Let’s be clear upfront: there will be no definitive answers here, nor any pixel-peeping sample photos of shots with and without filters. This is more about the types of filters and reasons to use them (or perhaps not). As with many of these posts, this is intended to be more informational/educational (with some opinions thrown in) than a simple “here’s the best and only way to do this.”
Finally, I’ll close with some recommendations of very good and excellent filters that I personally use.
One of the longest running debates on the Internet is the use of filtration – specifically UV (ultraviolet) filters. Some see UV filters as instruments of protection (of their lens) without any real consequences; others see them as sacrilegious pieces of inferior glass that compromise image quality. So, let’s get into it a little bit because there are merits to both camps; though I feel like it is almost universally agreed that any filters that come in a camera or lens bundle should best be left unopened and away from your lenses.
L39 UV Filter
Wikipedia Creative Commons License
What the hell is a UV filter? Good question. There’s a reason these filters aren’t just called “protection filters” or some such, even though that’s just about all they do (these days). First of all, we should denote the difference between a UV Filter and a UV Pass Filter because sometimes the former is used to describe the latter: a “UV filter” is nothing more than glass that filters/eliminates/blocks ultraviolet light from passing through into the lens and subsequently onto the sensor (or film). A “UV Pass Filter” is quite the opposite; it allows types of ultraviolet light through while blocking others. We’re talking specifically about UV Filters only.
Light presents itself in all manner of ways, many of which we cannot see. Ultraviolet light is one of those types that we cannot see – in fact, visible light is generally defined as landing somewhere between 400 and 700 nanometers. This happens to be the spectrum between ultraviolet (roughly 10-400 nm) and infrared (about 700nm up to something like 1400nm for near-infrared; other types have longer wavelengths). Below UV light we have x-rays, which can be as small as 0.01nm (aka 1 picometer), and gamma rays; above infrared we have microwaves and radio waves, among many others.
At any rate, there are many different types of light, but also many different types of ultraviolet light. So, in the olden days, film was particularly sensitive to ultraviolet light; most modern films fair just fine without filtration, though depending on the environment, color film can fall victim to hazing and overexposure in the blue layer (the blue layer is sensitive not only to blue light but also UV light). Most filters from the film days would filter out a specific threshold of light; an L39 UV filter (as seen above) was common among brands – its name indicating that it filtered out UV light below 390nm.
What about digital cameras? These days, many UV filters are nothing more than clear pieces of glass with some coatings – they don’t actually filter any light. Modern digital sensors now have UV (and IR) filtration in front of the sensor. This wasn’t always the case – many video cameras required the use of IR cut filters for a while, as did cameras like the Leica M8. Some people will actually have these filter stacks removed or replaced to allow their camera to capture a greater (or different) spectrum of light; I have a 720nm IR converted Sony a6000. Conversions can range from UV (to filter everything except ultraviolet light), 720nm IR (the most popular, as it makes for a nice balance of beautiful color photos as well as black and white), 590nm IR (best for color only), 850nm IR (best for b&w only), and even full-spectrum which allows the sensor to collect UV, visible, and IR light, though this type requires the use of lens-based filters to switch between whatever light you do/do not want to capture. Kolari Vision is famous for these modifications.
Long story short, unless you have an older digital camera that lacks filtration or you have a modified camera, a UV filter will not provide any image quality benefits. If you choose to use one, you’re using it as protection for your lens.
What are the downsides of filters? Well, there are many possible issues, with possibility nearing certainty if you use a cheap, poorly made filter. Issues can range from transmission loss (this is inevitable technically, but bad filters transmit even less), veiling flare, color cast, ghosting, decreased acuity, and/or vignetting. Many of these can become quickly apparent when used with wide angle lenses.
