Even in an age of digital photo editing, there are still some lens filters that either can’t be reproduced digitally or are more effective than their digital equivalents. These include protective UV filters, polarizing filters, graduated and neutral density filters.
UV filters are used these days purely as a protection for the front element of a lens. The UV filtering effect is no longer as important with digital sensors as with film, so these filters are purely for peace of mind. It’s a lot cheaper to replace a broken or scratched filter than a lens.
Polarizing filters are different. These are used to deepen blue skies and reduce glare and reflections – which can help increase color saturation too. Bluer skies can be simulated digitally, but the glare reduction can’t – it’s an optical effect that requires the polarisation of light, and that’s something that only a filter can do.
Neutral density filters are important for a different reason. These cut down the amount of light passing through the lens, and they’re used by videographers to control exposure without changing the lens iris setting (and hence the depth of field) and by landscape photographers who want to use long exposures in bright light to blur the movement of water and clouds. A physical filter is the only way to reduce the light by the amount needed.
Graduated filters consist of a clear lower part, a darker top part and a graduated transition in between. They’re used by landscape photographers to tone down bright skies so that they’re not overexposed. Sometimes you can do this digitally instead, but only if the sky falls within the dynamic range of the sensor – with very bright skies it might not.
UV filters screw directly into the filter thread of the lens. They are very straightforward to fit, but different lenses have different filter threads, so you may have to buy a number of different sized filters for your lens collection.
You can get polarizing filters that screw straight into the lens filter thread, but these are much more expensive, so buying several sizes quickly becomes uneconomic.
Instead, most photographers choose square filter systems. These consist of a square filter holder with slots for two or three filters and a lens adaptor ring to fit your lens filter thread. You can get a single holder and filter collection that fits all your lenses via different-sized adaptors.
This isn’t just more economical, it’s also necessary if you’re going to use graduated filters, because you need to be able to rotate these relative to the camera and slide them up and down to align the filter transition with the skyline in your scenes.
Square filter systems allow you to use filters in combination, such as and ND filter and a graduated filter at the same time, and most also allow you to fit a polarizing filter too.
Traditional black and white filters, or ‘contrast filters’ are a different case. It sounds paradoxical to use color filters for black and white, but they work by blocking some colors and allowing others through, which changes how these colors translate into shades of gray.
For example, a red filter allows red light to pass through but blocks its complementary color. This makes blue skies so dark they are almost black. You can achieve the same effect by adjusting the mix of colours when converting a color image to black and white digitally, and with a lot more control. In fact, there’s little value in using regular black and white filters on a lens because they will simply affect the response of the red, green and blue photosites on the sensor in the same way as a digital adjustment later.
The only exception is where cameras have completely monochromatic sensors designed solely for black and white photography. A couple of models made by Leica and Phase One have monochromatic sensors, but these are rare exceptions. With these cameras, you will need to use lens filters; with regular cameras with color sensors, there’s little point.