Clear and ultraviolet
Clear filters, also known as window glass filters or optical flats, are completely transparent, and (ideally) perform no filtering of incoming light at all. The only use of a clear filter is to protect the front of a
UV filters are used to reduce haziness created by ultraviolet light. A UV filter is mostly transparent to visible light, and can be left on the lens for nearly all shots. UV filters are often used for lens protection, much like clear filters. A strong UV filter, such as a Haze-2A or UV17, cuts off some visible light in the violet part of the spectrum, and so has a pale yellow color; these strong filters are more effective at cutting haze, and can reduce
purple fringing in
digital cameras. Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film.
While in certain cases (such as harsh environments) a protection filter may be necessary, there are also downsides to this practice. Arguments for and against use of protection filters include:
If the lens is dropped, the filter may well suffer scratches or breakage instead of the front lens element.
One can clean the filter frequently without having to worry about damaging the lens coatings; a filter scratched by cleaning is much less expensive to replace than a lens.
Adding another element degrades image quality due to
aberration and flare.
It may reduce the use of
lens hoods, since threading a lens hood on top of the clear filter might cause
vignetting on some lenses, and since not all clear filters would even have threads allowing a hood to be attached.
Additionally, users of UV filters must be careful about the quality of such filters. There is a wide variance in the performance of these filters with respect to their ability to block UV light. Also in lower quality filters, problems with
autofocus and image degradation have been noted.
Shop for Ultraviolet Filters at these stores
In their most successful applications, filter effects blend
in with the rest of the image to help get the message
across. Use caution when using a filter in a way that draws
attention to itself as an effect. Combined with all the
other elements of image-making, filters make visual
statements, manipulate emotions and thought, and make
believable what otherwise would not be. They get the viewer
UV filters are supposed to block UV light. So, for the newcomers to photography
let's first look at what UV light is and why you would want to block it.
The "traditional" visible spectrum runs from red to violet. Red light has the
longest wavelength and violet the shortest. Light which has a longer wavelength
than red is called infrared, and light which has a shorter wavelength than
violet is called ultra violet or UV. The wavelength of light is measure in units
of nanometers (abbreviated as nm), and 1nm is a billionth of a meter (that's a
US billion or 1000 million, not a UK billion which is a million million!). Light
shorter in wavelength than about 400nm is called ultra violet, light longer in wavelength than 700nm is called infrared.
So now we know what UV light is, why would be want to block it? Well the answer lies in the way that color film works. There are basically three color sensitive layers, one sensitive to red light, one to green light and one to blue light. The blue layer not only responds to blue light, but also to UV light, so if there is a lot of UV around the blue sensitive layer gets extra exposure and the final image takes on a blue color. Since film isn't normally sensitive to infrared, you don't need an infrared blocking filter. Interestingly though, digital sensors are infrared sensitive and most digital cameras have an infrared blocking filter built in.
Now there isn't usually a huge amount of UV around at sea level. There is some (that's what gives you a suntan or a sunburn) but most of it is scattered by the atmosphere. However as you gain altitude, for example by going up a mountain, the amount of UV increases. Under these conditions a UV filter can prevent a blue cast in photographs.
Since UV filters look clear and neutral to the naked eye, some people also use them as a protective filter which they leave on their lens at all times. Some people think this is a good idea, other question the wisdom placing a $20 filter in front of a $1000 lens and potentially affecting image quality. Both schools of thought have some valid points. It's your choice.
So if you buy a UV filter, you'd expect it to block UV right? Well, sometimes you'd be wrong as the results of this test show. I've looked at the range between 350nm and 400nm for UV blocking since the glass used in almost all lenses will itself block any light with a wavelength shorter than 350nm, so you don't need help from a filter there.