General Interference with optics. This can be due to a huge variety of reasons and the effects can manifest themselves in just as many different ways (some are described as you read on). But, to describe an example as simply as possible: consider a lens like the Nikkor AF-S 14-24/2.8G, or Tamron SP 15-30/2.8 G2, or Sigma 14/1.8; all of these are fast, wide-angle lenses with bulbous, convex front element. Now, think about putting a flat piece of glass over that (which you have to attach to lenses like these with a large 150mm square fixture due to that front element). It might change some things, right?* One of which is the way light hits and enters the front element (angle of incidence), of course altering the path of the light as it travels to the sensor in the process. In effect, you are changing the optical formula of the lens.
*Wide angle lenses are particularly susceptible to this – not just because of their large front elements and gargantuan filter holders, but also because they’re collecting light from such a huge field of view (and therefore from an absurd number of angles). This is precisely why a 14mm lens at f/2.8 only requires a pupil diameter of 5mm, whereas a 100mm lens at f/2.8 needs a 35.7mm pupil – they both gather the same amount of light, but the 100mm needs a much larger opening because it gathers light from a significantly smaller area.
Light loss. All glass has a certain level of transmittance* – light is reflected, scattered, and absorbed as it passes through glass. This is why we have both T-stops and F-stops and they are never the same, though the best lenses manage to get very close. Lens and filter manufacturers try to achieve transmission as near to 100% as possible through the use of specialized coatings and glass elements, but a true 100% transmission is purely theoretical.
*“Transmittance” and “transmission” are not the same. Transmission is the amount of light that passes through a material (glass in our case) – often expressed simply as a percentage. Transmittance is a measurement of the amount of light that the glass reflects, absorbs, or disperses. Ergo, transmittance is essentially the inverse of transmission; and thus, the higher the transmission, the lower the transmittance (and vice versa).
Ghosting. Filters can also cause light to bounce around internally in ways that it certainly isn’t meant to. For example, light entering the lens can bounce from the sensor, back off the rear of the filter, then once again back into camera – this can rear its head as ghosting due to the secondary “ghost” image it produces on the opposite side of the optical axis. This is particularly noticeable at night and/or wide open and can even happen without a filter attached at all if there are flaws in the lens’ optics. I learned the latter the hard way years ago when shooting a show with what was obviously a misaligned Sigma 18-35/1.8 – ghost images appeared in a handful of shots, many during the daytime and a few at night; some daytime shots had matte-box ND filters attached and night shots had no filtration at all yet still produced ghost images.
Acuity/Resolution Degradation. Any time you place a piece of glass in front of a lens, you run the risk of degrading the peak image quality (in this case, pixel acuity) that the lens is capable of achieving. This effect is more noticeable as you climb the ladder of sensor technology; what may appear to have no ill-effects on a 12MP sensor with an AA filter may well prove to be terrible on a 47MP AA-less camera. In the days of film, a decent filter would likely not noticeably degrade IQ – lenses weren’t as good, 35mm film resolved less detail than sensors of today, and grain would typically mask minor optical deficiencies. Plus, even UV filters actually had measurable benefits on film.
Color Cast/Vignetting. These are relatively simple to understand. There are a couple reasons a filter may introduce color cast:
1) If a filter is not spectrally neutral, it will have some sort of effect on the colors – fairly obvious.
2) Depending on the design of the lens and the filter being used, the optical formula of the lens may be compromised to the extent that you see the effects. Filters can imperil the telecentricity of the lens and therefore undermining its zero angle of incidence*; this in turn causes rays of light to strike the sensor at an angle non-parallel to the optical axis. The upshot of this is the potential for color crosstalk as the light interacts with the color-filter array.
*A lens with a zero angle of incident is purely a theoretical perfectly telecentric lens. I doubt any such lens truly exists. Lenses will have a non-zero angle of incidence by their nature, however this doesn’t change the aforementioned interactions.
3) When using an ND filter, there is a decreased amount of visible light passing into the lens, but infrared light is unaffected. Because of this, the ratio of infrared to visual light increases and can induce color casts.
