Imagine the scene. A young photographer, eager to learn at the feet of an expert. In his camera bag are several rolls of black and white film, with one pre-loaded in his camera. (Yes, film. This is a story set way back in the mists of time.) The expert looks at our tyro, a smile on his face. ‘Are you ready to begin shooting?’, he asks. ‘Yes’, comes the slightly nervous reply. ‘Excellent.’ The pro briefly looks at the scene and then drops a bombshell, ‘then I’d suggest using a red filter for this shot.’ What? A red filter? For a black and white photo!? The youngster, tears welling up in his eyes, is crestfallen. No-one mentioned anything about needing filters today, so he hadn’t brought any to the session.
This never happened to me. Nope. Never. It was a friend. Of a friend. Of a relative.
The usefulness of colour filtration when shooting black and white is one of those weirdly counterintuitive concepts photography occasionally throws at you. To understand why they’re needed we first need to step back and revisit a few basic concepts.
A photo is made by light falling onto a light-sensitive surface such as film or a digital sensor. The light is either from a direct source, such as the sun or a lightbulb, or has reached the camera after it’s been reflected by a surface – a person, or a landscape. Generally, it’s reflected light that’s most useful to a photographer – it’s only in a few cases when a direct light source is an important element of a shot.
Here’s the thing though. Different surfaces have different levels of reflectivity. My camera is largely black plastic and rubber and so doesn’t reflect much light at all. But the pages in the notebook I carry around in my rucksack are white and so reflect quite a lot of the light that falls onto them. Somewhere between these two extremes would be something that was mid-grey. (Weirdly, mid-grey surfaces don’t reflect 50% of the light that falls onto them, but roughly 18%. In fact grey-cards – used to determine exposure or accurate white balance – are often referred to as 18% cards for this very reason.)
The brightness of a pixel in a black and white photo is theoretically determined by the reflectivity of the subject. In a monochrome world this would be simple. Dark subjects would result in pixels close to black, light subjects would be close to white, and mid-grey surfaces would be somewhere in the middle. (The reflectivity of a subject is influenced by a number of factors, so I’ve simplified a bit here, but the basic point still stands.) However, we don’t live in a monochrome world, we live on a planet that is bursting with colour.
A black and white photo is one where colour has been rendered into a range of greyscale values. The problem comes when two or more objects have distinctively different colours but a similar reflectivity, a green apple and a red apple for example. In a colour photo you would easily be able to tell which apple was which. In a black and white photo the two apples would be indistinguishable as they’d have the same greyscale values.
This is where filters come to the rescue. A coloured filter lets through wavelengths of light similar in colour to the filter, but blocks dissimilar wavelengths – with the greatest effect seen with objects on the opposite side of a colour wheel to the colour of the filter. Let’s return to those apples and add a red filter to a camera’s lens. Shoot in black and white and the red apple will be lighter than the green apple (which will get slightly darker), which helps to separate the two visually. Swap to a green filter and the this time the green apple is lighter and the red darker.
So, which filters would you use and when? Green filters work well when shooting black and white portraits as they tend to make skin tones deeper; you wouldn’t use red filters as they have the opposite effect. You don’t get blue apples, but you do get blue skies and yellow, orange, and red filters can help to deepen the tones of in a sky, with yellow having the least effect and red the greatest. Strangely, blue filters are rarely used – they have an odd effect with skin tones and will tend to wash a sky out completely.
Great, you’re probably thinking, so now I need to rush out and buy a set of coloured filters? Fortunately, unless you’re still shooting film, the answer is no. Most cameras have a black and white mode with an option to apply a digital colour filter, which has the same effect as a physical filter attached to the lens.
Ultimately though, post-production software offers a greater range of adjustment than a camera. Software such as Lightroom, offers adjustments to a wide range of colours, not just the standard four of yellow, orange, red and green. To make use of this facility you do have to shoot in colour first. (At least when shooting JPEG. With Raw you can shoot in B&W which makes it easier to visualise your shot. You’d then start from a colour image in post-production, altering the B&W tonal range to suit.)
Whether you choose in-camera or post-production one thing is certain. You’ll never have to be embarrassed by a lack of filters ever again.