SUN FILTERS AND BLADES.
The task of photographic filters is to reproduce the color scale of nature on the film emulsion as faithfully as possible in a black and white image.
This emulsion - even the commonly used panchromatic emulsion, that is, all-color-sensitive — it renders some colors of nature much brighter or darker than our eye sees them.
To prevent this, we use filters put on the lens, which fully transmit colors corresponding to the color of the filter (or related), reducing the effects of rays of other colors (especially complementary). This results in some loss of the amount of light passing through the lens (hence the need to extend the exposure time using filters).
In universal practice, only certain types of filters are used, especially shades of yellow and yellow-green and orange; filters with a different color are for special purposes.
However, one cannot speak with complete accuracy about "double" or "three-fold" filters, because the extension of the exposure time depends not only on the density of the filter, but also on the type and quality of the film emulsion, from sunlight (np. in the mountains or by the sea), time of day (the color of sunlight), weather (clouds) itd.
In practice, it's enough, that by using a given filter and a panchromatic film, we increase the exposure time, what would be needed without using the filter, two- three times etc.
Admittedly, the main task of the filters is to correct the incorrect rendering of the colors of nature by the film emulsion, however, there may be cases, where the filter is used, to deliberately enhance a certain color of nature or to subdue another. If we take photos, np. green object on a red background and known, that if these colors are reproduced faithfully in black and white, both objects will come out in the same tone, you have to choose the filter accordingly, to make one of these colors in the photo much brighter, the other much darker.
In the first case, we will talk about the correction filter, in the second about the contrast filter. To highlight the difference in how they work, one can say very colloquially, that the correction filter is used to increase the fidelity of rendering in the black and white scale of individual colors yes, as our eye sees them. contrasting filter, kind of "fakes” this color ratio, highlighting these, that we care about, and suppressing others.