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# The Complete Guide to Lens Filters (Part Two)

Everything you wanted to know about lens filters, in one place. Part Tw covers ND, Diffusion and IR filters.

In Part One we looked at lens filter basics, and the UV and Polarizing Filter. In this part we’ll look at:

• Three more important kinds of filters.
• How filters are mounted to the lens, and the pros and cons of each system.
• Some suggestions for filters of each type.
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## The Neutral Density (ND) Filter

These work like sunglasses. They cut light. For outdoor video they are almost a necessity.

ND filters are measured in stops of light cut:

 ND Number Optical Density Stops of Light Cut No ND 0.0 0 ND2 0.3 1 ND4 0.6 2 ND8 0.9 3 ND16 1.2 4 ND32 1.5 5 ND64 1.8 6 ND128 2.1 7 ND256 2.4 8 ND512 2.7 9 ND1024 3.0 10

Finding it hard to remember? Use my method of 1-2-3:

 X stops = 2X ND = 0.3*X Optical Density

E.g., if you want to cut four stops of light, then:

• 4 stops of light = 24  ND = ND16
• 4 stops of light = 0.3 x 4 Optical Density = 1.2 Optical Density

Not that hard anymore, eh?

The three kinds of ND filters

There are three broad classes of ND filters:

• Fixed value ND filters
• Variable ND filters
• Split or Graduated ND filters

Fixed value ND filters can only cut a fixed amount of light. To cut more (or less) light, you’ll need another fixed value ND filter with a higher (or lower) ND number. If you work in constantly changing lighting conditions, you’ll need to carry many filters, each with different values of ND.

These filters offer the greatest precision. When it says ND32, you know for sure it’s going to cut exactly 5 stops of light. This means, if you’re lighting a set with a light meter, and you want to cut a certain amount of light, these filters will offer you greater control. Sometimes, a high-quality fixed ND filter will also help in IR filtration, as we’ll see later.

Variable ND filters cover a limited range of ND. These convenient because you can study your shooting conditions and buy just one filter that covers the range you want. When light changes, all you have to do is rotate the filter, which increases or decreases the amount of light cut. The disadvantage with these filters is that you don’t have the exact precision of fixed value ND filters. For this reason, if you’re going for this kind of filter, get the best. Sometimes its effect is almost imperceptible on small viewfinders or LCD screens.

Split or Graduated filters are ND filters with the ND over a limited area only, like if you want to stop down only the sky, for example:

Obviously, if you have moving elements in your frame, or if you’re moving the camera, a graduated ND filter might not work the way you want it to. But these filters have their place. The two most common scenarios for use are:

• Reducing a bright sky.
• Splitting an interior and exterior location (it can also be used vertically!).

The ND filters you choose will ultimately be based on your style of shooting. In interior locations, we are usually starved for light, and an ND filter will rarely be used. On the other hand, it would be ridiculous to venture outside without one. If you are new to filters, and are just starting out, I highly recommend you start with one variable ND filter and one split or graduated filter.

## Diffusion Filters

These filters soften the image, which can be a good thing when dealing with skin:

Like ND filters, diffusers have different ‘powers’, depending on the intensity of the effect you are after. E.g., the Tiffen 77mm Glimmer Glass Filters are numbered 1, 2, 3, etc – where 1 is the subtlest effect and it goes up from there.

Diffusion is a highly subjective matter. It is in your best interests to test multiple diffusion filters thoroughly before use. Not only must you look at the image on set, but also study its qualities on a large broadcast monitor in post production to see how it will match with shots that are cut before and after it. If you don’t take the discipline and pain to test diffusion filters, the results will be tacky at best.

You don’t always have to use glass to soften an image. You can use stockings, nets or petroleum jelly spread over a UV filter. I know you’re shaking your head at the last suggestion, but if you want to isolate only certain portions of your shot for diffusion, it’s a powerful DIY skill to have!

It’s tough to form generalized categories for diffusion filters, because each manufacturer comes up with unique solutions based on their target markets and experiences. In general, ask these questions:

• How much transparency or diffusion do they provide?
• Do they vignette? E.g., do they diffuse more at the center than at the sides, etc.?
• How many stops of light do they cut?
• Do they change colors?
• What kind of texture will you get at the end?
• How does it handle highlights and shadows?

Diffusion filters form an integral part of any cinematographer’s arsenal of tools. It’s so much easier and cheaper (and not to mention more artistically satisfying!) to get certain effects on camera rather than in a color grading suite. But it takes a lot of experience to use it creatively and with intent. This means, a certain brand of filter becomes as cherished as a lens is.

## The Infrared (IR) Filter

All camera sensors, whether they are CMOS or CCD, are sensitive to infrared (IR) radiation. This causes unwanted color casts on an image. Camera manufacturers already know this, which is why most modern cameras have in-built IR filters to prevent such issues.

However, not all sensors behave the same way. Cheaper camera models have poorer IR filtration. You don’t normally see this, until you start to use ND filters. When you use higher value ND filters, you cut out more light. Cheaper quality ND filters stop visible light, but not IR radiation.

