Archived posting to the Leica Users Group, 1998/10/09
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 10:17 PM 08/10/98 EDT, you wrote: >Hi all, > I just purchased a Peak grain focuser-it's a fantastic instrument. I do have >a question though. It came with a blue filter which the manufacturer says to >use for black and white. I know there has been some controversy about focus >shift especially with variable contrast paper ( which I use ). Does anyone >know if I should use the filter? If it's not important I'd rather not use it >as it's quite dark. TIA > Richard Wasserman > Richard, I use the same focuser, and I do not use the blue filter. It does make focusing difficult because of the dimness. Two good references to read regarding why not to use the blue filter, are Ctein (1997) "Post Exposure" by Focal Point Press, and Gainer "Hazards of the Grain Focuser" in the Jan/Feb 1997 issue of Photo Techniques. The reasons for hot using the blue filter are technically complicated, having to do with the chromatic aberration in the human eye, and how it contributes to a focus shift of an aerial image. _ [o] -GH