Last update: June 6, 2012 (to restore missing section on ND filters).

A. General Orientation

Q. What are filters?

A. Filters are simple items, little pieces of glass in metal or plastic mounts. You should use them if they are helpful. You should avoid them if they are not, because they will rattle around in the bottom of your camera bag and scratch something you really care about.

Q. What are normal uses for filters?

A. Filters have several legitimate uses. They usually address problems.

• Correcting color balance with color film. This can include a reddish cast at sunrise/sunset, a bluish cast at noon, or using color film for tungsten light and vice-versa. These goals correspond to the 81-series (amber) and 82-series (blue) filters.
• Eliminating bluish shadows on color film (Skylight 1A and 1B - also called L1A and L1B or KR1.5).
• Minimizing unwanted reflections from reflective objects (linear or circular polarizers).
• Changing tonal relationships between objects in black and white photography (blue, green, yellow, orange, and red contrast filters).
• Reducing effective film speed.
• Making effects that do not exist in nature.
• Cutting out specific types of light (infrared cut, UV).
• Some combination of the above.

Note that lens protection is not listed here - using a filter for protection is not an optical function of a filter, it is a mechanical one. Additionally, listing this (the vestigial function of a UV filter) may seem to legitimize the use of UV filters, which are in most ways less useful than mildly tinted skylight filters.

Q. Who makes the best filters?

A. Difficult to say, because the products vary wildly across manufacturers. Here are the top brands (and you need not look much further, since life is short, these are good and filters are on the whole pretty cheap). Moreover, I suspect that there are only a couple of Japanese producers making most brands you see.

Hoya (Japan) is the price/performance leader. Hoya owns one of the biggest glass works in the world; Tokina makes their filters. Hoya filters (blue package) are coated, solid optical glass. The black package is multicoated. Hoya makes most major types of filters for color and black and white. Hoya mounts are increasingly plastic, which although distasteful to puristss, is a much better material to screw into aluminum lens threads than... aluminum. Kenko is Hoya's companion brand in Japan.

B+W (Germany) is probably the best brand going. Their filters are made of solid, coated Schott glass and are meticulously tested. That said, their color/type identification system is non-standard and most of their mounts are too tall to use with ultrawideangle lenses. The mounts are luxuriously finished anodized brass. The downside is that although these filters screw into modern aluminum lenses smoothly, they are not so easy to screw into older brass lenses. The best B+W filters are the MRC line, which are multicoated and dust resistant (this has to be seen to be believed). Heliopan, also made of Schott glass, is not quite as well controlled-quality wise, at least if you believe Ken Ruth.

Nikon (Japan) makes relatively inexpensive multicoated filters from solid, optical glass. Relatively inexpensive compared to B+W. Nikon's sizes are constrained mainly to sizes that fit lenses Nikon sells. Not a lot of variety in colors.

Kodak (USA) makes (or sells) Wratten gel filters, which are the world standard for color accuracy and consistency. The Wratten gels are dyed gelatin on a polyester substrate. You mount them in little frames which attach to the front of your lens. They are adaptable to any camera system. Gels are very fragile and expensive, but they come in every color or density ever thought of. And they're so thin they have little or no refractive effect.

Tiffen (USA) takes Wratten gels, sandwiches and cements them between two sheets of uncoated glass (true of all of their filters except UV and Enhancing), and mounts them in metal rings that screw onto your lenses. Tiffen mounts are very heavy-duty, and they are a staple for motion picture work. Tiffen is most known for its soft filters, which are various types of softening materials (like fishnet) sandwiched between two pieces of glass. Tiffen filters are too thick to use in a lot of superwideangle applications.

Cokin/Lee/Kood/etc. These filters are typically made of CR39, which is the same thing most eyeglasses have. There is nothing inherently wrong with plastic, provided that you understand it is not coated and that scratches are somewhat easy to achieve (suffer?). Among these, the quality of the plastic does not seem to vary much, but the quality of the dye (particularly for ND grads) does.

