Leica Elmarit-R 28 mm f/ 2.8 Lens Manual
Chapter 1
Leica R-Lenses
2
__Aberrations
You can imagine that a scene is made up of many small patches
with different brightness. Their distribution is the basis for a
good composition and may be used to find an interesting com-
position. At the moment we are only interested in the fact that
the lens should reproduce these brightness spots faithfully.
Even the smallest image point must have some extension and
therefore a certain amount of energy as the light quanta are
themselves energy particles. The light rays of on object point
fall on the surface of a lens and are refracted by the spherical
form of the surface to form an image point as small as possible.
These image points on the negative are very small (typically
0.005mm in diameter). If you look at such a point very closely,
you will note that it consists of a very small bright core where
most of the light rays are focused, and a larger circle around
the core where the rest of the rays will fall. This is a rim caused
by flare. If we have a very small core and a large area around
the core, we have maximum resolution but contrast will be very
low. You can reduce the circle of flare around the core, with the
effect that more rays will be concentrated in that core, but we
now need a larger area for more rays. Now contrast is at opti-
mum, but resolution is slightly less (see illustration below).
The designer has clearly some influence on the shape and com-
position of this image point. He/she can try to find a certain
compromise between between both extremes and nudge the
design in a certain direction. The residual aberrations that are
always present, because of the fact that the optical errors can-
not be reduced to zero, will influence the final image quality.
Object points that are on or close to the optical axis will be
represented as circular image points and are relatively easy to
correct. That is why a good image quality in the center of the
image field is not so difficult to achieve provided you restrict
yourself to small apertures. When the maximum aperture is
quite large, say f/2 or f/1.4, it is no longer a simple task to get
good quality in the center. If you go to positions away from the
axis, the distance from image point to optical axis (image
height) will increase and the rays will fall obliquely on the image
plane (angle of entrance). Now we will encounter new types of
aberrations like astigmatism and coma that are quite difficult to
correct. And some of these aberrations will, depending on
image height and entrance angle, increase disproportionally in
magnitude. As an example we may use spherical aberration. If
we increase the aperture from f/2.8 to f/1.4, the magnitude of
error increases ninefold.
To complicate matters, we know that we can only correct a lens
optimally for a certain distance or magification. Mostly we use
the infinity distance as the reference, but for macro lenses
another distance should be selected. The image quality cannot
be evenly distributed over all distances, over the whole image
field and at all apertures. And the wider the aperture, the more
difficult it will be to find a satisfactory balance of error correc-
tion. And finally we have to face the chromatic aberrations. It is
well-known that every color is refracted in an optical system by
a different amount (dispersion). This means that every color
(red, blue, green and so on) will have its own optimal sharpness
plane and that all colors will be seen with differences in sharp-
ness. This so called longitudinal chromatic aberration must be
taken into consideration when you are searching for a compro-
mise between highest contrast and resolution of small textural
details.
These facts may illustrate that any lens is always a compromise
solution between many and often conflicting characteristics
decided upon by humans. Even the best computer program can
produce only numerical indications for this balancing. At the
end of the day the optical designer has to make the final deci-
sions. The three standard lenses in the Leica R-system therefo-
re offer a different set of capabilities and performances.
Image point A
Image point B