[Leica] Re:Noctilux abberations?

[henningw at archiphoto.com (Henning Wulff)]

At 8:56 PM +1030 3/24/06, Deveney, Marty \(PIRSA\) wrote:
>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

The great thing about the current Noctilux is that the choice of 
glass and coatings has resulted in a lens that, while exhibiting an 
expectable number of aberrations manages to produce images with 
remarkably little flare and medium to low contrast at wide apertures. 
Usually low flare results in high contrast, which would reduce shadow 
information and leave it black. Similarly, medium to low contrast 
with normal levels of flare would veil shadow information. It's this 
combination of characteristics that make it such a superb 'available 
darkness' lens.

The other lenses above, like the Canon aspheric 1.2 and Noct Nikkor 
are higher contrast and better corrected, but their flare levels are 
considerably higher, so that in the end they are better for taking 
pictures of city lights at night (less coma and astigmatism) but not 
so good for taking pictures in dark places where you have to see into 
shadows, past the bright lights.

In this type of lens low distortion is not a requirement, and while 
the vignetting in the Noctilux is rather great, the substantial 
increase in size and weight, and most likely flare levels would not 
be acceptable.

All in all, a great lens, and one not likely to be bettered in 
practice by an aspheric version.

-- 
    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
  |[ ]|     http://www.archiphoto.com