Details of their usage are given in the Kodak Professional Data Guide as shown above. They were listed in the form of a "Colour filter circle", which also gave the exposure factors to be applied for each. Most helpfully the advice given is that "Adjacents Lighten" and "Opposites Darken", so for example, when using a green filter, which is listed as a Wratten #58, the grey tone rendering of yellow, green and blue-green will be lightened and the opposite colours of red, magenta and violet will be be darkened in tone.
So why are filters used in black and white photography? There are three main reasons:
- to eliminate haze (yellow filter) or enhance atmospheric conditions (blue filter);
- to render tones in a black and white photographs in such a way as to make them look more natural; and
- emphasise objects due to their colour, classically, a yellow filter will darken the sky.
Filters are also used to correct colour casts or marks when photographing old documents, for example.
In digital photography, conversion to black and white can be achieved in a number of different ways, with considerable subtlety depending on the route chosen. Most software will have a "button" which represents the one stop solution, in my case, using Lightroom v4, in the Develop module, I can click on the Black & White tab in the Basic adjustment panel or I can select B&W in the HSL / Color / B&W panel. The effect on both is the same. This is the starting point from which further adjustments can be made using the sliders which adjust the following colours:
Red, Orange, Yellow, Green, Aqua, Blue, Purple and Magenta.
The nomenclature is that used by Adobe.
So to apply a yellow filter, say a Wratten #8, then would lighten yellow, by say taking the slider to say +40, but would also lighten Orange and Green, though to a lesser extent. It is necessary, though, to remember to deal with the "opposites", so darken Blue, to say -40 and then darken Aqua, Purple and Magenta in smaller proportions. Magenta would likely be darkened the least.
Most software will have a preset which will do this adjustment in a single step. Lightroom v4 has the following B&W filter presets:
- Blue Hi-Contrast;
- Red Hi-Contrast Filter; and
The effect of applying these filter presets is different to the more simplistic rule applied using the Wratten Colour Filter Circle, so in my example above, Red is lightened most as as well are both Magenta and, to a smaller extent Purple. Aqua is the "blue" colour darkened most. I suspect that other software will also apply similar presets differently.
This exercise requires us to demonstrate the effect of applying No filter, Yellow (Wratten #11), Red (Wratten #25), Blue (Wratten #47) and Green (Wratten #58)I have set up a scene which is likely to exagarate the impact of applying the 4 "filters".
ISO 100, 1/125, f/11
|Yellow - Manual|
|Yellow - Preset|
|Red - Manual|
|Red - Preset|
|Blue - Manual|
|Blue - Preset|
|Green - Manual|
|Green - Preset|
What have I learnt from this exercise? In the digital world I have a huge amount of control over how I want to adjust a black and white image. The preset filters are a great starting point, but can then make further adjustments from there in order to achieve the desired effect. At least I do not have to consider the panchromatic qualities of the B&W film as well!