Q. Should my filters be (multi)coated?

A. Coatings serve two purposes: (1) increase transmission by (2) decreasing flare. Flare (and light loss) can account for up to 6% per uncoated surface. Multicoating exists to collect fingerprints, frustrate cleaning efforts, and to cost more.

With black and white contrast filters (described below), transmission is so low (15-75% of what comes in) that not having a coating may not make any difference, except possibly for ghosting.

With "clear" filters like skylights, transmission is high and flare should be kept as low as possible (the light entering is stronger), so coatings are pretty important. Multicoating is nice, and at least theoretically reduces flare even more, but most multicoating on filters (green in color) is notable primarily because it is very difficult to clean. The exceptions are Contax MC filters and B+W MRC.

Note that due to some interactions between concave front elements and filters, you can get extra ghosts. A case in point is the kit zoom for the Sony NEX series.

Q. What are Series filters?

A. Back in the day, when dinosaurs roamed the earth, a number of American filter manufacturers started a system called Series, by which the hundreds of disparate filter thread sizes could be adapted to six different sizes. See a cross-reference table here. Typically, series filters are mounted in plan rings and drop into adapters which screw or clamp onto your lens. You then hold the filter in by screwing in a retaining ring or a lens hood. The most useful size is Series VI (41mm diameter), which fit adapters to go on lenses with thread sizes from 34mm to 49mm.

The advantage of Series filters today is that if you have a number of lenses with different, exotic filter sizes - say a Summar with 34mm thread, a Canon with 40mm thread, a Contax with 40.5mm thread, and a Nikon with 43mm thread - you can buy several adapters but only one of each type of filter. The downside is that Series filters are primarily designed for 50-100mm lenses. There are few effective adapters for wideangle lenses (Canon made some for its 35mm rangefinder lenses), and the total "stack" of adapter, filter, and lens hood is often too thick for lenses wider than 35mm.

Series filters follow one of two basic types of constructin. Wratten and Tiffen Series filters generally are constructed of a gel filter cemented between two pieces of optical glass. These gel filters are plentiful and cheap, but watch out for fading and fogging (one caused by sunlight, the other by humidity). The other variety is solid, coated optical glass. In this line, Ednalite Heritage filters ($2-5) are the best buy for the money. The best filters at any price are B+W and Leica ($30). Next comes Hoya (yes, they did make series filters for a short time) ($10-20 if you can find them). Then Spiratone (eerily similar to Hoyz) and Walz ($5).

Q. Where can I get specialized filter adapters?

A. You can get all the exotic adapters you want at SRB Film Service. For a price.

B. Filters for Color

Q. What is a polarizer?

A. A polarizer is a gel filter with millions of microscopic pigment lines in it. The name is derived from the fact that light travels along one axis (toward or away from the subject), but in its wave form, it oscillates (moves up and down) in another direction. When light reflects from a non-metallic object, it takes a specific orientaion - all of the light waves vibrate in substantially the same direction. The polarizer takes advantage of this by acting as a gate. Because most polarizerscan be rotated around the lens axis, you can turn it until the microscopic lines are perpendicular to the vibration of the wave. This knocks out the reflection. Not only does a polarizer allow the lens to see past reflections (as on glass or water), they can also be enlisted to darken a blue sky.

Among polarizers, there are linear polarizers and circular polarizers. The only thing you really need to remember is that the circular polarizer must be used where the camera meters or focuses through a semitransparent mirror (as on a 35mm SLR). Because circular polarizers are much more expensive, you should only buy one if you need it.

Most polarizers are sandwiched between two pieces of glass and set in a rotating double mount. This double mount typically has an index mark with which you can figure out how the polarizer is oriented. Very few polarizers are made with coated glass, because (1) they are already cutting out a huge amount of light and (2) their "gate" function helps prevent oblique light rays from entering the lens. For this reason, many polarizers also lack a front thread for lens hoods.

