[Leica] Re:Noctilux abberations?

[deveney.marty at saugov.sa.gov.au (Deveney, Marty (PIRSA))]

Hi, 

Ted wrote: 
>I've used a Noctilux since it was available in Canada, '72-73? And at times 
>it was - is my main lens indoor and out. 
>However I was never aware of this condition. Is this something I've missed 
>all these years that's good, bad or ugly? 
>Serious question. 

I love my Nocti.  I've shot a hundred or so rolls through mine since I 
bought it from George Lottermoser in January 2005.  it's the same version as 
yours Ted, the 58mm filter thread one that came with a separate hood.

You've raised something interesting here Ted - all lenses have aberrations, 
it's just a physical fact of optics.  Even the Leica M asph lenses (probably 
better corrected for aberrations as a family than any other group of lenses 
current manufactured for cameras) and the 50/2 Summicron (a very 
well-designed lens with conservative specifications)  have them, you just 
have to know what to look for.  With these lenses, most of the time most of 
these aberrations hardly matter (in a well-designed, adequately constructed 
lens) and are completely invisible to most viewers (who don't know what 
they're looking for).

The Noctilux is another story.  As Erwin Puts points out, wide open, the 
Noctilux displays coma, spherical aberation and chromatic aberrations from 
an image height of 9mm outwards at full aperture. He also notes 'some inward 
bending curvilinear distortion' in addition to the vignetting or light 
falloff that is quite pronounced.  These are, to some extent, inevitable 
when designing a lens this fast.

But what does all that mean? 

Coma, 
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/coma.html 
http://en.wikipedia.org/wiki/Lens_(optics)#Coma 
otherwise known as oblique spherical aberration, makes bright sources near 
the edge of the field look like a comet with a tail rather than a single 
spot.

Spherical aberration 
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/aber.html 
http://en.wikipedia.org/wiki/Spherical_aberration 
occurs because spheres are not the ideal optical surfaces (this is why 
aspherical lenses perform better if properly designed) and results in 
softness or blurring.  If you take a photo of a sheet of newspaper under a 
sheet of glass with a Nocti (actually somewhat more instructive than it may 
seem at first), it will be softer at the edges than in the centre.

Chromatic aberration 
There are several kinds of this.  The most common and the one the Nocti 
suffers most from is described here: 
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html 
http://en.wikipedia.org/wiki/Chromatic_aberration 
where the lens bends different colours of light to differing degrees.  It 
results in strange colour effects with colour films and a loss of sharpness 
in B&W.

'Inward bending curvilinerar distortion' simply means that if you take a 
picture of a brick wall square on with a Nocti at f1, the bricks at the top 
and the bottom will look like they're curving in towards the centre of the 
image rather than as straight lines.  This is often called 'barrel 
distortion' presumably from the corresponding outward bending aberration's 
propensity for making things that are composed of straight lines look like a 
barrel - fatter in the middle and thinner at the edges.

The vignetting or light falloff is an optical inevitability of the speed of 
the lens.  Despite the extremely high refractive index of some of the glass 
used in the Nocti (the higher the index the more the glass can bend light) 
the edges of the lens simply can't transmit the same amount of light from 
the edges of the field to the edges of the film frame.  In the case of the 
Nocti it is definitely optical vignetting (of this type) as opposed to 
physical vignetting where the barrel or lens hood gets in the way of the 
edges of the field.

The Noctilux is a truly magnificent design.  There are fewer aberrations 
wide open in the 55/1.2 asph breechblock lens for the Canon FD cameras, in 
the 58/1.2 Noct-Nikkor and of course in the 50/1.4 asph leica-M lens.  Wide 
open, even the original 1.2 aspherical Nocti performs better in some 
respects.  All these lenses are, of course, also a half to a whole stop 
slower.  Any lenses as fast or faster than the Nocti that have been 
manufactured are inferior.  The Canon EF 50/1 is the closest in performance, 
but it still lags behind in many areas.  it's also about the size of a piece 
of telegraph pole (well, maybe not, but if you think the Nocti is big, the 
Canon is huge).

It would be possible for Leica to design an asph version of the f1 Nocti 
that would improve the performance relative to the current version.  While 
the current Nocti is astoundingly good, lens design, glass manufacture and 
(especially) methods for making aspherical elements have improved enormously 
in the last 20 years.  Such a lens would probably be entirely unaffordable 
and anyone hoping for a 50mm lens with the least number of aberrations 
should buy a 50/1.4 asph.

That ended up way longer than I intended . . . hope it was useful. 

Marty