View Full Version : B&W filters

David Grandy
18-Apr-2000, 11:30
Recently I was reading an old Darkroom Techiques article on the use of filters w ith black and white film. The magazine published a colour wheel and explained t hat the filter would not "see" the colour opposite it on the colour wheel.

So far so good.

Then I noticed that opposite red is magenta; while opposite yellow is blue. So why does a red filter substantially lighten (on the neg) blue, even more so than a yellow filter? I've always used yellow/orange/red in that order, to darken t he sky (on a print).


Ellis Vener
18-Apr-2000, 11:42
Last timeI looked at a color wheel the oppositions were:

blue opposite yellow

green opposite magenta

red opposite cyan.

One of the things to keep in mind is that most "panchromatic" film does not see all colors equally. C yan and blue tend to be seen more weakly than things on the red end of the spectrum. this probably explains the phenomena we are all aware of.

Ellis Vener
18-Apr-2000, 11:48
Immediate correction! I meant to say One of the things to keep in mind is that m ost "panchromatic" film does not see all colors equally. Cyan and blue tend to be seen more strongly (hence the over exposure of skies and loss of de tail and contrast beteen say a blure sky and a white cloud in an unfiltered but otherwise correctly exposed photo) than things on the red end of the spectrum. "

I apologize for my confusion.

N Dhananjay
18-Apr-2000, 12:32
The sky is not only blue but also contains a fair bit of green, which is why a red which absorbs blue and green darkens the sky more than a yellow, which absorbs the blue but passes green. DJ

William Marderness
18-Apr-2000, 12:33
A yellow #12 filter removes all blue light. A yellow #8 removes some blue light. A #25 red removes all blue and green light. Blue objects usually reflect some green light too. A red filter will darken most blue objects more than a yellow filter because the red filter also removes the green light that most blue objects reflect.

Ellis Vener
18-Apr-2000, 14:40
What do filters do? they remove something from an aggregate. This is why, in sim ple terms, "the filter would not "see" the colour opposite it on the colour wheel."

More color wheel info: green+blue = cyan. red cancels out cyan. Skies are mostly blue +cyan, or put another way think of a sky as being mostly (2x blue) + green.

I think the red cancelling out magenta (and v ice versa) statement is a mistake on the part of the author and editors of the article.

David Grandy
18-Apr-2000, 17:43
I paraphrased the content of the article when I used the "couldn't see" the opposite colour phrase. It's more my statement(and the net effect)and I used this phrase to simplify my question as much as possible.

Richard Rankin
18-Apr-2000, 18:47
At the risk of being too sanely visual here... I would suggest what I think is probably the ultimate 'color filter circle'. It is located in Ansel Adams book 2, The Negative, chapter 5, 'Filters and Pre-Exposure' and is Figure 5-3. St Ansel says it was kindly provided from Kodak Publication F-5 (the filter handbook which seems to be currently out of stock). It is a simple 'wheel' of all the colors along with tungsten and daylight filter factors, filter numbers and, in the center of the circle, the rule to live by: Adjacent lightens; opposites darken.

I copied this and pasted it into my Kodak Prof. Photoguide (where it should have been published in the first place) and it gives instant resolution to these questions and all the info one might need on them. And, for the record, it says that 'red 25' directly darkens 'deep- bluish green 65'; 'Orange 21' directly darkens 'cyan 44'; and 'yellow 8' directly darkens 'deep blue 47B'. So, pick your sky and then pick your filter...

Probably more than anyone wants to know - this wheel also gives the 'width' of the filter. So you can see that 'deep yellow 15' is exactly the nemesis of 'blue 47' but 'light red 23A', for example, only covers part of the spectrum covered by its arch-rival 'cyan 44'.

Cheers, Richard