What this does to the image is, it introduces a reddish or brownish cast. The higher the ND, the greater the cast. Beware, this is not generic. Different camera sensors react differently to IR, so without testing there’s no way you are going to know how to begin correcting for IR. Here’s an example of the Blackmagic Cinema Camera being tested for IR pollution:

You can see the visible shift in colors at higher NDs. It’s better to correct these issues on location with a filter than try to fix them in post production. The major problem in trying to do the latter is, the color cast isn’t uniform throughout the image. Various colors and objects in the scene reflect IR differently.

A few top-of-the-line ND filters also have some kind if IR filtration in them, so if you know your camera sensor is only sensitive to IR at certain ND values, it’s good to get these ND filters separately. They’re worth it.

Ultimately, it’s up to you to use IR filters in one of these ways:

• With a variable ND, use a dedicated IR filter at certain values only.
• With a fixed ND, use a dedicated IR filter if necessary.
• Use a fixed ND filter with in-built IR filtration.
• Use a dedicated IR filter (no ND), if you feel the sensor is sensitive to IR.

Note: What I have described in this section is an IR blocking filter, the purpose of which is to block IR radiation. There are also IR filters that only lets in IR radiation, which leads to interesting results:

## Mounting Filters

Filters can be divided into two broad groups:

• those that screw directly on to the lenses, and
• those that are used on matte boxes or filter holders.

This is an important consideration. Which type you choose will depend on whether you need a matte box or not (if you don’t know what a matte box is, read The Complete Matte Box Guide).

A circular filter is screwed on to the front of the lens. Sometimes, circular filters themselves have threads so that you can screw another filter on it, forming a filter stack.

You’ll find the filter thread size on a lens at the front or the side, alongside the Greek letter ‘phi’ (?). It’ll look something like this (look at the top):

This value is what you need to match when you buy threaded filters. In the above case, if you want to buy a filter to screw on to the Canon 40mm lens, you’ll need a filter with a 52mm size.

What are some of the disadvantages of circular filters?

One problem with screw-filters is that you might have to buy different filters for each lens filter thread. E.g., the Canon EF 24-70mm f/2.8L USM has an 82mm filter size while the Canon EF 70-200mm f/2.8L IS II USM has a 77mm filter size. You can overcome this problem by using adapters called ‘Step Up Rings’ to match the filter with the thread of your lens. Talk about the tail wagging the dog.

Secondly, circular filters are difficult to stack. Let’s say you you’ve stacked three filters and have now decided to remove the one in the middle. How many filters will you have to unscrew to get the job done?

Traditionally, circular filters have been a good fit for photography. They are light, and along with lens hoods, are easy to carry around. If you know you’re going to use only one or two kinds of filters, with common thread sizes, then circular filters will save you a lot of time and hassle.

Filter Holders

Filter holders are also mostly used in photography:

You can use standard size filters and carry a few holders that match your lens filter thread size. The big advantage of filter holders is that you can stack and remove filters easily.

The big disadvantage of filter holders is that they are fragile, and you have to handle your camera, filter and holder with care. I wouldn’t recommend them for video production use. Good filters are not cheap, and some even cost more than a lens!

Matte boxes

A matte box is just another step up from a filter holder, but much more solid, versatile and robust. On the other hand, it is much larger, heavier and slower to work with.

Matte boxes don’t have a universal standard for filter sizes. Some take both rectangular and circular filters.

Common sizes (in inches) for rectangular filters are:

• 4×4
• 4×5.65 (Panavision size)
• 5×5
• 5.65×5.65
• 6.6×6.6

Before buying, check the filters to see if they are compatible with the lens or matte box you have chosen. A standard size will give you more options.

As a general rule of thumb: Lenses with smaller thread sizes will work well with 4×4″ filters and matte boxes. PL mount lenses tend to have large thread diameters and need bigger filter sizes.

Unfortunately, pairing filters to mounts is not that simple. If you have decided that your production will be better off with a matte box of a certain size, but can’t find your desired filter at that size, you’re going to have to make some compromises. The people who make lenses are not the people who make filters are not the people who make matte boxes.

Don’t assume a filter-mount-lens-sensor combination will work just because it looks solid on paper. Testing is the only solution to the ‘filter’ problem.

## Suggestions for Filters

I will list both screw mounted (mainly 77mm filter size) and 4″ x 4″ matte box mounted filters, but these are just suggestions. The principle is the same for any size.

Don’t forget testing. As I’ve stated above, it’s basically somewhat of a chicken and egg problem. Your choice of filters, lenses and matte boxes all form part of a single system, and that is how they should be put together – at the same time.

 Filter Type Circular (77mm) Rectangular (4″ x 4″) UV B+W 77mm UV Tiffen 77mm UV Tiffen 4×4 UV-2A Haze UV Polarizing B+W 77mm Kaesemann CPL Schneider Optics 4x 4 True-Pol CPL Fixed ND Tiffen 77mm ND 0.9 Schneider Optics 4x 4 ND 0.3 Variable ND Singh-Ray Vari-ND Variable ND Heliopan Variable ND Fader ND Digi Pro-HD Grad ND B+W 77mm Grad ND Schneider Optics 4x 4 ND 0.9, Soft Edge Graduated Filter Diffusion Tiffen 77mm Glimmer Glass Tiffen 4×4 Gold Diffusion Special Effects (FX) Filter #1 Lee Diffusion Pack