Because the effects of many on DSLR camera filters can now be emulated in software. It’s no longer necessary to buy a stack of them. Warm-up filter, which add a sunny amber glow to make a picture more attractive, were always a necessary purchase for film users, and colored grad filters were hugely popular for adding a hint of blue (or any other color) to a bland sky.
The latest versions of Photoshop and Elements include a Photo Filter (found in the Image Adjustments menu) and this allows you to add and control just about any filter effect you’d want to apply. There are three filters, though, which can’t really be created in Photoshop, and these are the ones worth buying if you’re serious about your landscape shots
ND filter
With landscape shooting, there are times when you want to reap the creative rewards of a long exposure. To obtain this, set the lowest ISO value, then switch to Aperture Priority mode and set your f/number to the highest possible. This is usually f/22 or f/32 on DSLR camera, and f/8 or f/11 on compacts. Your shutter speed will drop to slowest it can go.
Trouble is, on bright days this still may not give you a very long exposure, so you have to make the scene darker. A Neutral Density (ND) filter places a neutrally-colored “pair of shades” over the lens to allow less light in. ND filters come in various strengths, and the best bet is to get a three-stop mode (a 0.9 or 8xND). This allows only one-eighth of the light to get through, and consequently gives you shutter speeds 8 times longer.
With this, you’ll be able to make water “flow”, or if the shutter speed is slow enough, make moving people disappear from the scene. You’ll need to use a tripod to hold the DSLR camera still.
Polarising filter
A polariser suppresses reflections in glass and water, darkens blue skies, and intensifies colors in a scene. Polarising filters come in two types; linear and circular. This doesn’t refer to the shape of the filter, but in the way the pattern is etched onto its surface. Linear polarisers tend to be cheaper than their circular brethren, but they can confuse the AF and metering system in DSLR camera, so avoid them. After you’ve fitted a polarizer to your lens, you apply the effect by rotating the bezel on the filter. Maximum polarisation occurs when you shoot at 90 degrees to the sun’s position, so you’ll see the effect deepen and then back off when you rotate the filter. At the maximum point, a polarizer will steal around three stops of light from your DSLR camera, so unless conditions are really bright, that means you’ll need a tripod to avoid DSLR camera shake.
Neutral Density graduated filter
An “ND grad” filter is a sheet of glass or optical resin with a dark half like an ND filter and a clear half. The idea is to position the filter over the lens so the gradual divide runs roughly along the horizon. This results in a darkening of the sky, which helps balance exposure between sky and land. Depending on conditions, skies tend to be 2-3 times brighter than land, and although our eyes don’t see this, a DSLR camera sensor does. If you expose correctly for the sky, you’ll get an underexposed foreground, and if you expose for the land, you’ll be burn out the sky.
An ND grad filter will balance the two, and they come in different strengths for different conditions. They have numbers of 0.3 (1x), 0.6 (or 4x), and 0.9 (or 8x) corresponding to the amount of light held back in the dark half. Most useful are the 0.6 or 0.9, which hold back 2 or 3 stops or light.
They’re really easy to use – just position the filter and shoot as normal, then check your LCD to see if any highlights are burning out to white. If the burn-out is too prominent, dial in some negative exposure compensation (the +/- button), and shoot again, and continue until there are only a few small areas of peak white in the display. ND grads filter should be bigger than the front element of your largest lens.