Sunday, 10 March 2013

Colours into tones in black and white

When photographing in black and white, or now, more likely, converting the digital image from colour to black and white, either filters or a form of filtration is used.  The standard had been set by Kodak with their range of Wratten gelatine filters.

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;
  • Blue Hi-Contrast;
  • Green;
  • Infrared;
  • Orange;
  • Red;
  • Red Hi-Contrast Filter; and
  • Yellow.
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)

ISO 100, 1/125, f/11
I have set up a scene which is likely to exagarate the impact of applying the 4 "filters".

No filter
Yellow - Manual
Here I moved the Yellow slider to +40 and the Blue slider to -40.  Orange and Green to +22 and Aqua and Purple to -22.  The blue background is noticeably darkened and yellow is very light in tone.  Green is slightly lightened.

Yellow - Preset
In the Lightroom v4 preset, both the Orange and Red are lightened significantly with Red having a value of +63.  Aqua has the lowest value at -38 and Blue at -17.

No Filter
Red - Manual
As before, I moved the Red slider to +40 and the Aqua slider to -40.  Orange and Magenta being lightened at +22 and Green and Blue being set to -22.  The effect is quite visible with the red being lightened and blue slightly darker.

Red - Preset
Again the preset is significantly different to my manual interpretation.  Red (+50), Orange (+38) and Yellow (+36) are significantly lightened.  Surprisingly Aqua is also lightened (+25).  The sum of all of these means that my manual interpretation is significantly different when judged against the preset.

No filter
Blue - Manual
This time I lightened Blue to max (+100), Aqua and Purple to +40.  Yellow (-40) and Orange (-20) were darkened.  This left yellow, green and red very similar in tone.

Blue - Preset
A massive difference after applying the preset with yellow and red becoming almost the same in tone with values of -60.  Blue (+63) and Aqua (+38) are lightened, thouhgh not to the same extent as I did in the manual and this is clearly evident.

No Filter
Green - Manual
For the green filter, I went back to my "standard" approach lightening Green to +40, Yellow and Aqua both to +20.  Red (-20), Magenta (-40) and purple (-20) were all reduced.  The effect is predictable and the difference is visible.

Green - Preset
Visually, the preset presents the red somewhat lighter, with a positive lightening of + 25.  Yellow and Aqua are much lighter at +40 and +38, so all these differences become visible.

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!

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