Camera Lens Filters Explained | Chris Bray Photography
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Understanding Lens Filters

jonathan ives
Tutorial by Jonathan Ives

camera lens filters
Have you ever been out photographing when another photographer turns up and starts screwing random filters onto their lenses, or slipping semi-transparent squares in front? UV filters, polarising filters, neutral-density filters, graduated ND filters... What do all these filters do? How do you use them? And most frustratingly of all, "How come my photos don't look like that?"

The ability to confidently and successfully utilise filters is definitely a skill worth having. Filters allow the photographer to overcome limitations in the photographic process and to capture creative and beautiful effects in a photograph, such as the silky appearance of a flowing waterfall or the beauty of the coral just beneath the glare of the waves. Progressing by order of (in my opinion) most important to least, I hope this tutorial will help you understand the different types of filters out there, what they do and how to use them to get professional looking photos.
lens filter diameter
Filter diameter is written on the lens.

What are Filters?:

Filters are mostly made of high quality glass (or resin) and when attached to the front of a lens, are used to block (filter) certain colours or types of light coming into the camera. Some filters (eg ND filters) are designed to reduce the overall light intensity coming into the camera. Filters in general have lost a bit of popularity in our age of digital photography because many believe the effects can just be added in post-processing through editing software. While this is sometimes true (Adobe Lightroom for example allows you to put a 'graduated' filter over your images which is VERY handy!) many effects, such as that created by a polarising filter (see below), simply can't be replicated digitally. Besides, there is always a limit to the amount of digital manipulation possible before image quality is severely affected anyway.

Most filters are available in circular form and screw directly onto your lens. The size required depends on the diameter of the lens. Each lens you own will have its filter thread diameter marked in millimetres near the front element.

An alternative filter design is known as the 'system filter' whereby you purchase an entire system from the same manufacturer. The filter plates are usually square and can be changed by sliding them in and out of the holder in front of your lens. This is a necessary design for graduated filters (see below), but for the rest, the design you choose is up to you.
broken uv filter
UV filters protect your lens!

UV Filters:

Personally, I recommend everyone put a UV filter on all their lens AND keep it there as an almost permanent fixture. UV filters are basically just clear glass, which traditionally helped lessen the amount of UV light hitting film. Humans can't see UV light, but film was sensitive to it, resulting in unexpected blue tints. Today though, digital sensors are not sensitive to UV, so there is no real visual benefit from having a UV filter attached to your lens. So why have one?

Only weeks ago I was swapping my macro lens for a wider angle. I rested the macro lens 'safely' on the table, only for it to be bumped moments later by a family member and tumble spectacularly to the floor, landing 'face first' on the ground. I looked in dismay at the fragments of broken glass surrounding it. My lens, however, was completely intact. The (MUCH) cheaper and easily replaceable UV filter had saved the day - as it has for countless other people. In short: UV filters protect your lens! Anyone who shoots outside the safety of a clean, calm studio (which, let's face it, is pretty much everyone) should have one fixed to the front of every lens. It keeps dust, salt spray, rain, greasy fingers and stray branches off the expensive front glass of the lens. Lenses have very delicate, thin coatings on their front elements designed to reduce reflections etc, and not only is it way too easy to scratch these compared to the more durable glass of a filter, but it's also a lot easier to wipe marks from a filter. I personally always feel more secure in the knowledge that the stray speck of sand I just accidentally scraped across the front of my lens didn't actually scratch the lens at all, only my replaceable UV filter.

A couple of things to note, though. Firstly, you do get what you pay for. Some of the very cheap UV filters (e.g online for a couple of $s) do reduce image quality a little in terms of sharpness. Every extra piece of glass that light has to pass through may alter image quality, particularly if the glass is a little rough, not perfectly flat or not 'optically clear'. In my opinion, it's worth paying a little more - even the most expensive ones though are still MUCH cheaper than replacing an entire lens which got scratched or dropped! Secondly, every glass/air interface means there is another chance for reflection and lens 'flare'. Higher quality filters tend to be multicoated to reduce this. If you're getting unwanted flare in one particular shot, with the sun or moon in shot for example, you can always screw the filter off for just a moment.

