titrisol said:
I have to try the black background!
How did you compensate for focus?
I'm responding from my own experience. Some time ago I wrote the article "Hazards of the Grain Focuser" for Photo Techniques. I tested red, green and blue separation filters to determine if the spectrral sensitivity of the paper or the eye made a difference. I did a number of trials with each color and without any filter to find mean and standard deviations of focusing errors. I focused for each trial by moving the enlarger head until sharpest visual focus was obtained because movements of the lens carrier would have been too small to measure accurately. I measured focus distance from negative carrier to baseboard.
You might expect only a difference in standard deviation due to change in focusing color, but there was also a different mean error for each color. The exception was that white and green gave the same result: smallest random error as well as smallest mean error.
The question of which color was absolutely closest to true focus was answered by making greatly enlarged prints from each mean focus distance. This i did by first making a print on ortho lith film, then using that as a negative to square the enlargement from 10 to 100. The grain in the final photos was convincing: the sharpest focus was obtained at the point of least mean error.
The conclusion I reached was that the ancient reason for using blue filters to focus telescopes was not applicable to modern enlarging lenses. The focusing error was more likely due to chromatic abberation of the human eye, which is easily demonstrated. This effect has been known to astronomers for a long time: the aberration of the eye increases the apparent aberration of a refracting telescope.
Thus, the sharpest focus of an enlarger will be obtained by either white or green light because the acuity and sensitivity of the eye are both greater there. Modern enlarging lenses are achromatized very well over at least the blue and green, so the best strategy is to favor the optimum color for the eye.
Now as to thickness of the paper, I used in my tests the optimum aperture for the lens I had, an El Nikor f/2.8. The visible grain was sharpest at the 5.6 aperture. The smallest average error in focusing was considerable more than the thickness of the printing paper, and the mean square deviation was greater yet.
