[Leica] exploring the limits (part 2)

["Erwin Puts" <imxputs@knoware.nl>]

This study is not designed to be a lens test. The use of several lenses was
necessary to ensure that the employment of a specific lens would not unduly
impair the validity of the results. It may come as a surprise that the
humble Canon lens, which is a cheap noname in the market, is as good as a
current Summicron (when resolution on axis or close to the axis is
concerned). Optical (MTF) tests did show the Canon to be very well made (no
decentring etc) and indeed had a slightly higher aberration content than the
Summicron, specifically spherical aberration softness of smaller details.
For a resolution test this is not such a big problem. It does indicate the
the market value of a lens does not have to be related to its true optical
capabilities. Market value is related to scarcity and that elusive value of
What-Peope-Want-To-Believe.
Back to the real topic.
What I try to establish is system performance and not the absolute
performance of a lens or a film. Generally any photographer uses a film and
a lens for pictorial or continious tone pictures and so the absolute
resolution figure of a microfilm (like Gigabit=Agfa Copex and TechPan) of
600 or 400 linepairs/mm is a figure to be interpreted. This figure is based
on two assumptions: a film gradient of 3 and the possible resolution of
black-white patterns, like letters. To make these films useful for pictorial
photography, the gradient (characteristic curve or CI-value) has to drop to
a more normal value of CI=0.6. The Gigabit film just does this and uses a
specific developer that  gets a CI of 0.5 or even lower. The true speed
point of this film is ISO20 and when using it as EI=40 (as recommended) and
developing it to a CI of 0.5, we get the classical pattern of
underdevelopment and underexposure, good for the dynamic range (overexposure
latitude) and resolution, as overexposure will kill any attempt to record
very finedetails.
I used TechPan as a companion film and when developing both films (Gig and
TP) in Technidol for the same development time, I got identical results. The
much discussed dynamic range of the Gigabit film can be had with the TP too
if developed to this effect.
Studying resolution patterns at several densities, I noticed that all these
hi-res films are very sensitive to small variations in exposure (plus and
minus). Best resolution you get at densities around D=0.4 to D=0.6, which
generally is one stop below the grey card value. That may be the reason that
hi-res films often are underexposed.
You can only compare film capabilities when films are developed to the same
CI-value, so my first attempt was to find the correct exposure and
development time for all films to be comparable. Having established that I
selected the lens for a new series of tests. (My third attempt and many
rolls of film used).
Officially best results with a lens are at the infinity position (which is
defined optically as the location of the object where the rays are
perpendicular to each other and reach the lens at right angles). Normally
this at 50 to 100 times the focal length. For all lenses I made tests at a
distance of between 5 meter (50mm) and 13.5 meter (135m).
Now is 13.5 meter quite inconvenient, so I checked with Leica what would be
the closest range without dropping  significantly  in optical quality. They
proposed that 5 meter would be appropriate for the 135mm.
The apo-90mm  was also used, at 5 meter and 4 meter. You should realise that
films in the speed range of ISO 16, 25 and 40 are difficult to test. At
longer distances the flash or the tungsten lamps will be at low power and
using shutter speeds of 1/4 sec and an apaerture of 3.4 or 4 is not quite
convenenient.
So I did another thing and ordered new resolution charts (the same Leica are
using btw), which make for easier and more accurate testing at the distances
to be used.
With these new goodies (test patterns and film/exposure/development resuts
for optimal result and comparison), I made extensive series of tests with
both lenses at several distances and the following combinations of
film-developer.
Gigabit in Gigabit developer, in Rodinal, in Technidol and in FX39
TechPan in Rodinal, in Technidol and in FX39
Seven combinations in all, with two lenses at three distances and optimum
apertures, including exposure and focus bracketing. That kept me "happy" for
a while.
Here I have to include some observations about resolution figures in
general. The difference between 80 lp/mm and 100lp/mm looks big when you
interpret in numerically. If you look at the resolution patterns themselves
on which the calculation is based, you see a much smaller difference. In
other words, there is a law of diminishing returns when you progress in the
higher resolution numbers.
My test was setup as follows: the original test patterns have real patterns
of 1 and 2 linepairs /mm on the print. They go in steps of 1.1, 1.25, 1.4,
1.6 and 1.8 lp/mm. So if I use a reduction of 40 times and can clearly
detect  in the subsequent enlargement a pattern of 1 lp/mm and the next
pattern (1.1) is blurred, then my film/lens-resolution is 40 lp/mm.
I used reductions of 30 and 40 and 100 times, with the main attention to the
30 and 40 times enlargements: that would still imply a print of 1meter wide
from a 35mm negative and who does this on a daily basis.
The results? Next
Erwin