Quick tip:

While UV filters have a thread on their outer rim too, allowing you to screw on more and more filters at the same time, one after the other, when using a different filter (below) consider taking off the UV temporarily. This is partly to reduce the layers of glass that the light has to pass through and also prevents shadowing in the corners of your image (called Vignetting). Vignetting happens, particularly with wide-angle lenses, when you restrict the lens's field of view by a tunnel of filters.
polarizer filter comparison
Without Polariser (top)
and with (bottom)

Polarising Filters:

This type of filter will enhance an image in ways that cannot be achieved by any other method. As its name suggests, a polariser is a special type of filter that has the ability to block polarised (i.e reflected) light - often seen as 'glare'. Even once screwed on tight to your lens, the glass part is rotatable via the outside rim so that you can look through the camera's viewfinder while rotating it and choose how intensely you'd like to filter the polarised light. The resulting effects are the darkening of skies so that the rich blues are brought out vibrantly in contrast to clouds, as well as reducing reflections on the surface of water or many other reflective surfaces. They are also VERY handy for bringing out the colours of rainbows. You don't even have to put the polariser on your lens to see the effect this filter brings, just hold it up to your eye and rotate it... you'll be amazed. Just like wearing polarised sunglasses, it can make landscape scenes look much more impressive.

Polarisers show most effect when the camera is shooting 90 degrees from the sun (so the sun is at your right or left, not behind or in front). However don't use it constantly with the assumption that it will always enhance the image. Over-use of a polariser can sometimes have an unnatural effect, particularly when shooting on a very wide lens and the polarisation in the sky changes dramatically across the image.

It's important to note that because polarising filters are not clear (they look quite dark) they do actually cut out the amount of light reaching your camera - but not as dramatically as an ND filter (see below). This means that in order for your camera to create a properly exposed image it will need to compensate a little by either using a lower f-stop (larger aperture hole) if you're controlling your shutter speed (S or Tv mode), or using a slower shutter speed if you're in Aperture mode (A or Av). You can help make your shutter speed faster by selecting a higher ISO, enabling you to hand hold your camera. Regardless though, if the shutter speed becomes too slow to hand-hold (for example late afternoon when light is already low), and you still want the effects of the polariser, consider mounting your camera on a tripod.

Quick tip:

Polarising filters can sometimes be used effectively in situations where most people would overlook their benefit. For example, they can prove helpful when wanting to reduce glare on overcast days, when photographing after (or during) rain or when wanting to bring out more colour saturation. They are also handy for reducing the glare on plant leaves - a great tip for photographing in a rainforest - revealing richer, darker greens.
ND filter
An ND filter held up to a lens,
showing the light reduction.

Neutral Density (ND) Filters:

Unlike the polarising filter which changes the way the image looks, ND filters aim to remain 'neutral' and simply reduce the overall amount of light coming through the lens. Like dark sunglasses for a camera, by reducing the amount of light coming through the lens, ND filters allow the photographer to purposefully compensate by slowing the shutter speed down and achieving motion blur effects, even in situations of bright light. When elements in the scene are moving, such as a waterfall, waves, clouds or even people, a range of creative possibilities are opened up.

There are a number of ND filters on the market. Typically they come numbered (ND2, ND4, ND8 etc.) These numbers signify how much light the filter cuts out. An ND2 is supposed to transmit only 1/2 of the available light and is therefore the equivalent of 1 f-stop. An ND4 (twice as dark) will transmit ¼ of the light (allowing 4x longer shutter speeds) and is equivalent to 2 f-stops. ND8 (twice as dark again) will therefore only let in 1/8 of available light (8x longer shutter speeds) and is the equivalent to 3 f-stops and so on. Depending on the situation, these filters can be stacked on top of each other to reduce the light even further. If shooting on Av (aperture) mode, the camera will alter its selected shutter speed accordingly, lengthening it as the darker filter is applied.

