Archived posting to the Leica Users Group, 2001/01/03
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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