Archived posting to the Leica Users Group, 2000/11/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]When testing the first batches of Xtol versus D76, I was puzzled by some of the Kodak claims. To be able to compare developers, you need the same film (obviously) and you have first to do a series of experiments to find: a) the real speedpoint (base fog + 0.1 D) and ") the same CI value (in my case always 0.62). I used Tri-X as the comparison emulsion and experimented as long as needed to get identical speed points and CI values. Then I checked first the claim that Xtol delivers finer grain and sharpness. Here I found marginally finer grain for Xtol, but the same sharpness as with D76. The push capabilities of Xtol were very disappointing. At EI800 I found by densitometer that the emulsion was a full stop underdeveloped compared to the normal speed of 400. The curves of the pushed films (800 and specifically 1600) were steeper and gave higher density in the mid grey areas and highlights. In the shadows remarkably less happened. In fact the speed point moved to the right as it would when you simply underexpose. Also I noted that the development times given were generally too short for normal development and too regular when looking at the push steps. After consultation with Mrs Zawadski the following conclusions could be drawn. The differences between Xtol, D76 and even Tmax are very small and not true for every film. Some films are better in T-max, some may benefit when used in Xtol, some are best in D76. Room for personal experimentation! The research goal for Xtol was primarily environmental (away from hydrochinon) and as the BW processing is done worldwide with water of various levels of impurity great attention was given to buffering Xtol so as to give consistent results in any type of water. The second design goal was a different shape of the characteristic curve. Normally this curve is S-shaped with a very slowly rising start, then a fairly steep and straight part and then a rounding off at the shoulder. The typical Xtol curve has a slightly upward curving early part (the foot of the curve) improving shadow densities and contrast in the shadow parts and a less steep shoulder part, making it easier to get differentiation in the highlights. Measured curves with D76 and Xtol substantiated this claim With Tri-X at least. Here the curves approach a straight line as Zone System workers like it. With Delta400 the curves were straight too, but the shadow densities did not improve (different grain chemistry here). The push claim was the hardest part: I questioned their claim and data sheets here. I had to present her my densitometer readings and after discussing them , she said that indeed pushing is in fact nothing else than controlled underdevelopment. We agreed that the times used by Kodak were indications, based on some very thorough testing by Kodak with an accuracy of 1/60 of a stop!, but with some large interpolations between the test figures. The Kodak approach here is interesting: the level of pushing isdirectly related to the subject brightness and the desired level of details you wish to have in the shadows. The ISO speed is not all altered by pushing (choosing a different ISO value on your exposure meter). If your subject has a brightness range of 3 to 4 stops around the middle grey value, that you are interested in, then pushing may help you get a higher density in these areas. Kodaks figures are based on this typical situation which is indeed often encountered in available light photography. A direct comparison with D76 and Xtol with TriX at push stage One (one stop underexposure) gave somewhat higher density in the deep shadows for Xtol, but NOT because of speed improvements, but because of the different shape of the curve in the foot. Finer grain of Xtol versus D76 is not so much caused by smaller grainclumps, but by a tighter and more homogeneous distribution of the grain. Xtol is a definite improvement when compared to Tmax, whose push capabilities are also a bit overhyped, but compared to D76 differences are small, but might be significant for some workers. If you are sensitive to environmental issues, XTol is a must. Erwin