ND # Light
Collection
Optical
Density
f-stop
Reduction
Trans-
mittance
(%)
0 1 0.0 0 100%
2 1/2 0.3 1 50%
4 1/4 0.6 2 25%
8 1/8 0.9 3 12.5%
There is also the option of purchasing a variable ND filter. These screw onto the lens but have an adjustable outer ring (like the polarising filter design) which can be adjusted to darken the filter (i.e into an ND2, or ND4, whatever) depending on your needs. While less accurate than a fixed ND filter, it removes the risk of tunnelling and loss of image quality that can occur when stacking filters. (Tunnelling or vignetting, as mentioned earlier, is the problem that occurs with wide angled lenses when the filter starts to become visible in the corners or sides of the final image).

ND filters have limits, particularly in the variable ND filters. When pushed to the extreme (in bright light), when the ND filter needs to be exceptionally dark, the resulting image can sometimes become uneven. Occasionally the camera also assumes you're trying to take a dark image and you may have to adjust your exposure compensation a little until you get the settings you're after. Alternatively you could play around in manual (M) mode with test shots until you reach an exposure you're happy with.

Because everything looks so dark through exceptionally strong ND filters, you may find that a) it becomes impossible for your camera to auto-focus (AF) through it, and b) impossible for you to see your composition. In these cases, it's best to frame up your composition and pre-focus (using AF) before you put on the ND filter. As you'd be using a tripod anyway with such slow photos, neither the focus nor composition will change. To prevent the lens hunting for focus once the filter is applied, after you've pre-focused with AF it's best to switch your lens to manual focus (MF). This will 'lock' your focus. Alternatively, some lenses also have a focus distance scale physically displayed on the lens, allowing you twist the focus ring to approximately the right focus distance.

Quick tip:

Look for creative compositions that are created by movement eg. leading lines resulting from the water movement, or the flow of people.
graduated nd filter
Correct exposure across most of
the bottom image, rather than a correct
sky and darker rock [top image].

Graduated ND Filters:

ND Grads', as they are sometimes referred to, are ND filters which provide a gradually changing ND gradient across the glass plate i.e. a darker ND number at the top changing to clear glass at the bottom. They are always square or rectangular and therefore require a 'system' mount (or awkwardly hand-held in front of the lens). In today's world of digital post processing, ND grads are far less common, but shouldn't be entirely overlooked.

System filter with ND Grad
System filter with ND Grad
When watching a beautiful sunset through your own eyes, despite the light and colours being extreme, as you look around you'll find it easy to see into both the bright sky and the darker shadows equally well. Our minds are very intelligent and balance the bright areas and the shadows for us subconsciously. However for a camera, it's impossible to capture both the bright sky and dark foreground evenly. All cameras, of every make, continue to struggle when confronted with high-contrast conditions or scenes where the brightness range varies dramatically across the image. Unless something is used to reduce the brightness of the sky down to a similar level to that of the rest of the scene, the photographer is faced with the option of 'correctly' exposing the shadows (which results in bleaching the sky) or 'correctly' exposing the sky (blackening the shadows and creating silhouettes). ND Grads help to overcome this issue by their ND gradient, allowing the photographer to darken the brighter areas of the photograph (sky) through the filter while keeping the darker shadows unchanged, giving 'correct' exposure across the entire image.
digital Graduated ND filter
Graduated Filter in post processing,
bringing out the definition
in the mountain.

Quick tip:

Adobe Lightroom and Adobe Photoshop (RAW) provide a convenient graduated filter you can apply in post-production with a simple click and drag. It won't completely solve your problems, and post processing is always going to affect your image quality to some degree, but it's a very helpful tool nonetheless! You can also digitally apply other graduated effects too - not only ND. For example you could opt to make the sky more saturated or bluer. The creative options are numerous.

Tips for all filters:

Be selective in your filter use. While a UV should be a near permanent fixture, don't always just assume using a creative filter will make for a better photo. A filter won't necessarily bail you out from a taking a bad photo, but it will help you take GREAT photos.

Be keen to maintain image quality. Don't stack filters on top of each other unless you have to. The more pieces of glass that the light has to pass through, the lower the image quality will be.

Be wary of 'tunnelling' or vignetting when using wide angle lenses.

Be on the lookout for quality. You get what you pay for. There are cheap options out there which are great fun and perhaps good to start experimenting with if you're not too serious or are on a tight budget, but the old rule of thumb definitely applies here. The better the filter the more it will bring to the task, and the less image quality will degrade.

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