Re: 35mm lenses compared?(tech/optics)

["O.J. Anshus" <otto@cs.uit.no>]

> From: Erwin Puts <imxputs@knoware.nl>
> Date: Sun, 18 May 97 20:06:48 +0100
> Subject: Re: 35mm lenses compared?(tech/optics)
> 
>
<snip> 
>
> If your ASPH is 'significantly better' at f/2 than at f/1,4 I do not know
> what you mean by resolution, or how you measured it. On the optical bench
> and it practical shooting I noticed: (1) the performance of the ASPH is
> at 1,4 and at 2.0 virtually identical  

At f2 the results are clearly sharper than at f1.4. By sharper I mean
that the small details can more easily be made out (text, details in
faces somewhat in the background). In lack of a better word, I called
this "resolution".

I tried out the lenses by mounting the M6 on a tripod, shooting the same
object, and looked at the pictures in order to see if I could spot any
differences big enough to be seen by just using my eyes.

If the lens is supposed to be identical at f1.4 and f2, perhaps I should
complain to Leica, and get a new lens. I would be most happy if the
performance at f1.4 matched the performance at f2. But not the other way
around.

> (2) the f/2,0 performance of the
> Summicron-M and the Summlilux ASPH are the same. You are right as far as
> the very far edges are concerned. At f/2,0 the ASPH has slightly better
> performance at the very, very  edges (20 mm distance from the center).
> But the much more important outer zones from 12 to 18mm distance from the
> center are better represented with the Summicron. Furthermore the ASPH
> has slightly barrel distortion in the far corners. Resolution is not the
> only factor when judging the performance of a lens. In balance both
> lenses perform at f/2,0 admirably, but their fingerprint is a bit
> different.

I do agree that there are many parameters influencing the performance of
a lens. I did not mean to imply otherwise when I reported on what I had
seen regarding resolution (?), coma (?) and vignetting. However, these
were things I could easily see with my eyes. 

Perhaps I have a lens that is bad at 1.4 or extraordinarily good at f2.
When time permits I will redo the "test" of my ASPH to try to rule out
various sources of error.

<snip> 
>
> BTW, I would like to know how you established the presence of 'coma' at
> the apertures f/2-f/4 with the Summicron, as this is theoretically almost
> impossible.

With the Summicrons point light sources (stars, street lamps in the
distance) became elongated. This started to happen about half way from
the center of the pictures towards the borders. This effect I called
coma. What term is the correct to use?

> Greetings
> Erwin Puts

Otto