Colours in black and white

There’s more to creating a black and white photograph than just removing the colours because a vast difference in colour is no guarantee that there will be a vast difference in the values of the grey that the colours convert to. As an example, here’s a photograph of red roses and green leaves alongside the result of converting to greyscale in Photoshop:


The roses no longer stand out, having become the same colour as the leaves. When using black and white film, coloured filters are used to accentuate the differences between colours. They work by lightening the colours that are the same as the filter, and darkening the complementary colours; in the example above a red filter would darken the greens and lighten the reds, and a green filter would do the reverse. Photoshop lets us do the same thing when converting to black and white. Starting from the same coloured image and using channel mixer, I produced two very different versions, darkening the roses in one, and lightening them in the other.


If you want to realise the full potential of this type of photography, you should take control over how colours translate to black and white. If starting from a digital file, a jpg file is the worst starting point because it limits the tonal range available.

If you make a selection based on colour in Photoshop, you can fine tune the result; here I was able to control the effect so as to single out a single bloom:


The ability to control the tones to this degree gives a lot of scope for emphasising different parts of an image.

298 words

This article was really challenging to write. I’ve left the word count in above, and I was pushing the limits of space available. A more detailed treatment follows.

The article hopefully showed what happens when colours are converted automatically, but fell short in providing a full set of examples and discussion. I am writing a book, and what follows is an extract from the discussion on using filters in black and white photography. As the text makes clear, although the discussion deals with filters, exactly the same principles apply when using Photoshop – it’s just easier to write about if I assume something physical!

Filters for Black and White
From the graphs film makers publish for their films which show the spectral sensitivity, it can be seen that the response across the range is not even, and does not match the response of the human eye. This is one reason why coloured filters are the stock in trade of black and white film photographers to alter the relative tonal values of the colours. These filters are usually referred to as “contrast filters” because they are used to increase the contrast between objects that are different colours but would, if left unchanged, be rendered as the same or similar shades of grey. The digital world seems to be ill served with this level of information about sensors, so I’m having to assume that the situation is analogous.

Contrast filters are not needed with colour film or digital cameras (indeed, their use brings only disadvantages). In digital photography, all the coloured filters can be simulated in Photoshop, which is by far the best way to handle conversion to black and white. For film photographers who intend to scan and print digitally, a good case can be made for the use of colour negative film as the basis for black and white prints. Starting with a colour image allows the greater versatility of image processing software to be used, rather than the more basic in-camera options; it allows selections to be made based on the colours, and no filter factor is needed. It also saves the expense of buying the filters, and removes two surfaces that could increase flare. I’ll have more to say about this later in this chapter, where I’ll put the other side of the case.

A red filter will pass red light, and stop blue and green although as we’ve seen in the previous chapter in the section on black and white films, all objects will reflect at least a small amount of every wavelength contained in the light that illuminates them. We might be able to filter out all but the red light, but green and blue objects will still not be rendered as black.

The textbook example is a red rose with green leaves. Left to the normal rendering of colours in black and white, the red and green will appear as similar shades of grey as can be seen below.

RedRoseColourGreyScalePhotograph 151 Red rose converted to greyscale

In the pair of photographs above, the roses clearly stand out from the leaves when colour is present; converted to greyscale in Photoshop the result shows very little differentiation. Using a red filter lets the red pass freely, and reduces the brightness of the green, giving a pale flower against dark leaves. A green filter gives the reverse effect. If we simulate the effect of using red and green filters in Photoshop, the contrast is restored, but with very different end results.

RedRoseGreen100Red100Photograph 152 Roses filtered

A red filter lightens the roses considerably, whereas the green filter makes the petals almost black. Using less extreme filtration (or Photoshop effects) enables the effect to be precisely selected.

Many different coloured filters are used, each with their own effect. The one thing that they do have in common is that, as they absorb some of the light, an increase in the exposure is necessary. Cameras which have through the lens metering should allow for this automatically (assuming that the meter’s spectral sensitivity isn’t fooled by strong colours), but those setting the exposure using a hand held meter will have to make an exposure increase, which depends on both the filter and the colour of the light source.

With landscapes that include the far distance, and particularly at higher altitudes, heat haze can obscure the distant details. The haze is bluish in colour (distant views look bluish) and if you reduce the blue light reaching the film, the amount of haze can be reduced.

