In this guide we’ll look at the following:
- What are lens filters (and camera filters)?
- Why do we need them?
- What are the important types of filters and how do they help?
- Things to look out for in a good filter.
- Examples and suggestions for good filters for video.
What is a lens filter?
When we normally use the word ‘filter’, we think of it as something that takes away unwanted stuff. In photography and cinematography, though, a filter is something that manipulates light entering a camera system.
There are three main places you can put a filter:
- Before the lens.
- After the lens and before the lens mount.
- Before the sensor and housed in the camera body (It’s not part of the lens).
The first two are called Lens Filters, while the third are called Camera Filters. A lens filter can be fixed before or after the lens:
In this guide we’ll only cover lens filters.
Depending on physical characteristics and limitations, you can add more than one filter. The process of adding more than one filter on a lens is called filter stacking.
A few filters that are not filters
Sometimes, you get bogged down by technicalities. Technically, anything that filters light is a filter, but in popular usage we don’t consider the following to be lens filters:
- Lens coating – Every lens is coated with various chemicals to reduce scratches, flare, reflectance, etc. In addition, some manufacturers also apply coatings to manipulate the color (even invisible electromagnetic radiation) of the light. These are beyond our control and gives each lens its character. To learn more about lens coatings, read this primer.
- Lens hoods, scrims, barn doors, flags and other physical objects designed to block light.
- The black coating inside a lens – light should pass through a lens directly onto the sensor, without bouncing around anywhere. To this end, the interiors of lenses and sensors are painted jet black. The lens mount is designed to be a twist-lock (screw or bayonet) system to not only keep the connection firm, but also to assist in keeping out light.
- Adapters, bellows, tubes, shims, etc. – anything that increases the distance between the lens and the lens mount.
In popular usage, when people use the term lens filter, they mean something that looks like these things:
Lens filters are either circular or rectangular pieces of glass or plastic – each with a unique ability to manipulate light – abilities that allow the cinematographer to extend the capabilities of a lens to:
- Greater creative effect, or
- Deal with unwanted lighting conditions, or
- Correct problems in the camera-lens system.
The key defining characteristic of filters is that a user can remove or add them at will. And the most popular place to put filters is at the front of the lens. The objective here is to modify the light before it enters the lens.
On the other hand, some lenses come with filter holders at the back:
Putting filters at the back has the disadvantage that every time you want to change filters, you have to remove the lens and expose the sensor to the elements. Therefore, unless you have strong reasons to place filters at the back, stick to filters that go at the front of the lens.
Why filters are always ‘bad’
Filters are basically a slab of glass or plastic, and this is how light behaves with a slab of glass or plastic:
Light that comes in from the center along the optical axis goes straight through. Light coming in at an angle usually refracts (passes through, but with a deviation). Some light also reflects (not a good thing at all).
Let’s look at some disadvantages inherent in filter design:
- The above image is what happens in a perfect world. In real life, there are always manufacturing and design defects that make it tough to prejudge what the resulting effect is going to be. Minor variations in the glass will lead to unwanted effects like increased flare, ghosting, vignetting, loss of contrast, etc.
- The front element of a lens is spherical, while a filter will be flat, or flatter – never the same shape in any case.
- Because the filter frame protrudes outwards, any moisture, water, dirt, etc. that collects on the filter will remain on the filter unless you thoroughly clean it.
- Every subsequent filter in the stack will further degrade performance, especially the amount of light hitting the sensor and its contrast.
- Any coatings on the filter might interfere with the job of the lens coating.
Don’t get me wrong. Many of the ‘ill-effects’ of filters are rarely visible. You have to test each filter-lens-camera combination in different lighting conditions to be sure.
The lens is the designated tool to creatively manipulate light to form an image. If a filter interferes with this function, then it’s a problem. Modern filters (from good manufacturers) are designed to keep many such problems at bay. You can’t avoid filters always, so stick to this rule of thumb: Never buy cheap filters. What you want is a good synergy between the capabilities of your lens and your filter. Since the lens designers and filter designers don’t always share information with each other over a beer, it becomes your job to see how well they mate – after you have paid for both of them.
Let’s look at two popular ‘always-on’ filters (because they are always kept on, no matter what!).
The Ultraviolet (UV) Filter
A UV filter tries to cut out ultraviolet (UV) light, while passing everything else.
UV light might cause haziness in the sensor, though modern sensors are not very sensitive to UV light (film is, though).
Typically, UV filters are harmless filters left on the lens forever. Why would anyone want to do that? UV filters, because they don’t negatively impact the image, are usually left on the lens permanently to protect it from the elements, or from being scratched due to incorrect cleaning practices or the occasional bump or drop. A scratched filter is cheaper to replace than a scratched lens. Some lenses also need a UV filter to complete their weather protection system.
