Grain Focusing - any advice?

[Shmoo]

I also use a Peak focuser and had exactly the same problem, my course of action was to use the blue filter which came with the Peak focusing unit. Not only does it reduce light by about 2 stops, maybe a bit more, it aids focusing somewhat for me with my enlarger, which is a DeVere 504 with a dichroic head and single 250W globe.
...
Mick.

I found that using the blue filter got me close enough to focus that I would then take it off the Peak focuser and spent a relatively short time fine tuning the focus. By limiting my time with unfiltered light, the retina-toasting effect isn't as severe...and I'm using a condenser head on a Beseler 45 MCRX.

 

[RJS]

I come late - for various irrelevant reasons. Faced with this problem using a cold light and particularly wanting to print consistently with the optimal aperture of my enlarging lens I purchased a rheostat from Aristo. This enables me to focus more comfortably, using a dimmer setting, but also to always print two stops down from wide open.

 

[gainer]

If there is a focus shift of the projected image with different colored light then the enlarger lens has chromatic aberration.

When using a grain focuser with an arial image, if the reticle is in focus on the retina then chromatic aberration of the human eye will be compensated.

That is not true. There was no measurable focus shift of the lens itself. I proved this by making prints with red, green and blue light after focusing visually with green light, using the grain focuser. None of these prints was out of focus. On the other hand, refocusing with red, green and blue light using the same grain focuser produced out-of-focus prints with red and blue light. You see, the eye is most sensitive and has its highest resolution in green light. The resolution is markedly less in blue and red. Both the image and the crosshair are fuzzier in red or blue light. The image of the crosshair cannot be in sharpest focus on the retina in red or blue light if it was set in green light because of the chromatic abberation of the eye. The combination of focus shift of the eye, which you can easily demonstrate to your self by measuring the nearest point of clear vision in different colors of light, and reduced resolution gives a wider spread in focus errors as well as a bias of the mean.

There is no point in arguing the theory when the theory does not explain the experimental results. The lens I used was a Rodagon apochromatic which is optimized for reg, green and blue.

 

[gainer]

PS: the back issue of Photo Techniques containing my article is still available at
http://www.phototechmag.com/back_issues.htm
if anyone is interested. It is 1997 Vol. 18 No. 1.