To keep it simple, accept that the 100-300nm part will be totally blocked from space to the altitude of about 7km. Some of the 315nm may reach the lower altitudes and even the surface and longer ones will reach it, depending on the latitude, season, solar activity, pollution and other factors.
|Schneider B+W 010 UV Haze transmission curve|
(C) Schneider Kreuznach
Both filters are extremely useful for film photography, specially for mountain/snow/beach areas.
FILTERS FOR B&W PHOTOGRAPHY
Black and White FILM photography can be tricky if we're talking about contrast and tones.
It's easy to imagine a scene in plain colors, but not so easy to have a precise idea about the final look on black and white film.
B&W photography is all about contrast and tones and to make things worse, there are color tones that will be rendered exactly the same way on film, even being quite different colors.
There are four main filter types for B&W photography:
There are other ones, like blue, lime, but let's talk about the four main ones for now.
Take a look at the spectrum image above and let's talk about the work horse of the b&w filters, the yellow one.
|Transmission curves for filters (C) Hoya|
Y2 = Yellow
YA3 = Orange
R1 = Red
X1 = Green
X0 = Lime
Filters like the yellow, orange and red are lowpass filters. They attenuates higher frequencies and let pass the lower ones with sight or no attenuation.
Take a look at the figure and let's talk about the transmission curve of a yellow (Y2) filter. The cut-off frequency of this filter is about 475nm. It will block all wavelengths below, let's say, 425nm. But the blue is at something like 470nm and will be just partially blocked. Looking at the graph I would guess that the blue light will be reduced by something like 10-15%.
This slight reduction on blue light can result in a slight contrast enhancement between a cloud and the blue sky. The sky will look a bit darker than the clouds.
The orange filter transmission curve "knee" is shifted to a longer wavelength, and will reduce dramatically violet, blue and some green.
And the red filter will cut from green to violet.
A yellow filter makes colors with shorter wavelengths than yellow to appear darker than the longer wavelengths and all the longer wavelengths will have the same look as without the filter. The orange and the red ones follow the same rule .
Green filters have a different transmission curves. The Green (X1) is a band-pass filter, with the central point at something like 540 nanometers and it will attenuate any wavelengths in both directions but in a not so steep way.
Green filters makes green objects lighter (or brighter if you prefer) than objects of other colors. They're also nice for portraits of people with white skin, giving a more natural tone.
The lime filter is something like a combination of a band pass and a low pass filter, and can be used as a general purpose filter for b&w photography, if you plan to photograph people and vegetation.
The BLUE filter does the opposite of what the Red does. It will lighten the colors at the blue side of the spectrum and darken the ones at the red side. Blue filters are high-pass or band-pass filters, depending on the manufacturing process.
Important to mention is the fact all the above mentioned filters blocks 100% of the UV light.
All filters blocks some light and this quantity is called FILTER FACTOR. It's expressed in how many times the light will be attenuated. It's simple, if you're using a red filter it will reduce de light to 1/8 or -3 stops. Remember to compensate the exposure if you're not using TTL metering or if the filter is not covering the light sensor of your camera.
|B&W Filter effect table|
The skylight filter is like the Y1 filter, but for color photography.
Do NOT use physical Y/R/O/G filters on digital cameras with Bayer or X-Trans sensors, even if the camera is set to monochrome. This WILL cause interpolation errors during the demosaic process and may give very strange results. In this case you need to use software based filters.
But if you have a Sigma camera with the Foveon sensor and plan to shoot in b&w, you need the filters, like you were using b&w film. By the way, Foveon's pictures shot this way are amazing.
|Original image in color|
|Yellow filter, slight blue attenuation|
|Blue filter, severe yellow to red attenuation|
Another set with more green.
|Color, no filter|
|B&N no filter|