Are there any negatives to having a UV filter on?
Some argue that having an additional optical element unnecessarily degrades the image, especially if it is a cheap UV filter. If you use a good filter, this point is a non-issue, in my experience.
One important negative is that people tend to forget they have a UV filter on a lens, until its time to remove it. A filter can get stuck to the lens thread if not regularly removed and cleaned. As long as you don’t forget this important step you’re gold.
My recommendation is, if you’re not using a matte box or any other filter, keep a UV filter on. But no matter what you do, don’t put on a cheap UV filter. It’s the height of stupidity to buy an expensive lens and degrade its quality with a crappy filter.
Ratings on UV filters
Just like sunscreen, UV filters come in different ratings. Three common ratings are:
- L37 – removes UV light with a wavelength shorter than 370 nm.
- L39 – removes UV light with a wavelength shorter than 390 nm.
- No rating! Nowadays you’ll find UV filters with no ratings, because sensors aren’t affected much by UV radiation. When in doubt, choose what you have.
Note: There are also ‘UV filters’ that only let in UV light, while blocking everything else. This is not generally used in cinematography so we won’t cover them here.
The Polarizing Filter, or Polarizer
Light is a funny thing. Without going into technical details, let’s just say that one aspect of light, while moving forward, has the tendency to bob up and down at the same time. Think 12 o’clock and 6 o’clock, up and down, while moving forward.
Strangely, it can also bob up and down in different angles or orientations – like 1 o’clock and 7 o’clock, 3 o’clock and 9 o’clock, and so on. Naturally occurring light can bob up and down in 12-6 in one cycle and then change to 3-9 in another cycle, and so on. It doesn’t stick to one orientation. This is what it looks like:
This kind of light is called unpolarized light. To ‘discipline’ this light into one orientation, you can pass the wave through a slit, and get this:
We don’t need to get any more technical than this. Now imagine this: What if the scene you are shooting has unpolarized light (it has, almost always), and you wanted to just isolate light of one particular orientation, while ignoring the rest? Or, what if there are naturally occurring polarized light that you can block? Can you do this?
Yes, you can. But the question is, why on earth would you want to do something like this?
Thankfully, some kinds of light in nature are polarized (it’s not 100% guaranteed!):
- Some light from the sky
So, a polarizer can block certain reflections and certain kinds of sky light. If you’re shooting a glass surface or a shiny floor or a still pool, and want to eliminate reflections, a polarizing filter is your weapon of choice.
For light from the sky, a polarizer only works generally in one angle, somewhat 90o from where the sun is. What does it do? It generally adds more saturation and ‘pop’, for lack of a better word:
Notice the shadows in the above image, which shows you the sun is at an angle of 90o (perpendicular) to the direction the lens is pointed at.
You’ll see its best effect only at certain angles. At other angles the effect will hardly matter. If you can master your orientation, then you can take the first steps towards mastering light’s orientations!
So, how does a polarizing filter work? It’s simple. A polarizing filter can be rotated independently of it being screwed on to the lens:
I included the above text so that you realize the importance of angles in polarization. Your angle in relation to the sun, and the angle of the lens in relation to the horizon and the magnetic poles will determine how useful the polarizing effect will be.
Now, there’s a problem with linearly polarized light – for the camera sensor:
- It might throw off the sensor’s autofocus mechanism.
- It might affect the optical low-pass filter in front of the sensor.
- It might affect the metering.
Is there a way to linearly polarize light, and then throw it out of whack again just before it hits the sensor? Sure, such a filter is called a Circular Polarizing Filter. What these do, is they first linearly polarize light, and then change the polarization to a circular pattern (not unpolarized) which is okay for sensors:
- Darken the sky.
- Remove reflections from water, mirrors, shiny surfaces, etc.
- Take the gloss off shiny surfaces.
- Increase color saturation.
I’ll keep it simple – stick to circular polarizers (CPL). They do everything linear polarizers do, and they are not hard on your camera’s exposure meter or sensor. Those who shoot exteriors often have a CPL permanently screwed on to their lens, which is why a CPL is the second-most favorite ‘always-on’ filter.
Is there a disadvantage to CPLs?
Of course! Look at the moving GIF image above. Notice how the light gets blocked the more you polarize something? CPLs cut down light, which is not something you want in an interior location. For this reason, I recommend you only use polarizers when you need something specific out of them. And when you do use them, always know how much light they cut out.
In Part Two we’ll look at more kinds of filters, how they are attached, and some examples and recommendations.