It's time for another photography tutorial.
Desaturating a photo
Desaturating a photo is not a big deal. Just click the correct option in the image manipulation software of choice. There are a few points to be remembered though.
Usually you can get rather good results just by desaturating the picture. Often you need to adjust levels afterwords though, since color contrast is lost, increasing the contrast in luminosity is usually a good idea. Otherwise the image may seem a bit bland.
Then there's an issue that's very often neglected much because it's not that relevant. Spectral sensitivity of human eye is not linear. It decreases towards both infrared and ultraviolet range, leaving the eye at its most sensitive to yellowish green light. Black and white films are mostly panchromatic so the luminosity reproduction is rather linear through the spectral range. In this it's very similar to what simply desaturating a color photo produces. Photographers used to use green filters with black and white films to compensate the spectral sensitivity of the human eye. As other than green colors were slightly filtered, the relative luminosity of the shades in the picture resembled more to what a human eye would see in color.
Don't run to your nearest camera store just yet, to buy filters. Digital age has made actually pretty much all filters useless. You can do the same in for example Photoshop. Here's slightly exaggerated example of what you can do if you want to make the grayscale conversion that emulates human eyes sensitivity. Generally this is not very important but can be useful if the desaturated image looks odd.
We'll start with a color photo. In this case the photo is quite saturated and has a high contrast already. So as seen in the plain desaturated picture the result is actually quite pleasing.
Here's what the picture looks like after color correction. As said, this is slightly exaggerated to illustrate the effect a bit better.
I made the color corrections using curves-tool. Green channel is unchanged but the maximum luminosity of both blue and red channels is reduced.
Now here are just plain desaturated and the color adjusted photo. Adjusted on the left. (or bottom if they don't fit your browser window side by side) Now when you adjust the levels, green parts of the picture will stand out more.
As said, this is very rarely needed. However if you have a part or a detail of some certain color in the picture that you want to bring forth you can do the same to the color in question. So you can use this method for much more than getting all anal about emulating the spectral sensitivity of the human eye.
Now lets get really really oldskool. This is just a trick which is somewhat related to my last tutorial. Old, I mean really old, films were practically only sensitive to blue light. These films are nowadays pretty much extinct as panchromatic films have been available since the 1930s. Still you may sometimes want to emulate the spectral qualities of these films. You can use the same method.
The color version again as a reminder.
Here's the color adjusted version in color.
Curves for the adjustment. You could actually remove red and green channels completely but it wouldn't be completely realistic since the old films were actually slightly sensitive to a wider spectral range. Luminosity curve wasn't linear either as seen in the blue channel's curve.
Here's the comparison again with the plain desaturated photo and the oldskool museum stuff. I made the appropriate level corrections to this one already to illustrate what the photo would actually somewhat look like taken with 1920s film.
A few words about filters
In digital photography colored filters are pretty much useless. Cons supersede the pros practically always, as the color adjustments can just as easily be made by software. UV-filters have indisputable benefits as they can increase the contrast of the picture. This is because digital sensors are slightly sensitive to ultraviolet light but can't process it correctly.
However, there's a pitfall of coating. Uncoated filters are extremely susceptible to flaring and reflections which on the other hand reduce the contrast of the photo. Single coated filters are a bit better but because of the physical properties of the coating, the coating is effective to a very limited wavelength range. Optical properties of triple coated filters are very good but they tend to be really pricey. Generally it's better not to use a filter than to use a bad one. I'd personally advice to stay away from all others than triple coated ones.
There's one very useful filter though, a polarizing filter. It's the kind of a case where pros supersede the cons despite of the coating. Polarizing filters are used to remove diffused light. In general they increase the contrast a bit. As a downside they do shade the lens about 1/3 stop. Removing diffused light becomes really handy when you're photographing for example through windows or taking pictures of water in strong lighting, as polarizing filter reduces and quite often completely removes reflections in these air to glass or water interfaces. But as using it affects the draw I'd recommend using it only when you really need it, just as any other filter.
There are also a bunch of special effect filters, like IR-filters, fishnets and cross screens. These are basically playthings but the same effects are quite difficult or even completely impossible to emulate in software so in special cases they might be useful.
Overall I'd avoid using filters, at least if you're not as rich as Croesus and you'd have to settle for single coated or uncoated ones. The ones to buy if you decide to get them are UV-filter and polarizing filter. Out of normal filters there's not much use for other ones.