You’ve probably heard of filters, but like a lot of subjects in photography, there’s some controversy about which ones are good to use or even necessary. I’m here to demystify things a bit.
Before digital photography took hold, there was a huge variety of filters being produced and used. Most of them had very specific and niche use cases, and the majority of them are totally unnecessary for digital photography. I’ll start off by talking about the small handful of filters that are still arguably useful for digital work, as virtually any filter that simply adds or removes a color cast to the image can be easily replicated in post processing.
UV/Protection/Skylight filters. I’m lumping these together into a single “category” because despite having very small technical differences, they are effectively the same thing: a (more or less) clear filter that won’t really change anything about your image, it’s just there to serve as a layer of protection between your lens’s front element and the outside world. If you drop your $2500 lens, instead of scratching it, you might scratch up a $100 filter instead. In fact, you might not even do that - some UV filters are made from extremely tough tempered glass that is very difficult to damage.
Not all UV filters are created equal. If you have a very nice, sharp lens, and put a $15 filter on the front, you should expect to see a slight decrease in image quality… which kind of defeats the purpose of having a high-end lens in the first place.
There’s also debate on whether UV filters are a good idea at all. Many keyboard warriors insist that any clear filter is just unnecessary and a weak point in your optical setup. Others say they use a UV filter instead of a lens cap. Photojournalists often use this technique--when taking off a lens cap could make the difference between getting an important shot or missing it, the ability to just pull out the camera and shoot immediately becomes pretty important.
As for me, I have UV filters permanently on the front of most of my lenses. I haven’t seen any loss in image quality, and for me, I’d rather be safe than sorry. My UV filters only come off when I’m using a different filter.
Neutral Density filters. Neutral density or “ND” filters are filters that simply block out light, theoretically without affecting your photo. If you’re out in the bright sun, already shooting at f/22 and your lowest ISO, but you still want a slower shutter speed, you can use a neutral density filter. They come in all sorts of “strengths” (or densities). Common ones block out 1 stop, 3 stops, or even 9 stops of light or more. I even saw some photos once by a guy using a welder’s mask as a 13-stop ND filter. If you’ve ever seen a seascape photo where the ocean just looks like a bunch of mist or fog, that was probably taken with a 9-stop ND filter. It blocked out so much light that the shutter speed was probably upwards of a few minutes, allowing the waves to come in and out of the frame several times. This is how the misty look in the ocean is achieved.
Cheap ND filters, particularly the denser ones, have a tendency to impart a color cast, despite being “neutral.” Whether or not this is a problem is something you need to decide for yourself when shopping for an ND filter. Personally, I don’t find it too difficult to correct a little extra magenta in my photo if the price is right on the filter.
There are variable neutral density (VND) filters, though I don’t have much personal experience with those. The filter rotates on your lens to adjust how much light it’s blocking out. I love the theory of those filters, since you would only need one of them instead of a full set, but have heard anecdotal reports that they don’t block the light evenly across the entire frame. If you know of a good VND filter that works really well, let me know in the comments!One other type of filter worth mentioning here is a graduated neutral density filter, or GND. These are usually squares that mount into something like a Cokin system. They have some amount of neutral density on the top of the frame, which gradually (or abruptly) changes to clear as you go down. This allows you to lower the exposure of only part of the frame while leaving the rest untouched--particularly useful if you’re shooting a high-contrast landscape where the sky or sunset is a few orders of magnitude brighter than the foreground, and you’d like the exposure to come out more balanced.
Polarizers/Circular Polarizers. Without getting into the complex technical side of how polarizers interact with light, I’ll just say that polarizer filters cut reflections and glare from water and metallic surfaces, as well as making colors a little more saturated. For these reasons, they’re popular with landscape photographers. It’s a little difficult to picture what I’m talking about without a visual example, so I’ll link one in a second. But if you’ve ever worn polarized sunglasses, that is the exact same thing.
Polarizers for photography are often sold with two layers of polarized glass that can be rotated in relation to each other to change the strength of the effect. This is called a circular polarizer or CPL. To see the effect of a polarizer, check out the video below. Notice how as he rotates the filter, you can see reflections appearing and disappearing on the glass, and the sky goes from a lighter blue to a deeper blue.
It’s worth noting that polarizers will usually cut out about ⅔ of stop of light, so expect to need a slightly longer shutter speed, higher ISO, or wider aperture to compensate for that. Sometimes this is advantageous. I’ve used a polarizer as a weak neutral density filter to get a longer shutter speed for shooting moving water, for example, even in situations where the polarized effect was minimal or unnecessary.
A circular polarizer dialed up to nearly maximum polarization allowed me to isolate the clouds from what would otherwise have been a pretty bland sky. It made the blues deeper and darker to contrast with the wispy white clouds, so even though this image was converted to black & white, the polarizer is what made it work.
That about covers it for filters that are useful for digital photography. I’ll have another post covering the range of filters applicable to film photography. Questions or comments? Have a favorite filter? Did you find a VND filter that actually works really well? Leave me a comment.