When it comes to photography, and in particular digital photography, filters can be roughly split into two groups. It’s tough to name these groups but I’m going to call them ‘pre-capture’ and ‘post-capture’.
Precapture filters are physical filters that you pop into or onto your lens or camera. These range from simple UV filters to coloured filters to special effect filters.
Postcapture filters are the digital jiggery-pokery that either you or your camera does to make your image sepia, black and white, or watercolor – or something more complex like gradients, lightening and darkening etc.
Here, I am more concerned with precapture filters, so from now on, that’s what I’m talking about.
Types of Filters
UV Filters
The simplest filter you can get is a UV filter. This is a piece of optical glass that cuts down the amount of UV coming in through the lens. UV is partially responsible for haze in your image, but the primary use for these filters today is to protect your lens. There is a raging debate as to whether you should have a UV filter for protection or not, so I’m not going to tell you whether you should or not. Suffice it to say, the optical glass used in modern lenses already cuts out most UV, so it is purely for protection that you would install a UV filter. Cheap filters will degrade your image quality – making your lens more prone to flare and reducing clarity – and even UV filters can be quite pricey depending on the size you require.
Tinted and Gradient Filters
The next group of filters are tinted filters of one sort or another. This may be a warming filter to give your image a warm feeling, or a gradient filter that darkens just one half of your image (to retain sky detail for example). You can get gradient filters that are orange at the top and yellow at the bottom for dramatic sunsets, or strong red filters that make your skys turn black for incredible black and white photos. In the digital age, many of these filters have become obsolete because all the effects are obtainable in the computer after the fact.
The one filter amongst these that is probably still most useful is the gray gradient. Without getting into photographic terminology, the sky is a lot brighter than the ground. About 60 times brighter much of the time, but even up to 250 times brighter in the right conditions. The sensor in your digital camera only has a dynamic range of about 300:1 – and many digital cameras have lower range (your eyes are good up to about 1 million:1 in the right conditions). This means that when taking a photograph with the sky as an important feature, either the sky appears very bright, or the ground very dark. Now there are various post-processing tricks you can pull to overcome this, but they all lose image quality. The use oif a gray gradient filter will darken your sky and bring it closer to the brightness of the ground, allowing your camera to capture detail in both.
Polarising Filters
All the effects caused by a tinted filter can be achieved on a computer in post production. There may be a certain degradation in image quality, but the effect is achievable.
One filter that cannot be reproduced in the computer is the polarising filter. It is difficult even to approximate the effect of this filter digitally. It is probably the easiest filter to use effectively. Unfortunately it is one of the most expensive filters.
Light waves have an orientation, and as they bounce off surfaces their orientation is altered. The polariser allows only light of a particular orientation through. You can turn the polariser as it sets on your camera and alter which orientation is let through. The effect is wonderful. Clouds reflect light in all orientations, so no matter how you set the polariser, they are always white. But the blue in the sky is heavily biased in a particular orientation. By turning the polariser to block that light out you can get deep blue skys but still have bright white fluffy clouds.
Another use for the polariser is for increasing colour saturation the natural way. Foliage, buildings, skin, clothing – these all produce a certain amount of glare. If you look at a leaf, you can see a soft shine on it. This shine is made up entirely of polarised light. By setting the polariser to cut this out you see the natural colour of your subject - foliage gets greener, skin looks healthier, and clothes get more vibrant.
Finally, reflections off the surface of water is highly polarised. Especially reflections at about a 30 degree angle. Using a polariser allows you to either cut these out and see clearly into the water. Or you can turn the polariser the other way to maximize the reflections, allowing your to capture images with great reflections of whatever might be on top of the water.
As you turn the polariser on your camera you can see the effect it is having through the lcd on your camera or through the viewfinder. So you always have control and know what you are going to get.
Special Effect Filters
Finally, there are oodles of special effect filters for things like soft focus, starburst effects, segmented vision etc. In a digital age, these only limit what you can do - all these effects can be reproduced much more controllably on a computer.
Other Filters
Okay, so there are couple of specialised types of filters I have left off, but these are really more like lens augmentations and are best left for another discussion.
