For anyone with an smartphone who’s young enough to not have grown up with film cameras, a “filter” means an effect that you apply in Instagram or Photoshop. It could be as simple as a “blur” filter, or as complex as a “vintage look” that applies a series of complex algorithms to an image, changing color, texture, exposure values, even adding borders and more.

But of course, that isn’t what a filter started as! Before digital, a filter was simply a piece of glass that you put on your camera lens (and it still is, of course) to create an effect. At its simplest, a filter could be a UV or Skylight filter which had minimal affect on the image and is mainly used as protection between your expensive lens and the elements.

Using common analog filters

For B&W photography, color filters are quite common; whatever color filter you put on, those colors become lighter/brighter in the scene and their opposites become darker. So a yellow or orange filter will brighten skin tones, and a red filter will brighten red tones, while making blue skies almost black.

Other common filters are polarizers, which minimize reflection and can remove glare from the surface of water, a car’s windshield, or even the glare off of a wet leave. 
Neutral Density filters reduce the amount of light coming through the lens, allowing for longer exposures so you can create things like blurry waterfall photos.

The neutral density filter allowed for a long, multi-second exposure

Applying digital filters

Now that we are mostly shooting digital, you can of course still use analog filters. In some cases it’s better to use analog (you can’t magically see through the surface of a lake using a computer filter to replace a polarizing filter, for example), and in some it’s better to use digital (a red glass filter for B&W has one, permanent setting — but doing it digitally give you infinite options with much more control).

Software like DxO FilmPack gives you incredible control over B&W conversions, allowing you to channel mix individual colors as black and white to brighten or darken specific tones.

Here’s a before and after of an image converted to black and white using the Kodak T-Max 100 film preset with no additional affects applied, compared to the same image with the yellow and green channels raised (to brighten foliage), and the blue channel reduced (to add contrast to the clouds).

But again, filters can be entirely for creative purposes, too. And those can be really fun! In this sense, the term “filter” can be used quite loosely. Some apps may call what’s more a preset of a series of settings a filter, while others may use the term for a single, adjustable effect. No matter how it’s used, the idea is that you have a quick and easy way to apply a unique look to your photos. 

In this screenshot of the Presets page in DxO PhotoLab, you see a lot of very different “looks”. These aren’t technically filters, but actual presets of settings that combine together into an easy to apply, well, filter!

In DxO FilmPack, the presets once again are actually a complex combination of settings, including film grain and tone settings, resulting in unique looks that are so easy to apply, you may as well call them a filter, too!
No matter what you call it, or how you use it, the “filter” has evolved in a spectacular way. You can use them for creative or corrective work, whether you’re talking about putting glass in front of your lens, or applying a slider to your digital photo. However you look at it, you’re modifying the image, and that’s something is just about as old as photography itself!
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