Typically, it is difficult to take properly exposed landscape photos because the sky is much brighter than the land so a camera’s exposure meter averages the exposure. In the resulting photo, the sky is usually blown out with no cloud detail and the sky isn’t deep blue. A polarizing filter will help darken the sky but what is often needed is a graduated filter to get the proper exposure.
Before I get into how to use the Cokin filter system, I first need to describe the difference between a typical screw-on filter and the Cokin filter system.
Cokin makes filter systems for digital and film SLR cameras. Unlike screw on filters, Cokin’s filters are mostly square or rectangular. They can be used with many different size lenses. The main advantage of the Cokin system is that you should only have to buy one set of filters to use with all your lenses. The Cokin system uses a filter holder that can hold up to three square or rectangular filters at the same time. There is also a low profile holder for wide angle lenses. Adapters are available for various size lenses. Let’s say you have a 58mm lens and you buy a 52mm lens. With the Cokin system, you would only have to buy a 52mm adapter instead of having to buy a new set of filters.
Another advantage of the Cokin system has to do with their graduated filters. Circular, screw-on filters require that the horizon line is centered top to bottom in the frame which is usually an amateur’s choice because they aren’t thinking about composition. Most of the time, this isn’t where you will want the horizon line. Cokin graduated filters solve this problem. Cokin filters are rectangular and fit into a filter holder mounted to the front of your lens. The filters can slide up and down in the filter holder. This means that you can put the gradation anywhere you want. You aren’t stuck with a boring, typical composition.
The only downside of using Cokin graduated filters is that it can sometimes be difficult to determine where to put the bottom of the gradation. The bottom of the gradation fades out to nothing. The easiest thing to do is to use a piece of cardboard cut to the same size as the filter, slide the cardboard into the filter holder, and line it up with the horizon line while looking through the viewfinder. You can then approach your camera from the front and position a filter in front of the cardboard. It is much easier to see the gradation and ensure that it lined up properly when viewed from the front instead of viewing it through the viewfinder. After positioning the filter, remove the cardboard and take the photo. Having your camera mounted on a tripod will make this process easy and accurate.
I have heard that some photographers forgo the holder and just handhold the filter in front of the lens. Unless you have a really steady hand, this method won’t produce accurate results so I can’t recommend it.