Q. What is an Enhancing (Intensifier) Filter?

A. An Enhancing (or Intensifier) filter is a piece of didymium glass. Didymium glass was originally used for welding goggles and jewelry. It is made of one or more types of rare earth elements mixed with normal optical glass. Rather than cutting the entire end of the light spectrum (leading to an overall red tint), Didymium glass has a flat response across the spectrum, with a "spike" for a specific frequency (like red). It can be made to enhance red, green or blue. You can tell if you have a piece of it by looking at it against sunlight, room light, and florescent light. You will see that the glass changes color.

The red Enhancing Filter was invented by Howard Ross, a retired Ford Motor Company engineer. His filters are very thick and made of precision-ground optical glass. This glass normally has a pinkish tone. It transmits all wavelengths at 50% except for red, which is about 95%. According to Ross (who discussed this with me in person), Tiffen got the idea from him.

Tiffen Enhancing Filters were something of a "me-too" product. They do not have nearly as strong an effect as the Ross and look bluish. It is my understanding that they are not coated and not made from optical-grade glass, though this is a claim often made about Tiffen's substrate glass. There does not seem to be any degradation, in any event.

Hoya Intensifying Filters come in red, blue and green flavors. They are coated optical glass. Their effect is a little bit weaker than the Ross filters.

Q. What is a Mired/Decamired filter set?

A. This is a fancy way of saying light balancing filter set for color film. They typically come in sets of 3 or more filters in amber and 3 or more in blue, with different densities of amber and blue designed to compensate for different lighting conditions (like electronic flash, daylight, morning/evening light, etc.). The term "decamired" is now used as a generic term for these sets.

Strictly speaking, a "mired" is a uniform unit of color correction. "Decamired" technically refers to 10 mireds (mireds are themselves quite small units, not as noticeable).  A mired (microreciprocal-degree) is a color temperture unit defined such that a one unit shift creates an equivalent color shift at any color temperature (thanks Henning Wulff).

Because people use mostly digital or negative film now (easily corrected at the printing or scanning stage for small differences in color temperature) most of these filters are obsolete. The two big exceptions are the darkest blue, which is used for shooting tungsten light on daylight (most) film. The other, the lightest amber (now called an 81A), can be used as an all-purpose light warming filter for overcast days or as a substitute for a skylight.

Q. What is a warming (812) filter?

A. This is Tiffen's name for a proprietary light amber filter used to enhance skin tones.

Q. What is a Hot Mirror filter (generic name: UV/IR cut)?

A. This is a filter designed to reflect infrared wavelengths and transmit visible light. This was used for a lot of early digital cameras whose sensors were overly sensitive to IR radiation, mounted on the front of the lens. The newer cameras have this filter (also confusingly called an infrared filter) on the front of the sensor or somwhere else in the optical path, but inside the camera (thanks Henning Wulff).

A Tiffen Hot Mirror filter is at the light end. A Leica UV/IR is a moderately powerful hot mirror filter. And a B+W 486 is the strongest type you would normally buy.

As you go from weakest to strongest, you get more and more pronounced color shifts toward the edges of the frame. You can gauge relative color shifts by holding the lens at arm's length and rotating it. The faster the filter goes to a greenish tinge, the stronger it is. Color shifts were not a big issue with the original hot mirror filters, which were designed for small sensors. With the Leica M8, the camera manufacturer built in automatic correction dependent on the lens. With the B+W equivalent (which Leica owners are forced to use for lenses with non-Leica filter thread sizes like 40.5mm, 43mm, 52mm and 62mm), it often helps to use a program like Cornerfix.

Q. What is an Infrared filter?

A. Infrared filters block most (or almost all) visible light, so that infrared film (which is sensitive to all visible and nonvisible light) will pick up only infrared waves. A red 25A filter is where the fun starts; true IR filters look like they are a very dark red - or opaque. Infrared filters require a focus correction, so the correct way to use one is to focus (an SLR) without the filter, place the filter, and then turn the indicated distance from the center mark to the infrared index mark (usually red). If you do not have this mark, and your lens extends when it focuses (prime lenses only), the infrared correction is a fixed percentage of lens barrel extension (I believe 1/70 or thereabouts). You can use a micrometer to measure lens extension.