A lot of side-by-side tests of filters – and this is certainly not exclusive to UV filters, in fact ND filters may be the worst offender here – will show how much a non-spectrally neutral filter can shift the color balance of the image. Obviously, this doesn’t pertain to filters whose intended purpose is to do that exactly that. Vignetting is less of an issue here as it is one of the least worrisome of optical deficiencies, at least in my opinion. However, we really don’t want a filter to introduce additional vignetting on top of what the lens inevitably already exhibits. This is mostly an issue with wider angle lenses and/or thick filters. My suggestion is to always get the thinnest filter – Formatt Hitech, for example, calls these “Superslim.” Step up rings to use a larger size filter than your lens thread size is another option – this also allows you to buy fewer filters but still use them across multiple lenses.
So should you use a UV filter? I don’t know. Up to you. Personally, I use them some of the time and not at all most of the time. I certainly don’t use them in any type of controlled environment such as product, portraiture, etc. I will use them in more volatile environments – dusty, sandy, otherwise dirty, anything where there’s a good chance of abrasive particles/materials hitting the front element. I’ve shot in machine shops where small pieces of wood/metal/sparks are flying everywhere and I’ll definitely have one on in that situation. As for general protection? Meh. The front element of a lens is much more durable than most people think and modern repellent coatings are quite good at making minor dust, rain, snow, etc. easy to clean. I find that a lens hood and of course the cap are more than enough to protect from accidental bumps or dings.
Filters other than UV. Okay, so let’s hop in to the other types of filters – at least a few of them. There are tons of types of filters (and subvariants within those types) and I won’t cover them all. Pretty much every downside covered above is equally applicable to non-UV filters, though some of those effects (e.g. light loss in the case of neutral density filters) are the very purpose of the filter. So, the big ones:
Neutral Density (ND) Filters
These are filters that cut the amount of light that enters the lens by an order of a specific magnitude – an ND 1.8 (aka ND64) filter cuts light by six stops, for example. Ideally, these filters do this with a neutral transmission, meaning they cut the amount of light but do not affect the color, tint, white balance, etc. In practice, this is often not the case, especially with cheaper filters. A bit of a color cast can be dealt with and is not the end of the world in most cases, as long as the cast is uniform across the frame, which it ought to be with a standard ND filter.
Water smoothing effect of a lower shutter speed
Wikimedia Commons
These filters are usually used to lower the exposure enough to shoot at a wider aperture in bright light or to shoot at very slow shutter speeds. An ND64 (six stops) filter is the difference between a 1/60th second exposure and a 1 second exposure; or the difference between 1 second and 1 minute. In the latter, you may get a small amount of motion blur from that one second exposure, but a full minute will give you a lot more. Photos of waterfalls or ocean waves that have a smooth, silky appearance are examples of long exposure photography likely taken with an ND filter. Without the filter, these photos would have been grossly overexposed.
There are also variable ND filters, where the “darkness” of the filter changes as you turn the filter ring, and graduated ND filters, where the filter fades with a gradient from lighter to dark (usually used to darken a sky while not affecting the lower portion of the image). Uniformity becomes a potentially more serious issue with variable ND filters – these use two polarizing elements to allow you to vary the density by turning the filter. Again, due to the high number of glass elements, a poor quality variable ND will probably have some significant negative impact. With filters you really do often get what you pay for.
Colored Filters (Red, Yellow, Orange, Blue, Green of varying intensities)
These are almost never used today; most digital sensors have made them irrelevant. Colored filters were typically used in the film era with black and white photography.* A red filter darkens skies and creates a more gritty aesthetic; yellow darkens blues to separate the sky from the clouds and renders pleasing skin-tones; orange sits in the middle of those two, of course; blue lightens skies and darkens skin tones; and green filters were often used for landscapes or anything with a lot of foliage – they would lighten the surrounding-greens to give a more natural feel.
