Back in the pre-digital days, it was typical for photographers to carry an assortment of filters in their bags - especially if they were shooting in black and white. B&W filters in red, yellow, green and orange were used to enhance various colors (i.e. a red filter significantly darkens a blue sky and makes clouds pop), create better contrast by separating tones, and could also be used to highlight depth and texture.
Photographers who worked in color also carried filters. In addition to the polarizers, neutral density and split and/or graduated neutral density that black and white artists also used, they might have added color enhancing, color correcting, warming, and 80 series (tungsten).
When I started my photography journey, film was the only game in town, so my kit included filters, too. The one that got the most use was probably the enhancing filter. Like the 81B (which I used before I got the enhancer), it boosts the saturation of reds and cleans up earth tones: a great tool with which to capture spectacular New England autumnal scenes.
After transitioning to digital I removed most filters from my bag.
With the inherent capabilities of Lightroom, Photoshop, et.al., which can create many of the effects we used to depend on filters to achieve, are they relevant anymore?
Yes!
I wouldn't be without a few of them:
Circular Polarizer
Once upon a time, the polarizer was used mainly to create deep blue skies and enhance clouds. While that is still something the polarizer can do for you, this is quickly and easily accomplished in post-processing. Another reason to perform this task after capture: you'll avoid uneven polarization. Especially when using a wide angle lens, a patch of sky will render as weirdly "too dark" blue if you've turned the ring too far.
The filter has other more important applications. Foremost, it removes glare. On drizzly and/or rainy days you will be surrounded by glare as light is reflected off wet surfaces. Your eyes might look past all that shiny stuff, but your camera won't - and this is something which cannot be corrected after the fact.
Glare isn't just an issue when it's wet; it's present even on sunny days. Look at foliage or rocks with and without a polarizer on a sunny afternoon; you'll see a difference. It isn't as dramatic as when everything is wet, but it's there. By removing glare, colors appear to be more vibrant and details are clearer.
When shooting water, the polarizer will enable you to see beneath the surface - again, because it's reducing glare and reflections. Manipulate the ring to determine how much or how little of the effect you wish to employ. Let's say you're photographing a water lily in a pond. You can make the water go completely black, or pull back if you'd like to retain some of the plant's reflection.
On cloudy days, the polarizer can cut through haze.
On moonlit nights, you can use your polarizer (surprise!) to enhance starry skies.
The polarizer can also function as a mini neutral density filter. Because it works by blocking light, you'll lose as much as 1.5 stops when it's fully rotated.
If you so desire, you can purchase combination polarizers i.e. polarizer + warming or polarizer + diffusing. I use a regular polarizer and am never without it.
Neutral Density
The job of the ND is to absorb light - all wavelengths equally. These filters work in increments of stops: for example, ranging from 2-10. You can either purchase them as a set or opt for a single variable ND (which works the same as a circular polarizer and enables you to dial in the setting of choice).
ND filters enable you to shoot at wider f-stops in brighter light. This can come in handy when working with water. Imagine flat light on an overcast day - in theory, great conditions to photograph waterfalls. However, overcast can still be way too bright for the kind of slow shutter speeds you'll need if you want to render the water as silky. The ND provides the exposure control required to achieve the desired result.
I used to use a set of Cokin square ND filters because that was pretty much the only format you could get at the time. I didn't like having to carry that many ND filters, and I'm not a big fan of the square format. Just my personal preference; there's nothing wrong with square filters. When Singh-Ray's variable circular ND hit the market (2009-ish), I bit the bullet and spent the money. I won't lie - it was expensive. They retail now for nearly $400, so that'll give you an idea what I mean when I say it was spendy. There are other options now from other manufacturers, though I've been happy with my Singh-Ray.
[Graduated neutral density filters are another option. Clear on one end, they transition to block light on the other end (again, in increments of stops). Use these to even out scenes with a great deal of variation in contrast, like a sunrise. I used to carry graduated NDs, too, but the capabilities of both cameras and processing have come a long way since then. I haven't used these for a long time.]
I use my variable ND much less frequently than the polarizer, but am never without this filter, either.
UV
As the name suggests, these filters are used to manage atmospheric haze (often prevalent at high altitudes). Some will reduce blue cast and improve image clarity. There's no shortage of UV options on the market.
Most people, if they have UVs, use them to protect their lenses (from moisture, scratches, dust) and not so much to improve image quality.
Personally, I think UV filters are particularly useful when shooting around spray, especially salt water. It's easier to clean the filter than the lens. I haven't found them to be significantly helpful in terms of mitigating haze.
As for protection from damage, lens hoods will provide as much protection as a UV filter. In terms of catastrophic damage, a filter isn't going to make much of a difference. That's why I carry insurance on my gear.
I've got UV filters on some of my lenses, but not all of them.
If you never had to find room in your bag for a bulky pouch full of filters, consider yourself lucky. These days you can go much lighter - but I would never head out on a shoot without my polarizer and variable ND. As indicated, UVs are represented in my kit, too, though I wouldn't consider them absolutely essential.
When it comes to choosing filters, there is one mistake to avoid: don't cheap out.
After plunking down $3,000 or $4,000 (or more) on the body and a couple thousand more for a lens, why on earth opt for bargain-basement filters? Put premium glass in front of your premium glass. More expensive filters are thinner and optically purer, which means they will not interfere with your lens.
Inexpensive filters can result in noticeably degraded images: issues like loss of sharpness, vignetting, ghosting, flare and color cast.
Shop for quality.
One way you can economize is with a step-up ring. As you know, filter thread sizes vary by lens. Maybe most of your lenses have 77mm threads, but you've got one that's 67mm. Rather than investing in two sets of circular polarizers and variable NDs (which not only gets expensive but also will take up more room in your bag), purchase a 67mm-77mm step-up ring for the 67mm lens.
The only drawback will be the hood for the 67mm lens, which will obviously not fit when the step-up ring is attached.
Step up (and down) rings can be purchased in single specific sizes, or in sets.
Another word of caution with filters: try to avoid stacking them. This can degrade image quality (vignetting, flare, ghosting).
In closing, filters can help you make great photos. Fortunately, you no longer need to carry an armload of them: good news for the pocketbook, and it'll free up space in your bag.