C. Skylights, UVs, Window Glass, and ND filters

Q. What is the difference between a skylight, a UV, and a Clear Protector?

A. Skylight filters (also called 1A, 1B, L1A, L1Bc, and KR1.5) have a mild pink tint. They are designed for color photography and compensate for the fact that skylight is very blue and causes blue shadows (shadows have a larger proportion of this kind of light). They have no palpable effect on b/w shots.

A. UV filters have a yellowish tint. They absorb UV light, which at altitude can cause haze in color pictures. These are largely obsolete, both because lens cement and lens glass tends to cut a lot of UV out of the equation. Some testing has revealted that some manufacturers' UV filters (particularly low-end filters) actually do nothing! No effect on b/w shots.

A. Clear Protectors serve no purpose except to protect your lens from your fingers and sometimes create flare. No effect on b/w.

Q. Should I keep a skylight or UV on my lens at all times?

A. Ask ten people and you will get ten different answers. Here are some pointers on why you might want to keep a clear (but at least somewhat) functional filter on your lens at all times. Even the anti-filter people, to my knowledge, agree with these grounds:

• High atmospheric pollution in your area
• You can't keep your fingers off the front of the lens
• You might encounter sand or spray
• You drop lenses or bang them into things often

If you are not careful, a skylight or UV can actually degrade lens performance:

• When it gets fingerprinted or dusty
• When it gets scratched up
• When a lens is well-shielded from flare (as with a Macro) and the filter essentially makes for a flush-mounted "flare catcher"

It is cheaper to clean or replace a filter than a front lens element, but today, many front coatings are exceptionally hard - hard enough to stand up to typical cleaning.

Q. What is a neutral density filter?

A. A neutral density filter cuts down on the amount of light reaching the film or imager. This effectively lowers film speed and allows you to use a wider aperture (for less depth of field), a slower shutter speed (to synch flash or to induce motion blue) or both. The "neutral" part is supposed to correspond to neutral in wavelength. These filters are shades of grey or black. ND filters are denoted in effect on exposure (2x, 4x, 8x, 100x, etc.) or density (0.3, 0.6, 0.9, etc.). A 2x (0.3) requires you to open the aperture one stop (or slow down the shutter one stop); an 8x (0.9) is three stops or three shutter speeds. Where the denotation is in density, every 0.3 increment equals one stop. Good ND filters are color-neutral; the color balance of one of these filters can make a big difference on very long exposures.

In general, ND filters impact focusing on SLRs and mirrorless cameras because less light reaches the mirror or sensor. At low gradations, this is not a problem, but it can be an annoyance.

One variation on ND filters is the graduated neutral density, which either takes the form of a square filter that can be skewed and rotated to darken part of the picture (the sky, for example) or a round one that puts the "horizon" in the middle and allows rotation around the center axis. In general, it is desirable to have a 0.6 or 0.9 to make a big enough difference to see. Whether a ND grad transitions gently or sharply depends on your lens focal length; wides should have soft gradations and telephotos should have harder ones. ND grads are best used on cameras with TTL viewing but can be rigged to rangefinders and other indirect-view cameras.

Another variation is the "fader." This is not a neutral-density filter at all; rather, it is two polarizers that can be rotated relative to each other. The theory is that the more the screens cross, the less light gets through. When two polarizers are at right angles to each other, all light should be blocked. But because no polarizer is 100% efficient, some light will come through. The minimum filter factor is about 2.5x on these; maximum is generally 400x. Good faders are expensive; the cheap ones will have color casts that will affect color photography.

