[Leica] Hexanon 1,2/60, part 1

[Erwin Puts <imxputs@knoware.nl>]

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The first f/2 lenses for the 35mm format have been designed in the 
thirties and soon became the workhorse of the photographer. Scores of 
designs (5-, 6- and 7-elements) and a wide range of performance 
indicate that is not so very easy to design and build a true high 
performance standard lens.
The next step ( a f/1,4 aperture) produces an even wider bandwidth of 
design types and performance. The amount of light energy that passes 
through the lens is doubled. Many optical aberrations grow at a 
factor of 4 and some even at a factor of 8 when the maximum aperture 
doubles. It is quite clear that this much higher aberration content 
cannot be corrected as rigorously as in the case of the f/2 lenses. 
The  balancing of residual aberrations becomes more important. 
Secondly the correction of lower order aberrations should be pursued, 
taking into account the effect of the higher aberrations.
One stop wider produces the f/1 designs.  Aberrations again grow with 
a factor of 4 and 8 and now loom so large in the designer's eye that 
the  balancing and correcting act becomes a walk on a tightrope.
Many approaches have been tried. Note that these examples are 
restricted to RF type cameras.
One could retract a bit and  do a half stop (f1.2) and one could make 
the angle of view slightly less (60mm), the approach of the Hexanon 
1:1.2/60.
Another approach is to use more glass elements for more correctional 
freedom as Nikon did in 1956 and produced a 1.1/50 with 9 lens 
elements.
One could go for a little lower image quality as Canon did with its 
f/1.2 in 1957 with 7 lenses.
Leitz  invoked the help of aspherics in 1966, when introducing the 
Noctilux 1:1.2/50mm and countered the most troublesome aberration 
(spherical aberration) with two aspherical surfaces.  Still some SA 
was left in the design and by shifting the plane of best focus a bit, 
they enhanced contrast.
In 1976 Leitz resorted to glass with a very high refractive index 
(>1.9) and a 'classical' 7-element design and a fair amount of 
vignetting to control aberrations.
The Noct 1.0/50 then has 7 elements in 6 groups, as has the Hexanon 
1.2/60. The difference is the front lens of the Hexanon which is 
split in two elements, where the Noctilux has the last lens split in 
two elements.

Through the Hexanon 1.2/60mm at full aperure flows less energy than 
through the Noctilux 1.0/50mm. In fact the Hexanon at 1,2 is equal to 
the  Noctilux at f/1.4. When comparing the two lenses at their full 
aperture, one should be aware of the fact that it is a f/1.0 against 
f/1.4 game.

On test the Hexanon  at full aperture  produced medium to high 
overall contrast with excellent on axis performance. Very fine detail 
is clearly recorded, but tangentially oriented structured are 
recorded very softly. The overall effect of both orientations would 
be a softening of the contrast of fine detail. In the field chromatic 
aberrations can be detected as color fringes around the edges of 
outlines. Some  curvature of field is also noticeable. The on axis 
performance extends over a circle with a diameter of about 8mm. 
Beyond this circle,  image quality rapidly falls away, and in  a 
small zone beyond the 8mm area, fine detail has good visibility with 
a low contrast. In the outer zones and far edges the outlines of 
bigger objects are quite fuzzy.
At a focus distance of 1 meter, this general behavior holds, but 
contrast drops over the whole image field.
Some of the  lens-elements  of the Hexanon are  large and have very 
thin edges. I could note some decentring.  The report is based on the 
correct centring.

Erwin

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<fontfamily><param>Arial</param>The first f/2 lenses for the 35mm
format have been designed in the thirties and soon became the workhorse
of the photographer. Scores of designs (5-, 6- and 7-elements) and a
wide range of performance indicate that is not so very easy to design
and build a true high performance standard lens. 

The next step ( a f/1,4 aperture) produces an even wider bandwidth of
design types and performance. The amount of light energy that passes
through the lens is doubled. Many optical aberrations grow at a factor
of 4 and some even at a factor of 8 when the maximum aperture doubles.
It is quite clear that this much higher aberration content cannot be
corrected as rigorously as in the case of the f/2 lenses. The 
balancing of residual aberrations becomes more important. Secondly the
correction of lower order aberrations should be pursued, taking into
account the effect of the higher aberrations. 

One stop wider produces the f/1 designs.  Aberrations again grow with a
factor of 4 and 8 and now loom so large in the designer's eye that the 
balancing and correcting act becomes a walk on a tightrope. 

Many approaches have been tried. Note that these examples are
restricted to RF type cameras. 

One could retract a bit and  do a half stop (f1.2) and one could make
the angle of view slightly less (60mm), the approach of the Hexanon
1:1.2/60. 

Another approach is to use more glass elements for more correctional
freedom as Nikon did in 1956 and produced a 1.1/50 with 9 lens
elements. 

One could go for a little lower image quality as Canon did with its
f/1.2 in 1957 with 7 lenses.

Leitz  invoked the help of aspherics in 1966, when introducing the
Noctilux 1:1.2/50mm and countered the most troublesome aberration
(spherical aberration) with two aspherical surfaces.  Still some SA 
was left in the design and by shifting the plane of best focus a bit,
they enhanced contrast. 

In 1976 Leitz resorted to glass with a very high refractive index
(>1.9) and a 'classical' 7-element design and a fair amount of
vignetting to control aberrations. 

The Noct 1.0/50 then has 7 elements in 6 groups, as has the Hexanon
1.2/60. The difference is the front lens of the Hexanon which is split
in two elements, where the Noctilux has the last lens split in two
elements. 


Through the Hexanon 1.2/60mm at full aperure flows less energy than
through the Noctilux 1.0/50mm. In fact the Hexanon at 1,2 is equal to
the  Noctilux at f/1.4. When comparing the two lenses at their full
aperture, one should be aware of the fact that it is a f/1.0 against
f/1.4 game.


On test the Hexanon  at full aperture  produced medium to high overall
contrast with excellent on axis performance. Very fine detail is
clearly recorded, but tangentially oriented structured are recorded
very softly. The overall effect of both orientations would be a
softening of the contrast of fine detail. In the field chromatic
aberrations can be detected as color fringes around the edges of
outlines. Some  curvature of field is also noticeable. The on axis
performance extends over a circle with a diameter of about 8mm. Beyond
this circle,  image quality rapidly falls away, and in  a small zone
beyond the 8mm area, fine detail has good visibility with a low
contrast. In the outer zones and far edges the outlines of bigger
objects are quite fuzzy.

At a focus distance of 1 meter, this general behavior holds, but
contrast drops over the whole image field.  

Some of the  lens-elements  of the Hexanon are  large and have very
thin edges. I could note some decentring.  The report is based on the
correct centring.


Erwin   
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