There are two filters that are essential to landscape photography. A Circular Polarising Filter (CPL) and a Graduated Neutral Density filter (GND). Why do we call these filters essential? Well simply because the effect of a polarising filter cannot be replicated in post-processing. On the other hand, a Graduated Neutral Density filter helps give your photograph the extra oomph. Let us discuss these filters and the need for them in more detail.
Circular Polarising Filter (CPL)
A polarising filter reduces reflected light. It works by filtering out sunlight which has been directly reflected toward the camera at specific angles. By absorbing reflected light, a polarizer improves colour saturation and contrast. While software post-processing can simulate many other types of filter, a photograph does not record the light polarisation, so the effects of controlling polarisation at the time of exposure cannot be replicated in software. Therefore, if you have water, sky, vegetation or metallic surfaces in your photo frame then a CPL will stand you in good stead.
There are two types of polarising filter: linear and circular. These terms do not refer to the shape of the filter, but rather the way in which the filter modifies the light waves that pass through it. The circular polarising filter, as opposed to a linear polarizer, is designed in such a way that the camera’s metring and autofocus systems can still function. So if you are using a digital camera it makes sense to buy a circular polarizer.
“ If you own multiple lenses, do not buy filters for each individual lens. Instead, pick a filter that fits the largest of your lenses so you can use the same filter for your smaller lenses as well by using Step-Up Rings. Step-up rings cost a tiny fraction of what a good filter does. This practice has saved us a lot of money and heartburn in the long run.”
Graduated Neutral Density Filter (GND / Grad)
A GND is a filter where one edge of the filter is Neutral Density aka Grey and the other edge is 100% clear edge. A GND is built to equalise the exposure in the bright and dark areas of a photograph. This is essential for scenes where you have the sky or the horizon as part of your image. Usually, the lit-up sky is orders of magnitude brighter than the ground below it. If you expose your image for the bright sky the ground below it turns into in a dark mush with poor detail. Conversely, if you expose your image for the dark ground below the sky turns white with little or no detail. The grey area in a GND helps equalise the dark and light parts of an image by reducing the amount of light in the bright part of the image.
A GND’s strength is the difference in light transmission between the grey edge of the filter and the clear edge. Each F stop of difference is referred to as 0.3. Thus, 2 F-Stops is 0.6 and 3 F-stops is 0.9. This means a 0.6 GND has a darker grey edge than a 0.3 GND and provides 2 stops of light difference between the grey edge and the clear edge.