D. Black and White Contrast Filtration

Q. What are contrast filters?

Contrast filters are generally red, orange, yellow, and light green. They tend to allow transmission of light that is the same color as the filter and promote tonal separation in that color. They tend to suppress other colors. This drives some tones into being more "black," thus skewing the tonal range of the black and white negative.

Q. What are filter factors?

A. Everything and nothing. Absent a context for discussing them, filter factors are completely meaningless. In broad brush, filter factors are the exposure compensation that is typically applied when using filters. They are fairly straightforward when using neutral density, skylight, or polarizing filters, but they become much more complex in contrast filtration for black and white work.

A filter factor describes the nominal exposure multiplier required to properly render a greyscale subject using a given filter. Think of it this way: white (or grey or dark grey) light is made up of all colors. When you slap a contrast filter on the front of your lens, you suppress some of the colors. Thus, white and all greys down to black are affected, even if they are not complimentary colors. For example, if you use a dark yellow filter, it will suppress blue - but it also suppresses part of what is needed to correctly render greys and whites. So say you are using a G filter - if what you are worried about is a full range of greys, you need to increase the exposure (typically) by 2-1/2 stops.

Here is some nominal data for pictorial filters (information from Kodak), what they block, and what they let through.

Now here is Kodak's recommendation for exposure compensation for various standard filters (this is a current Kodak chart):

Filter Factors for KODAK Black-and-White Films
Filter Number	Color of Filter	Plus-X Pan and Tri-X Pan Films (Values in parentheses are for T-Max Professional Films)
		Daylight		Tungsten
		Increase the exposure by this factor	** OR** Open the lens by (f-stops)	Increase the exposure by this factor	** OR** Open the lens by (f-stops)
3 (K1)	Light Yellow	1.5	2/3	---	---
8 (K2)	Yellow	2 (1.5)	1 (2/3)	1.5 (none)	2/3 (1/3)
9	Deep Yellow	2	1	1.5	2/3
11	Yellowish-Green	4 (3)	2 (1 2/3)	4 (3)	2 (1 2/3)
12	Deep Yellow	2 (2)	1 (1)	1.5 (none)	2/3 (1/3)
15	Deep Yellow	2.5 (2)	1 1/3 (1)	1.5 (1.5)	2/3 (2/3)
21 (G)	Orange	5	2 1/3	4	2
25 (25A)	Red	8 (8)	3 (3)	5 (4)	2 1/3 (2)
29	Deep Red	16	4	8	3
47	Blue	6 (8)	2 2/3 (3)	12 (25)	3 2/3 (4 2/3)
47B	Deep Blue	8	3	16	4
58	Green	6 (6)	2 2/3 (2 2/3)	6 (6)	2 2/3 (2 2/3)
61	Deep Green	12	3 2/3	12	3 2/3

Now you probably have a couple more questions.

Q. What do I do with black and white filters?

A. Here is a brief rundown. Note that these are my classification of colors; often, manufacturers use the same name for different items. If you dig into B+W or Heliopan catalogs, there are many more variations, but these are pretty much the only ones that matter.

• Green (looks like RGB green; Hoya X1, B+W 061, Heliopan Green 13) (3x). This is a foliage filter and one for taking pictures of people with tungsten light.
• Yellow-Green (looks light a light lime green; Hoya X0, Nikon X0, B+W 060, Heliopan Green 11) (2x). This is a surprisingly useful filter. It enhances skin tones outdoors, enhances foliage tones, and darkens the sky a little bit.
• Light yellow (Rollei Sport Yellow, B+W 021, Heliopan Yellow 5) (1.5x). These filters were once de rigeur for ortho films that had trouble seeing yellow tones. They are useful as protective filters, mildly darken skies, and are otherwise benign. I would not pay any significant amount of money for one of these.
• Yellow (yellow) (Hoya K2, B+W 022, Heliopan Yellow 8) (2x). This has been the standard black-and-white filter for years, particularly the years where orthochromatic film had a hard time seeing anything but blue and green light.
• Dark Yellow (lager beer-colored; "minus blue", Tiffen 15, Wratten G). The true "G" filter is designed to hit yellow by two stops and not much else. It seems to be extinct except in gelatin filters and the Tiffen line (which uses them). It is functionally very similar to yellow-orange.
• Yellow-Orange (Hoya G, B+W 023) (3x). This is probably the limit for contrast filters that are useful in normal conditions. If you push your film at all, this is the absolute limit. Skies go dark; grass goes dark; shadows become empty. Blemishes on human subjects lighten, but their lips do too.
• Orange (Rollei orange, B+W 040, Nikon 056) (4x). The B+W and Nikon designations as "orange" are probably on the yellow-side; these are in reality a rich red-orange. They dramatize clouds but really kill shadow detail and foliage.
• Red (Hoya 25, Rollei Red, B+W 090) (5x). There was a time when red filters were used to make lunar-style landscapes. That said, these filters radically reduce the tonality of just about every type of black-and-white film, slow down exposures, and even throw focus off. Today, their major useful purpose is as a mild IR filter for use with infrared.

