Peak Focuser Blue filter mystery : LUSENET : Large format photography : One Thread

Hello friends,

My Peak enlarging focuser (I) comes with a blue filter that can screw onto the eyepiece. The minimal instructions, translated in the usual stilted style from the Japaneese, calls it a "BG filter". Furthermore, the instructions explain the filter as follows:

"When you want to obtain more correct enlarging of the black and white film, use the focuser after fitting the BG filter to its eyepiece. The use of this filter will permit the coincidence between the wavelength sensitive to your eyes and that to be enlarged."

Ok, I don't get it!

What does "BG" stand for and what is this all about?

Thanks as usual for all your great help...


-- Scott Jones (, June 03, 2001


Maybe because it's filtering out UV which can give you false focus?

-- Jim (, June 03, 2001.

The idea is to match the color of the light you're using to visually focus with the color that graded paper's sensitive to.

While in theory that may be proper, in practice it's irrelevent. Also, since VC paper has a mixed emulsion that's sensitive to different colors (hence the VC effect) the blue filter wouldn't be the proper filter anyway.

A few years ago, _Photo Techniques_ published an article in which the author tested visual vs "chemical" focus with and without various filters and found that the most accurate focus resulted from not using any filter.

-- John Hicks (, June 03, 2001.

I'm guessing that this is a throwback to the days when enlarging lenses might have had a lot of chromatic aberation (basically, the lens has a different focal length for different wavelengths [colors] of light) and people were printing on blue-sensitive paper.

Regular tungsten bulbs, of course, produce a lot of red light and not too much blue so you might tend to focus for red and ignore the blue fringes. By filtering away the red, you avoid the chromatic problem and focus with the light the paper is actually recording.

Of course, your enlarging lens probably doesn't have a chromatic problem and you're probably using paper that is sensitive to colors other than just blue. So, just like everyone else said, you can just ignore the filter.


-- Nathaniel Paust (, June 03, 2001.

Thanks for all!

The filter just got put in the back of the drawer to be rediscovered years from now with the comment: "I wonder what the Hell this thing is for?"


-- Scott Jones (, June 03, 2001.

The complete response to this is a real quagmire. Ctein addresses it in his book "Post-Exposure". In short, lenses have chromatic aberration, our eyes have chromatic aberration and certain papers are much more sensitive in the U.V. and near-U.V. region than others. The short answer is to ditch the filter. Ctein's book makes interesting reading on this and other matters.

-- Wayne DeWitt (, June 03, 2001.

Scott I think "BG" stands for Blue Glass, and is supposed to make focussing more accurate when using VC papers, whether it works.....who knows! Regards Paul

-- paul owen (, June 04, 2001.

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