench.  But if the
Leica lens outperforms the competion in real life, then why go to the
trouble of reporting these results in their magazine.  These optical bench
tests are not a useful criteria for PHOTOGRAPHERS to compare lens systems.
The Leica lens costs 10 times the Minolta (or Nikon or Canon) equivalent.
If POP (or other) tests were truly meaningful, Leica would be out of
business tomorrow.  

Dan C.