*Colored filters (in the broadest definition of that term) were not limited to black and white photography. 80 and 82 series filters were often used to warm an image, particularly if you needed to tone down the blue of a daylight film (keep in mind, most photographic film was balanced for daylight, but cinema film had either daylight or tungsten options). 81 and 85 series filters would cool the image – used to tone down the effects of tungsten lighting, or the golden tones at sunset, or any number of other situations. With digital, well, we have white balance in camera and in post. Warming and cooling filters are also used by cinematographers to correct tungsten-balanced film (cinema film comes in tungsten and daylight flavors) in daylight or vice versa, as some directors/cinematographers like the look of a tungsten film with an 85 filter during daylight, for example.
The lone exception here are monochrome sensors. With Bayer – or X-Trans or Foveon – we have the wonderful advantage of channel mixers; basically, we have the ability to use a multitude of colored filters and precisely control their effect/strength. Monochrome sensors, naturally, lack that ability – they’re no different than b&w film in this way. So, if you have a Leica Monochrom, you may be well-served by investing in some colored filters, though you still will never have the latitude of a channel mixer from a CFA – filters are global and channel mixing allows for precise local adjustments.
Polarizing Filters
There are two types of polarizing filters – linear and circular – but we’re just concerned with circular polarizers, often abbreviated CPL. Linear polarizers wreak havoc on the autofocus and metering systems of DSLRs and don’t play nicely with AA filters, among other issues.
Polarizing filters are also helpful for landscapes (before/after)
Image by David Joyce, Creative Commons License
A CPL is a comprised of a linear polarization element plus a waveplate – the former polarizes the selected light linearly and the waveplate converts this into circularly polarized light. The effect changes as you rotate the filter and is most often used to cut the polarized light from skies, which enhances contrast, and to remove or reduce unwanted reflections off water, glass, or other reflective substances. The CPL must be adjusted appropriately for every scene and angle – what most effectively reduces reflections from one point of view likely will not with another.
The biggest disadvantage of (most) CPLs is that, due to their design, you cannot easily stack other filters on top of them – if you need to combine, say, an ND and CPL, the CPL might need to be attached last. It also makes the use of a variable ND more difficult, if not impossible, because of this; both filters must be rotated to achieve the proper setting. I try to avoid variable NDs in general (except for video work), because they can have poor cross-frame consistency issues, especially at the extreme end.
Along with ND filters, CPLs are probably the most valuable and useful filters you can get – though naturally it depends on the type of photography you do. CPLs would hardly be of use for in-studio fashion photography. But if you do any landscape or nature photography especially, I’d highly recommend investing in one; better to spend a lot of money on a high-quality filter at the largest size you need and use step-down rings than multiple cheap filters.
Other Filter Types
There are dozens upon dozens of filters that do all manner of things, some of them nothing more than toy effects. But here’s a quick rundown of the most commonly useful filters:
Diffusion: These are filters typically used to soften the sometimes harsh and biting resolution of digital photography, or more commonly, cinema. They can be useful for portraiture, though you can usually accomplish similar effects with more control in post. Video is the main use for diffusion filters.
IR (Infrared): These are used with modified full-spectrum sensors to block visible light (usually up to around 720nm) for infrared photography. With full-spectrum sensors, you must use various filters to block out the unwanted light. There are also IR cut filters (mentioned above).
Light Pollution: These have a number of names - my Haida Nanopro is called “Clear-Night”, NiSi calls theirs “Natural Light,” Schneider has “True Cut” - but all of them are designed with the same purpose: to block the light emitted by sodium vapor lamps, which we commonly know as basic street lights. These filters have a strong magenta/pinkish color cast, but work wonderful for astrophotography if you are anywhere near a city. I have only used the Haida square Clear Night filters, which are excellent.
Brands I Like (Please Use Sidebar Links!)
B + W
Breakthrough Photography
Formatt Hitech Firecrest
Gobe (budget option)
Haida
Heliopan
Zeiss