Despite the advances of post processing thanks to Photoshop, filters are anything but redundant. So, what do they do and which lenses might a digital photographer need in his or her kit?
UV Filters
This is probably the most commonly used and useful digital SLR camera filters to purchase. It will filter out excess ultra violet light, whilst having no effect on exposure. Ultraviolet light can be a real problem for anybody taking landscape photographs as it can manifest as a haze that diminishes contrast from an image and becomes noticeable over a long distance.
However, given that this is much more of a problem with film rather than digital SLR cameras, because image sensors in digital SLR cameras are less sensitive UV light, why do you still need a UV filter? To protect your lens. A UV filter, unlike many other lenses, has no effect on exposure required or on the colours and will protect your expensive lens from knocks and scratches.
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Neutral Density filters
ND filters cut down the amount of colours and light entering the lens in equal measures so allowing you more creative freedom choice of aperture and shutter speed.
Therefore you can select a slower shutter speed (i.e. longer exposure) than might be achievable in bright conditions, for example, if you wanted to achieve a blurred waterfall effect on a sunny day without over exposing the image. Similarly one can be used when photographing action shots with blur to convey speed. Neutral density filters can also be employed in situations where one part of the scene is darker or brighter than others.
Polarizing Filters
Landscape photographers frequently use polarizing filters to diminish light reflected from outdoor surfaces like glass and water, for example if you want to capture images of water which doesn't lose its colour to the reflected sunlight on its surface. They can also saturate and darken colours, which make them useful in black and white as well as colour photography. A polarizing filter can intensify a field of oil seed rape, the green of a meadow or add richness to a blue sky. Similarly, in architectural photography, a polarizing filter helps with shots of modern glass buildings.
Graduated Colour Filters
To a certain extent, use of graduated colour filters can be avoided with use of Photoshop. However, if you want to cut out the post processing, or if you're not au fait with Photoshop, one of these can be very helpful in emphasizing the colour and tone of parts of a photograph. One part of the filter has a certain colour, which either gradually or abruptly changes to clear elsewhere on the filter. Commonly used graduated colour filters include orange to clear, for enhancing the appearance of sunsets, and blue to clear, which can enhance either water or sky depending on which way round it is used.
Contrast Filters
Because it can be difficult balance scenes where the contrast between light and dark is severe, there are two types of contrast filters available. A low contrast filters makes highlighted areas brighten up darker shadows around, while a soft contrast filter darkens the highlights. Both of these things can be difficult to pull off convincingly in Photoshop.
IR (Infrared) Filter
This type of filter blocks visible light and allows only infrared light to pass through to the camera lens. Skies darken and greens can become almost white.
Star filters
These have a thin lines etched onto a clear surface, creating therefore points of light that streak outwards from a central source, creating drama and visual interest.
Watch out for a few things. Light reflected from the inside of your filter can create flare. Stacking filters can create black shadows around the image, known as vignetting. You will lose clarity if filters are scratched or dirty.
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