Lens filters modify the way light enters the camera towards the sensor. Even when current applications allow for (almost) any desirable edition, image pre-processing may be the optimal solution under some specific circumstances. In this article, find out how to use lens filters.
Transmission of colour Coloured filters allow for the transmission of its own colour, and avoid the transference of the inverse one. For example, yellow and orange filters will keep sand and skin almost unchanged, but blue skies will be strongly darkened (blue light blocked). This kind of filters is used mostly in black and white photography, since obtained colours are usually not realistic. Clouds, in the given example, introduce a dramatic contrast with the (darkened) sky. Grey filters A special case is grey filters, either uniform or degraded. Those are used only to reduce light intensity, and colour is maintained. This way, exposition can be made longer (waves and waterfall pictures) or rather, illumination above the horizon (sky) can be made even with the darker landscapes. Other colours (blue, green, etc) will have specific uses, in a similar way as warm ones.
Special case: UV / IR
Special cases of coloured filters are UV (ultraviolet) and IR (infra-red).
UV filters block the ultraviolet light so that only effective light intensity is used. Blueish or violet halos are eliminated in high-contrast images. The effect may not be visible in standard, amateur cameras. However, many people uses those filters as protection for the objective. IR filters On the contrary, IR filters will block all light except the infra-red one. This way, photos are produced by the heat information in objects. Typical IR pictures have a red tint, and living objects (plants, people) come out brighter that static ones (ground, walls, buildings).
Finally, a third family of filters is the polarised ones. In many cases, those include the glass piece in a rotating ring so that the effect can be maximised or removed, without disassembling the filter. Circular polariser Circular polariser filters only allow for the transmission of light in one plane. Basically, sunlight is polarised and it is reflected in the same manner by all objects except for metallic ones, which produce a diffuse reflection in all planes. When the filter is placed parallel to the light plane, all light is transmitted. Then, glares and gloss are fully visible. On the contrary, if the filter is placed perpendicular to the light plane, glare is eliminated so that underwater rocks or fish, for example, can be visible. Window or glass reflections can also be removed.