LEICA APO-ELMARIT-R - Manual (Page 9)

Chapter 3
Leica R-Lenses 8
Even so, the optical quality of the 280 mm lens is higher. Here
we can detect the limit of the MTF graphs when we restrict our-
selves to 40 Lp/mm as the highest frequency. There are sound
arguments for this limit, but when dealing with very high perfor-
mance lenses, the information may not be as we want it to be.
The 280 mm f/4 Apo-Telyt-R lens is one of the every few lenses
that is truly diffraction-limited. This means that the optical
aberrations are so small that the size and shape of the image
point is governed solely by physical laws. The absolute limit can
be found at 450 Lp/mm. The most amazing feature is the follo-
wing: a contrast value of 50% for 50 Lp/mm is the normal limit
for high quality 35 mm photography.
The 280 mm f/4 Apo-Telyt-R lens delivers a resolution of 150
Lp/mm with 50% contrast. Often the lower limit for usable con-
trast is set at 20%. At this value this lens still delivers an out-
standing 300 Lp/mm.
The big question is: how do we obtain this performance on the
negative?
__ High-resolution photography
Let us make it clear from the start. Under practical circumstan-
ces, we can achieve a visible and usable resolution of more
than 150 Lp/mm on microfilm (Agfa Copex and Kodak Techni-
cal Pan).
At first sight this may appear to be a bit disappointing. But 150
Lp/mm are 300 separate lines in one millimeter and that
means that every single line has a width of 0.003 mm – an
exceedingly small number!
Between two black lines there is a single white separation of a
mere 0.003 mm in width. The smallest halo caused by the lens
or by the grain in the emulsion, will reduce that separation line
to a dark gray one, making the difference between black and
white disappear. The same holds for the slightest movement of
camera or subject.
Occasionally you will read about film emulsions that are capa-
ble of resolving 700 Lp/mm or more in normal photographic
situations (film-lens combination). In this case we have a line
width of less than 0.0007 mm and that is minute in the extre-
me. But these theoretical claims are not so important because
the results have never been seen or documented.
The 280 mm f/4 Apo-Telyt-R, which has a theoretical (i.e. com-
puted) resolving power of 450 Lp/mm (depending on the wave-
length that is being used), can resolve 250 Lp/mm with a con-
trast of 50%, of which approximately 150 Lp/mm can actually
be recorded on film. The 180 mm f/2.8 Apo-Elmarit-R has
values that are a bit lower.
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