Whether you’re a long-time expert in the field of photography, or fairly new to picking up a camera, you've likely heard of the importance of using filters at some stage in the process. Since the advent of digital photography, it's become easier to use digital editing processes in correcting your shots, but it hasn't removed the benefits of physical filters. Physical filters attach to the lens of the camera, and immediately affect the picture you take, while digital filters are added in post-production as you tweak your image in Lightroom or Photoshop. Each type has its own set of benefits and drawbacks, but which one is better overall?
To start off, it’s worth mentioning that there are several types of each, and both physical filters and digital editing can do things the other can't do. Within the physical category, the most common types are polarizers (which improve saturation and reduce glare on reflective surfaces), neutral density and graduated neutral density filters (which extend the length of exposure, and control and reduce strong light), and UV filters (which protect the lens and improve picture clarity, exclusively for film). Of course, some of these benefits can be replicated using digital filters, but which one is better? The answer depends on your preference, and your goal.
A huge benefit that physical polarizers have that digital filters simply can't match is the reduction of reflections. By restricting the direction of light hitting your lens (sort of like louvers on a window) the physical polarizer can enable you to see to the bottom of the river without any surface glare, or see the details in clouds without haze caused by moisture in the air, or shoot through windows while eliminating glare from lights. A circular polarizer can rotate, letting the photographer choose what level of reduction and from what angle.
Graduated ND Filters
Graduated neutral density (GND) filters also reduce the glare on particularly bright parts of an image, allowing parts that would normally be overexposed to show more detail. The physical filters are more transparent on the bottom and slightly darker on the top, as the sky is normally the source of glare. This is why this filter is typically used for landscape photos, because it allows you to put the transition between the lighter and darker sections (which can be either gradient or a more abrupt switch) right on the horizon, and create a more seamless transition as a whole. If you’ve seen a landscape picture where the ground and the sky have equally exposed tones, there’s a good chance a graduated ND filter was used.
Physical graduated ND filters can be bought in various "stops", or intensities, referring to how large the contrast between top opacity and bottom clarity is. For example, a one-stop filter darkens the top by a factor of two, or twice as much as the bottom. A two-stop filter darkens it by four times, and so on. This allows the sky and objects in it, such as clouds, to have more structure and contrast. Using a digital filter might have a similar effect, and depending on how well you know Photoshop, you will be able to lighten and darken various parts of the image that are more isolated.
However, you’ve no doubt heard that using a physical filter out in the field is best, and there is some truth to this. While putting anything — even a filter — in front of your lens does reduce the image quality to some effect, image quality stays closer to the desired effect when using a physical filter. You can probably achieve the same results with a digital filter, but chances are, it will take quite a bit more time and effort. And when it comes to overblown images, post-processing simply can’t reduce glare the same way a good old-fashioned graduated ND filter can. And of course, we’ve all seen the painfully grainy over-edited images that can come from too much post-editing.
If you're not a hardcore retro film & filter sort of photographer, your best bet is likely a balance between a physical filter and digital editing. There’s no better way to improve your photography than by practice, and physical filters achieve this more immediately, forcing you to think about the composition of your shot, lighting angles, and so on. However, if you’re more adept at post-editing software like Adobe Lightroom or Photoshop, you can achieve structure, saturation, and many more things on a much more specific and personalized level. Of course, the choice is up to you and your photography needs.