Professional photographers use a variety of filters to capture the perfect shot. The main types of filters used are UV filters, polarizing filters, and ND (neutral density) filters. Neutral density filters reduce light in varying amounts depending on the intensity of the filter. ND filter intensity ranges from ND2 to ND1000 and beyond.
These filters are mainly used for portrait and landscape work, with the strongest ND filters used in landscape photography. Graduated Neutral Density (GND) filters are practically only used by landscape photographers. They darken the sky during sunrise and sunset to balance the exposure of the sky relative to the foreground. While it is possible to replicate the effects of a GND filter through HDR techniques, some photographers prefer to create their photos from a single image and often use GND filters in the field.
A GND filter allows you to capture the shot in one go, which can be very satisfying and makes post-processing easier. Graduated color filters work similarly to GND filters, but instead of reducing light, they add color. In general, these filters are used to improve color in the sky. A popular filter for infrared photography is the Hoya R72 infrared filter.
Transparent filters are also used by professional photographers in their lenses. Polarizers and neutral density filters are often part of an additional package from the camera store when you buy your camera or lens. This is an old technology that has long been obsolete, but remains alive due to its ability to increase retailers' profit margins. UV filters were originally used with film, as the blue layer was quite sensitive to ultraviolet light.
However, even before digital cameras, most films had overcome this problem to a large extent. Digital cameras have eliminated it completely by using internal filters that block ultraviolet and infrared light. UV filters are completely useless when it comes to image quality and can cause or increase lens flare in some cases due to an additional piece of glass in front of the camera for light to bounce off before being captured with the sensor. Additionally, less expensive UV filters can decrease sharpness and contrast.
The reason most people use a UV filter is to protect their lenses, but even this seems to be less useful than previously thought as a lens with a UV filter may not be more protected than one without it. Circular polarizers are also used by photographers and work similarly to polarized sunglasses. The lens filter type consists of two pieces of glass connected together and rotated independently to adjust the amount of polarization. Circular polarizers can be used for several things that can help your photo, such as effects that are difficult or impossible to recreate through software editing.
Square pieces of glass that some landscape photographers use are usually graduated ND and ND filters, but there are also square polarizers (although it may sound confusing since it is technically a square circular polarizer). A great starter combo is a circular polarizer and a 6-point ND filter; it can give you those great landscape effects and still probably give you the benefits that a portrait photographer needs from an ND filter. If you slide a GND filter in front of the lens, you can darken the sky while keeping the foreground well exposed without having to take multiple exposures and merge them in post-processing. On the other hand, GND filters are expensive, and you'll often want to have at least a few (with different amounts of gradient, as well as some hard-edged and some soft-edged options).If you mount this filter holder to a polarizing filter, you could end up with vignetting even at 35 mm or more.
Even with a filter, you'll need a camera that can work well with this filter attached, and some cameras are better than others.