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Filter factor

It tells you how much to increase exposure


Filter factors & corresponding stop values for daylight film.
Filter factors & corresponding stop values for daylight film.

EXPOSURE COMPENSATION

Most filters absorb some of the light that would reach the film or a digital camera's sensor if there was no filter in its path. Since less light strikes the film/sensor, exposure must be increased to compensate for this loss. This is achieved by opening the aperture wider or by increasing the time that the shutter is open. When you do this, it is known as “exposure compensation,” because you are compensating for the filter’s effect on exposure.

This additional exposure varies with the particular filter in use. Some filters, like the Wratten No. 25 deep red filter for use with black and white film, are so dense that they require exposure compensation of four stops, while others like the UV filter are transparent and completely-neutral with respect to visible light, and therefore require no compensation at all. Neutral density filters, on the other hand, are designed to greatly reduce light transmission, and have very high filter factors that permit very slow shutter speeds, even in bright daylight.

FILTER FACTOR

The amount of exposure compensation has been predetermined for every filter, and is expressed as a “filter factor” (sometimes also called an exposure factor, and also referred to as Exposure Magnification or EM values). A filter factor is a number that indicates to what extent you must increase exposure when you use a particular filter (by multiplying the unfiltered exposure by the filter factor number). A filter factor of 2, for example, means you will need twice as much exposure, and a filter factor of 3 means you will three times as much exposure.

HOW TO USE A FILTER FACTOR

You must first obtain a meter reading of the scene you wish to photograph without the filter. If you have a hand-held meter, this is easily done. If you are using a through-the-lens meter, take your reading with no filter attached.

If your filter has a filter factor of, say 2, which you know requires twice as much exposure, you must increase exposure by one stop, which allows twice as much light to reach the film. So, if your meter reading without the filter was 1/250 sec. at ƒ11, you must either decrease the shutter speed by one stop to 1/125 sec. or increase the aperture by one stop to ƒ8. If you take your picture at the new exposure setting with the filter attached, the film/sensor will be properly exposed.

A filter factor of 4 means the film/sensor require four times as much light to be properly exposed. You must therefore increase exposure by two stops, since each stop doubles the amount of light that gets through. Using the example from the paragraph above, if your meter reading without the filter was 1/250 sec. at ƒ11, you must either decrease the shutter speed by two stops to 1/60 sec. or increase the aperture by two stops to ƒ5.6 for correct exposure with a filter that has a factor of 4.

But what if the filter factor is 3? How many stops is that? Well, a filter factor of 3 requires three-times more exposure. Since a one-stop increase doubles the amount of light reaching the film/sensor (for a filter with a factor of 2), and a two-stop increase quadruples the amount of light reaching the film/sensor (for a filter with a factor of 4), your exposure increase for a filter with a factor of 3 will be between one and two stops. It is, in fact, 1 2/3 stops. (See the table on the left.)


THE EASY WAY

There is an easy way to compensate for filter factor. Take a normal exposure reading as if you were shooting without a filter, and adjust your camera settings accordingly. Now, multiply the filter factor by the shutter speed. For example, if the filter factor is 4 and your shutter speed is 1/500 sec, multiply 4 X 1/500 = 4/500 or 1/125. You can now use that filter by changing your shutter speed to 1/125 sec while keeping your aperture setting the same, and your picture will be properly exposed.

AN EVEN EASIER WAY

A fast, efficient way to compensate for the use of a filter is to divide the factor into the film speed or your digital camera's ISO sensitivity setting. For instance, ISO 100 divided by a filter factor of 2 equals 50. Set your camera's light meter at this new ISO rating when using the filter (unless your camera is an SLR with TTL (through-the-lens)metering, in which case, the camera's meter will automatically compensate for the filter, and provide you with a proper exposure reading).

WHAT IF I USE MORE THAN ONE FILTER?

Using two or three filters at the same time will require an increase in exposure based on the factors of each filter. It is calculated by multiplying the factors together. For example, the combined factor when stacking three filters that each have a filter factor of 2 is 2 X 2 X 2 = 8, which requires an increase in exposure of three additional stops.

FILTER FACTORS & CORRESPONDING STOP VALUES

The table above shows the number of stops to increase exposure for various filter factors when using daylight film.

Except for completely-neutral varieties, the use of filters requires additional exposure because they prevent some of the light from reaching the film.
Except for completely-neutral varieties, the use of filters requires additional exposure because they prevent some of the light from reaching the film.

DO I REALLY NEED TO KNOW HOW TO COMPENSATE FOR FILTER FACTORS?

Not if your camera has a built-in TTL (through-the-lens) light meter, since the camera’s light meter accurately measures the light with the filter attached and automatically compensates for the light absorbed by the filter. (Be aware, though, that some filters, especially deep red, can fool your automatic light meter, and it is a good idea to check your camera's manual for information on metering through filters.)

If you use a hand-held light meter, however, or if your camera’s light meter should fail and you still want to take pictures using, say, the Sunny 16 rule to figure correct exposure, you need to compensate for the filter factor.

FILTER FACTORS DIFFER IN TUNGSTEN LIGHT

Manufacturers usually provide one set of filter factors for use with daylight film (including electronic flash illumination, which simulates daylight) and one set for artificial illumination. The different wavelengths of tungsten light, which contains more red light, require different filter factors.

WHAT IF I DON’T KNOW THE FILTER FACTOR?

If you have no indication what the factor is for a particular filter and you aren’t using TTL metering, you can use a hand-held meter, preferably with the translucent dome removed (or ideally the meter will have a flat diffuser). Take a normal exposure reading of the sky or another unchanging light source, then take another reading with the filter over the meter’s sensor, making sure no light is getting in at the sides. The difference in the readings will give you the increase in the number of ƒ-stops necessary for you to use the filter, and you can easily convert this to a filter factor number for future reference. Of course, if your light meter is set for the film speed you are using, you can use the second reading to obtain the required exposure settings. This is a bit rough and ready, but is better than guessing.

HOW ACCURATE ARE FILTER FACTORS?

Filter factors are applicable under average lighting conditions, and therefore should be considered as a reliable guideline, but should not necessarily be treated as definitive. You may find that applying a particular filter factor results in over or under-exposure most times you use a given filter, which means that the filter factor is inaccurate for much of your photography. You will need to further adjust your exposure one way or the other when using that filter. Once you know how much of an adjustment is needed, then you can assign your own factor to that particular filter for future reference.

The other thing about filter factors is that you can use some leeway in applying them. Intentional underexposure by half a stop or so will often improve a scene by adding more contrast when using certain filters. Experimentation will let you know how far you can veer away from the recommended filter factor, and what the effects are on your pictures when you do. Keep in mind, too, that a filter factor of 2 can even be ignored completely with the wide exposure latitude of most black and white films without affecting the film too much. Your negatives won't be perfect, but you will be hard-pressed to detect the differences, and the prints you make from them will be fine. To some extent, the same principle applies to digital cameras, since a good image-editing program like Adobe PhotoShop will allow you to make adjustments to a digital image file to compensate for exposure variations, provided they are not too severe.