This podcast is in Spanish but here is a summary of it!
In this digital age it is questionable whether if it is necessary to have any filters at all. Most of the traditional filters can be recreated in Photoshop or other editing software. The objective of digital photography during capture should be to gather as much usable information for postprocessing and alter the image the least possible. This way, all changes that you do to the image can be applied and reversed. Most filters can be replicated in software and you don really need to buy these (Nay), some others cannot so you may want to consider getting these (Yay), depending on what you photograph. Let's take a look at them:
Colored filters. (Nay) The first application of filters was to create contrast in B&W photography. When images are converted into black and white, how dark an area is depends on the density of the colors. In some cases, contrasting colors (e.g. Red and Green) will render on B&W film as the same shade of gray. In this case a colored filter can be used to create separation, a red filter for example would let the red wavelengths through easily than the green ones, reason why the red objects would come out lighter than the green ones. This is the same reasoning as yellow filters to darken the sky, etc. ALL of these effects can be replicated in software.
White balance filters. (Nay) When using daylight film indoors, the yellowish incandescent light would render everything with a yellow cast. Different correction filters were developed for different type of lighting. These are no longer necessary, since we have the powerful white balance functions.
Effect filters. (Nay) In the film days, one of the only ways to change the look of a photograph was by using filters. There are many filters to create effects, like soft focus, fog, sparkles, etc. Almost all of these can also be recreated with software.
Neutral Density filters. (Yay) Neutral density filters are sunglasses for your camera. Now, you would ask: Why do we need that if we have a diafragm and a shutter and we can control light by those means? Neutral density filters allow you to get the correct exposure even with strong lighting with otherwise impossible low f-stop or shutter speed settings. These may be required for creative purposes like maintaining a shallow depth of field or motion blur. Neutral density filters DO have a place in Digital photography.
Gradient filters. (Yay) Similar to the above but in a gradient, they look half transparent, half dark, in a gradient. When these are used to darken an area just for the look, that can be achieved with software too. But there may be situations where the scene we are trying to photograph has a very bright portion and a darker one. In some cases the difference can be so big that it will not be possible to capture with one single exposure the whole dynamic range of the image. In this case we can only resort to something like HDR photography -which can be tricky- or use a gradient filter. With a gradient filter you can darken the brightest area and bring everything to a manageable dynamic range.
Polarizer. (Yay) When light hits a reflecting surface gets "polarized", which means it gets in a certain direction. This happens only with "specular" reflections, the ones that we can see as shining. Polarizers are like louvers, they let in light that moves in a certain direction, so they can -within limits- block out the reflected light. Polarizers are also mounted on a free moving ring, so we can turn them until they have the most effect. Some effects from polarizers are being able to see past the surface of water and saturating color in dark skin or foliage. This CANNOT be replicated in software. Shining surfaces will be captured in digital as a lighter, or even white area that cannot be removed in post.
UV/Clear. (Yay) A good idea for protection purpose. However, we have to say that your $1,500 24-70mm probaly has a better coating than the $5 filter you bought, so watch for flare!
So these last four are probably the only filters you'll ever need.