Q. Why doesn't the overall transmission number match the filter factor given by the manufacturer?

A. The difference in f/stops has little to do with the rated filter factor for each filter. This is a product of the fact that filter factors are computed using an "average scene," not a pure white light containing all colors in equal proportions. A scene taken in bright noon sunlight, for example, has a suprising amount of blue light in it. In addition, modern panchromatic film is not uniformly sensitive to every wavelength of light. You can see from the chart that T-Max films are a little bit less sensitive to blue than the older films (Plus-X, Tri-X) are.

Q. Why doesn't an orange filter make the blue sky black?

A. Second, if this chart is right, then an orange filter should make the blue sky black, or Zone 1. Why doesn't this happen in practice? The easy answer is that the sky is not completely blue. The sky is blue, but it also contains red, orange and yellow light in smaller proportions. The same is true of any object that has a color. There are virtually no real-world objects that reflect excatly the wavelength that corresponds to their color. A red ball rarely, if ever, reflects only 700 µm light.

Q. When are filter factors actually useful?

A. Filter factors are useful on neutral (grey or white) subjects.

Q. Can applying a manufacturer's filter factor hurt my photography?

A. Yes. Filter factors are useless when your subject contains large swathes of color that you are trying to eliminate and perfect greyscale reproduction (rather than relationship) is relatively unimportant. For example, grey cement or limestone buildings against a blue sky on a sunny day can cause tonal merger without a filter. However, using a blue-cutting filter and then applying the full filter factor may bring the blue sky back up in brightness to the point that the filter is largely useless. The correct approach is to view this as an exercise in tonal relationships rather than reproduction and to apply less than the full filter factor (if at all). Mechanical application of filter factors takes black and white photography out of what it is - rendering perception, not robotically reproducing reality. When you see a middle-to-light-grey objection against a sky, you think that they sky is darker. The reality is that the sky is lighter - which is why people have such problems when attempting to use filters to darken it.

These factors are also (egregiously) useless when you are using extended development to increase the tonal range of the image). If you are using N+1 or N+2 expansion, you need to make sure that subject colors you are trying to suppress remain as close as possible to middle grey - so that these will not be lifted several tonal values when you extend development. In this situation, there is a good case to be made for applying no filter factor.

Q. How should I meter with filters?

A. Divorce your exposure readings from the filter itself, unless your filter is color-neutral (polarizer, skylight, etc) or has a very mild effect on exposure (color correction filters). Most exposure meters are sensitive to one color of light only; using a yellow filter in front of a blue-sensitive meter will cause an incorrect compensation. As noted about, wildly-varying subjet color distributions will lead to inconsistent and unwanted results. The sole exception is a Zone VI spotmeter, which is calibrated to react like black and white film - and reads only one degree at a time. If your exposure meter is outside the optical path that includes the filter, don't worry about this. For example, if your camera's exposure sensor is in a non SLR-type viewfinder or somewhere on the body other than behind the filter, you need not worry.