The final general point to make is that although I talk about “yellow” filters, “red” filters and so on, in practice all contrast filters come in more than one variety even for the same colour. You’ll find filters designated “light” or “medium” or “dark”, depending on the strength of the effect, and you may wish to have more than one variety available to use depending on the circumstances.

The effect of using filters
Before looking at the different filters, let’s see the simplest possible example of three coloured circles passed through Photoshop.
Figure 86 Coloured circles

The circles are shown “as is” and then as they appear when converted to greyscale in Photoshop. The colours, while not being exactly the same shade of grey, are very similar. If we photograph the circles through blue, green and red filters, and then convert to greyscale the results are rather different.

Photograph 153 3 circles filtered

The blue filter has darkened the red, while leaving the green more or less the same; the green has lightened the green circle, and darkened the red more than the blue; and red filter has clearly made the blue circle much darker, while the red is lighter

That illustrates the general principle that a filter will lighten its own colour, but is hardly a real world example. So let’s see an actual example with a colour photograph passed through Photoshop.

E1Photograph 154 Original and greyscale

I chose the above photo because it contains red, green and blue elements, which will make it easier to see the effects of adjusting the relative brightness of colours. The left hand photograph is the original coloured image, and on the right is seen the result of converting to greyscale in Photoshop. At first glance, this seems a reasonable interpretation of the original. But an interpretation is what it is; and an interpretation made by the software and not by the photographer. You may have other ideas of what you’d like to emphasise or distinguish.

Channel Mixer in Photoshop lets you vary the proportion of the red, green and blue channels in a coloured image to change the relative brightness of colours when converting to black and white, and the next pair have used this to simulate the effect of filters, a red filter on the left hand image and a blue one on the right. The left hand one has a noticeably lighter boat, and has been given values of R100, G0 B0. The right hand one has darker water (which was a yellow brown) and a lighter cap (although you may have to look carefully to see that) and used values of R0 G0 B100.

E2Photograph 155 Simulated red and blue filters

Despite this, you can see that no parts have turned completely black which shows that even though normal objects may have a very distinct colour, they are rarely monochromatic. Even the red boat is reflecting some blue light. The other point to note in this pair is the deep shadows. The rocks have been completely lost in the shadow in the red filtered photograph (remember shadow areas are illuminated by blue skylight) whereas they can be seen in the blue filtered photograph.

E3Photograph 156 Simulated green filter

The left hand image here is a green filtered one (R0 G100 B0) and the foliage has become lighter and airier. In the right hand image above, the values used were R0 G50 B50. The remaining examples are the results of mixing the values a little more.

E4Photograph 157 Other filter effects

The final pair have the values for the left R30 G110 B50 and for the right hand side R30 G110 B0. Note the foliage and the water tones, and compare the results from the other settings.

Is a colour image the best starting point for black and white?
The ability to vary the colour in this way when using an image editing program, and perhaps even more importantly, being able to make such changes selectively (applying a different filter to each part of the image as you convert it to black and white), gives an extra degree of freedom to those who either use colour film and then scan or use digital cameras and capture a colour image. An additional advantage of using colour for black and white is that selections can be made based on the colour, rather than just the tone. If you can easily distinguish sky from leaves in this way, a selective adjustment is made far easier.

Going back to the rose photograph earlier, by selecting a single bloom it can be made to stand out from the others in black and white:

WhiteRosePhotograph 158 White rose

If you have a digital camera, the answer then is obvious – you start with a colour image and make the changes out of the camera. Film users face a more difficult choice. If you start with a colour negative, you have all the advantages above of being able to apply filters in a more controlled manner, and with a wider range of colours, than you would ever have available as physical filters. That is a very positive advantage. The much lower dynamic range of colour slide film and higher contrast of the resulting image makes it the worst choice for both image capture and scanning.

On the other hand, using black and white film has advantages as well. As we’ve seen, black and white film has a greater resolving power than colour for the same ISO rating; and black and white film also offers the major advantage of being able to control the dynamic range of the film by processing. In conditions of very high or very low contrast, this could be enough to tip the scale in favour of the black and white. Black and white film is also cheaper to buy and easier to process yourself. Conventional black and white film does not depend on dyes for the image (unlike colour films and chromogenic black and white films) and therefore has a greater longevity.



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