See the difference between the two pictures below? The one on the left was shot without a filter, but the one on the right was shot using a ‘polarising’ filter. These darken blue skies, subdue reflections off shiny surfaces and increase colour saturation. These are optical effects which can’t be duplicated properly in Photoshop. Polarising filters are one of two filter types which are needed just as much in the digital age as they ever were with film.

Why do we still need filters? It’s possible to achieve a huge variety of special effects in Photoshop and other image-editors, but there are two reasons why traditional filters are still vital, even for digital photographers.

1. 1.They’re quicker! Even if Photoshop can produce a myriad different effects, it’s still quicker much of the time simply to slide a special effects filter over the lens when you shoot. Why waste time in Photoshop that could be spent taking more pictures?
   

2. 2.More importantly, you can only apply effects convincingly in Photoshop if the image contains the necessary raw material in the first place. And for this there are two types of filter which should be in every digital photographer’s kit bag: polarisers and graduates.
   

What does a polariser do? The physics behind polarising filters is quite complex, but they have two main effects. The first is the ability to darken blue skies. This makes outdoor shots look much more vivid and saturated, and can help keep the tones in the image within the sensor’s dynamic range. Many outdoor shots taken under blue skies are spoilt when the skies come out a blank white or an unhealthy-looking cyan colour. A polarising filter will often darken the sky enough to prevent this from happening.

The second effect of a polarising filter is to subdue reflections. Polarisers are often used for studio photography to cut down the glare from objects with shiny surfaces. This also increases the colour saturation to produce much more vivid-looking pictures. This has an effect out of doors, too. The darkening of blue skies is the most obvious improvement, but you’ll also see a general increase in colour saturation.

How do you use them? Polarising filters must be rotated very carefully to find the angle at which their effect is at its strongest. The angle of rotation will depend on the direction of the light relative to your subject and the position of the sun in the sky. You will notice that the image in the viewfinder is much darker. This is because polarising filters do block quite a lot of the light. In fact, they usually need an exposure increase of around 3-4 stops.

This Cokin circular polariser fits into a special, narrower slot at the back of the filter holder. It can be rotated within the holder to produce the required polarisation effect thanks to the knurled outer rim. This means the filter holder itself stays in the same position and that the polariser can be used at the same time as graduated filters, if necessary.

What’s the difference between linear and circular polarisers? Linear polarisers are optically simpler and cheaper to buy, but they clash with the complex mirror arrangements used with modern metering and autofocus systems. Circular polarisers get round this, but they do cost more. If you’re using an old film camera with manual focus and a simple metering system, a linear polariser is fine. If you’re using a modern digital SLR, you need a circular polariser.

What does a grad do? Graduated filters are clear at the bottom and darkened at the top. They may be a neutral colour (‘grey’, or ‘neutral density (ND)’ grads) or a yellow, orange, red, blue or other colour (for example a yellow/red ‘sunset’ filter). The clear bottom half and darkened top half are ‘blended’ together to produce a smooth, gradual transition (hence the name).

Graduated filters are used mostly for reducing the brightness of skies in outdoor shots. Outdoors, photographers often face a dilemma. If they set the exposure for the ground, the sky is overexposed; if they set the exposure for the sky, the ground comes out too dark. A graduated filter evens up the tonal difference so that both come out well. Grads are even more important with digital cameras than they were with film because digital cameras lose highlight detail much more readily. 

How do you use them? Graduated filters are rectangular and designed to be used with so-called ‘square’ filter holders. These typically have three filter ‘slots’, and this enables you to use up to three filters in combination. For example, if you have a 2x graduated filter (which reduces the brightness of the sky by one stop) and a 4x grad (which reduces it by two stops), you can use them in combination to produce a 6x grad.

With the Cokin square filter system, graduated filters slide up and down in the slots in the filter holder. This enables you to position them carefully to line up with the horizon in the photograph. You can also rotate the holder and hence the graduated effect if you need to, though it’s seldom useful because an angled darkening effect usually looks unnatural. The extra slots in the filter holder means you can use grads (and other filters) in combination if you need to.

Graduated filters do require special techniques. First, you need to slide them carefully up and down in the filter holder to place the ‘transition’ in exactly the right position - usually, just above the horizon. Second, you need to make sure the filter holder is horizontal because if the change in tone isn’t parallel with the horizon it will stick out like a sore thumb. Third, you need to measure the exposure before you fit the filter. (You can use auto-exposure with a grad fitted, but this can produce images where the foreground is too light - it’s safer to meter beforehand.) If you have time, switch to manual mode, take an exposure reading from the foreground, then fit the filter, reframe and shoot.

Graduated filters are just as useful in black and white as they are in colour. Here, a 6x grad has been used to turn a heavy overcast into a dramatic, stormy sky. The main thing is to keep the sky tones within the camera’s dynamic range. Without that, not even Photoshop can help you.

What strength do I need? At first, it’s easy to use too strong a grad. What looks right in the viewfinder and even on the LCD afterwards can prove to be overpowering when you look at it properly on the computer screen or in a print. Here’s a general guide:

1. 1.To control a pale blue sky or light overcast, use a 2x grad. Anything stronger will make the sky look ‘muddy’.
   

2. 2.For heavier overcasts, use a 4x grad. In these conditions, the sky is proportionally much brighter than the ground than it is on a sunny day.
   

3. 3.Only use a 6x grad or stronger if you want to produce a heavy, stormy effect on an overcast day or if you’re shooting into the light at sunset or dawn.
   

It’s easy to ‘overdo’ graduated filters. The unfiltered image (left) is a prime candidate for a grad because the sky is badly overexposed. The picture was re-taken using a 2x grad (centre) which was enough to bring the sky back into the sensor’s brightness range very effectively. But going for a 6x grad (right) was a mistake. The sky has a deep, intense tone but it looks unrealistically dark compared to the bright, sunny foreground.

Should I get circular or square filters? Many filters are available in traditional circular mounts which screw directly on to the filter thread on your lens. These are more compact and easier to handle than square filters, but there are some serious limitations. First, they will only fit a specific filter thread size. If you have more than one lens (and hence more than one filter size) you’ll need to buy more filters. Second, you can’t get circular grads. Graduated filters need to be moved up and down, and you can’t do that with a circular mount.

Square filter systems such as Cokin’s are bulkier but much more versatile. They attach to the lens using an inexpensive adaptor ring, so that if you have more than one lens you still need only one filter holder - you just need to buy an extra adaptor ring. What’s more, any filter type available in a circular mount can also be bought as a square filter, and this even includes traditional black and white filters.

Copyright © Rod Lawton 2008