Pixiq

With so much possible in post–production is there any point in carrying filters these days? I always divided filters into two categories – those you often use and those you try once. In the first category, came polarizers, warm up filters, graduated neutral density filters….

In the other category belong those filters described as ‘effects’ filters – starbursts, color graduates (those awful tobacco filters), multiple image filters and so on cliché after cliché– if you have the urge you can do much more creative/weird things in Photoshop.

How Polarizers work

What we call visible light occupies a very small portion of a map called the electromagnetic spectrum – which arranges electromagnetic waves according to their wavelength. Radio waves lie at the long wavelength end and as wavelength decreases we move through, microwaves, infra-red, visible light, ultra violet to X rays and gamma radiation. Light travels as waves and associated with the light waves is an electric field that varies at right angles to the direction of travel – it is what is called a  transverse wave. In normal light the variation is in all directions perpendicular to travel – with polarized light just one direction is selected by the polarizer. The plastics used in a polarizer have their molecules in long chains so and we can think of the polarizer as a kind of molecular grid or set of railings through which the light passes. When the grid and the electric field are aligned everything passes through - when they are at 90° (crossed polarizers) nothing gets through.

Polarizers are useful in photography because light when scattered or reflected is partly polarized. And these filters can be used to select or cut this light.

a. Blue Skies

The process of light scattering by molecules in the upper atmosphere creates a blue sky. This light is partly plane-polarized and the effect is most pronounced when you stand at 90° to the sun – face the sun and then turn through a right angle either to your left or right. The effect is dramatic - as a polarizer is turned it deepens the blue and seems to show up whispy clouds you did not even know were there. It is great for those ‘inviting travel shots but should be used with care: with  a low angled sun it can produce skies which are almost black.

At one stage, I used a polarizer for those blue-skied happy shots beloved of travel brochures Nowadays, I far prefer what I can do with the amazing ND (neutral density) filter in Lightroom 3.  However, a polarizer still works well for some things... call it nostalgia but I still carry a circular polarizer so read on…

There are two other areas where a polarizer can still come in handy:

b - Reflections from Water

When light is reflected from all non-metallic surfaces, it is partly polarized. As you rotate a polarizer whilst looking into water, reflections are cut and you are able to look below the surface - especially if the light reaching you comes at about 53° measured with the vertical to the surface (The Brewster angle or polarization angle). You can alter the effect to produce partial transparency when photographing rivers and streams.

If you want to do the same with glass and cut surface reflections you change the angle slightly (56°) because the refractive index ‘n’ (the bending power) of the light is different from that of water and that slightly affects the Brewster angle (q)

For those with a mathematical bent – or even bent mathematically

tan q = n 

where n = refractive index: for common window glass this is 1.5  for water 1.33

The idea is behind such things as the way polaroid sun glasses cut reflections from road surfaces or water and also the transmission of light through the windows at the ends of laser tubes.

C - Intensifying Colors in nature.

Surfaces of leaves, grass and flowers can all reflect light - careful use of a polarizer cuts down some of the reflected light and make colors appear more intense. This is especially good for large areas of vegetation in a landscape with a variety of greens – in bright light the surface reflection from leaves makes then appear grey and a polarizer makes the colors seem far warmer.

If you are trying to get a deeper blue to the sky and cut refection from the surface of water in a lake you have to set a compromise because it is unlikely both will be polarized in the same direction.

Linear and Circular Polarizers

Circular polarizers are recommended for use with autofocus cameras since ‘linear polarizers can affect the function of the autofocus system. These incorporate an extra layer - a 1/4 wave filter - which changes the ‘phase’ of the light and allows the autofocus system to function. Polarizers are expensive and if you use both autofocus and manual focus bodies it make sense to standardize on circular polarizers. The problem I have found is that unless you pay a premium price for one of these bits of glass you get an unwanted side effect – a color tint that changes as you rotate the filter from slight green to magenta.

Graduated Neutral Density Filters

Whilst on the subject of filters I used to carry a set of Neutral Density (ND) flters when doing medium and large format film work…no more. It was always tricky with smaller formats and you had to align horizontals. When the contrast difference between land and sky is too great a ND filter can help. I find in my work that the ND function in Adobe Lightroom – one of much that is great about that program works amazing well…and I can add several at different angles…Those expensive filters are now in what has become my museum cupboard along with a series of film bodies.