[Leica] limits part 4

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

I started this investigation to find the truth about the claim of the
Gigabit people that by providing a film with a resolution of 600 to 900
lp/mm would improve the quality of current BW photography significantly if
not dramatically.
The use of document film in a POTA type developer for pictorial results is
not new and indeed with TechPan we have a solution that is claimed to
provide the 35mm user with medium format quality.
So I compared the Gigabit with the Techpan as a natural comparison. My
selection of Tm100, instead of D100, was motivated by the fact that TM100
has slightly finer grain than D100 and a straigher curve, making it more in
line with the GB and TP film curves, which are quite straight too. As GB
stresses its capabilities to not diminish in quality when overexposing, the
TM100 was the better comparison.
A second argument was the role the TM100 (and Tgrains) has played in
emulsion technology and its boost in quality. I also wanted to make readers
aware of its excellent quality and induce them to use this film. There is no
need to cultivate a mono-culture here in film choices.
Resolution does not tell all of a film/lens capability. As I have often
noted, no single number or one-dimensional approach can capture the full
spectrum of a lens/film combo. It is wellknown that the lens MTF is a
excellent representation of the residual aberrations of a lens. Film MTF is
based on acutance measures. The combination of film and lens MTF (the
socalled cascading function) gives you a number, which is the result of
combining the MTF value of the lens (at a certain resolution) and the MTF
value of the film (at a certain resolution).  MTF values and resolution
values are (as I noted in my Viewfinder articles) proportionally releated. A
high MTF value is als a high resolution value. But if I would say that the
film/lens combo would provide at 50 lp/mm a contrast transfer value of 20%
and at 100 lp/mm a value of 3%, most readers would have trouble interpreting
these values.
So for practical evaluations of film/lens systems, the resolution figure is
a fine approximation of useable image quality when doing high resolution and
high magnification photography, providing high quality optics are used
(those with high MTF values over the relevant spatial frequencies).
 It does not make sense to do this excercise for Tri-X films and Leica
lenses as the resolution of the TX will be reduced significantly by grain
patterns to a low level. Generally my preliminary conclusions are these:
(subject of course to further knowledge I may get during the rest of the
study).
If you use films with a relative low resolution figure of 100lp/m  and less,
the quality of the optics is significant for image quality and with a
moderate control of all components, you may get 10 to 30 lp/mm on the
negative.
When using films that have a resolution figure of 200 lp/mm, the quality of
the optics is decisive and here with a tight control of all components, you
get around 60 to 70 lp/mm on the negative.
Films with even higher resolution (which automatically implies a very high
MTF value at the lower spatial frequencies) can deliver up to 100lp/mm, but
with topclass lenses and an extremely high level of control of all
components.
I also noted that the grain pattern is more likely to influence the result
than does resolution. So TP may be as good as TM in practical resolution,
but will win in the fine grain area when enlarging above 15 times. SO the
end result may be more pleasing with TP, but you do get the same sharpness
impression and detail definition.
Quite surprising is the fact that slight errors in focussing accuracy and
exposure have more impact than film flatness. At least so it seems in the
middle of the negative. I did not check the corners of the negative area,
because I have not yet found a reliable way to align the camera back and the
test charts absolutely parallel. Some people have reported that their Leica
negs are always less sharp in the left side of the negative, where the film
chamber is located. This could indicate problems with the film feed. I have
not been able to check this in a reliable way, so do not see this as fact.
If you note these problems, let me know.
Some other results. The BJP tested recently the TP, PanF, TM100, D100, APX25
and old Pan-X and these are the results at 20 tims enlargement: finest grain
TP, and with only marginal differences in grain size the rest.
In sharpness it is again TP and then the rest within very close distance.
The BJP summed it up as follows: Tgarins have given the medium speed films
like TM100 and D100 properties very close to those of a previous generation
(APX, PanF and Pan-X). With TP a fraction ahead but at ISO16 to 25 ths is
not honest when compared to ISO100!
A German magazine (no longer in existance: it was too good and informative
for the market). tested these films also 10 years earlier and used an
elaborate direct contact masking to test resolution. Their results:
TP ans APX25 and Agfa Ortho and TM100 could all resolve patterns around 100
lp/mm. But the quality of the pattern was different: clearly differentiated
with TP and just visible with TM100 and not visible with PanF.
Here lies the danger of using resolution patterns without some additional
qualifications. TP and TM100 both resolve 100 lp/mm, but where the TM100 is
here at its limit, the TP could go to 250lp/mm. but this value is not
useable in pictorial 35mm photography with the current best lenses!Just
resolvable also implies that defocusing, camera shake, overexposure etc have
a much greater impact on the result than when using TP.
It is not as easy as you think and to close now with a paraphrase of a great
optcal/emulsion scientist: it is  selfdefeating to try to capture a film or
a lens in one number!

Erwin