Blue filter on a grain focuser?

[jstraw]

I was rereading one of Gene Nocon's earliest articles on f-stop printing and in passing, he recommended using a blue filter over the eyepiece of a grain focuser for "tack sharp focusing."

Has anyone ever tried this? It's counterintuitive, as the whole premise of the visual acuity provided by *blocking* blue wavelengths suggests doing the opposite, if anything.

 

[Curt]

Interesting thought, Can you tell me where the article is located I would like to read it?

 

[Jim Noel]

I have watched Gene print on several occasions. He swears by the use of the blue filter since that is the part of the spectrum to which the paper is sensitive.

 

[jstraw]

The article isn't online and it's just a passing recommendation. It's an old magazine...would you like the title and date?

 

[MattKing]

Isn't this similar to what is being discussed in this thread:

(there was a url link here which no longer exists)

Matt

 

[jstraw]

Isn't this similar to what is being discussed in this thread:

(there was a url link here which no longer exists)

Matt

Thanks for the pointer. Somehow I completely missed that thread.

 

[Loose Gravel]

My grain focuser came with a #47 filter. The idea is that if there is any chromatic problem with your lens, the #47 will allow you to focus on the light that the paper sees and not what your eye sees (green).

I never use mine.

 

[Dan Henderson]

I believe that Barry Thornton dismissed this idea in his book, "Edge of Darkness." Of course, he also cast aspersions on split grade printing, too...

 

[PHOTOTONE]

If you use a grain focuser, and then stop down two stops, if there was any chromatic aberration, which I don't think there is in modern enlarging lenses, then the depth of field enabled by the two stop move would easily cover the so called residiual un-correct-ness.

 

[PHOTOTONE]

However..I might add, if you are printing VC paper with filters, one should focus with the filtration in place.

 

[Mick Fagan]

My personal experience with focusing wide open and then stopping down a couple of stops, is that there is an extremely small focus shift.

This shift is noticable in the prints produced, but not really easily detected when looking through the grain focuser.

I have had quite a few sessions with my Opthalmologist in the last few years. Lately I have been in discussion with her regarding this actual subject, my sister in-law in Germany is married to an Opthalmologist and he and I have had some interesting discusions regarding just what the eye can and supposedly cannot see.

My experience in my own darkroom is that there is a difference using a BG filter and I'm waiting the use of an APO enlarging lens to test another theory that was suggested in the above mentioned thread.

As far as focusing with multigrade filters in place, I wouldn't think it's a good idea, various reasons, but the most compelling came from my Opthalmologist who suggested that the wavelength they are situated in is a difficult one for the human eye to see in, therefore our judgement of correct focus is more than likely going to be shifted slightly.

This shift in focus, is a like shift in focus that I believe happens when I use the BG filter on my grain focuser.

Mick.