Filters are one of the least expensive accessories you can use to make big changes in the way your pictures look. Obviously, digital photography has changed the use of filters and photo-editing software has eliminated the need for using many of the filters. For the purposes of this discussion, I’ll limit the scope of my information to filters that can be used to alter your photographs, be they digital, color, or B/W film.
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Filters come in two basic forms. The most common is a transparent disk of colored glass or plastic that is mounted in a metal frame and screwed into the threads on the front of your lens. What size filter you choose depends on the maker of the lens and the lens you are putting it on. They come in sizes ranging from 48mm to 77mm. Look on the front of your lens and you should see a number like 52, 56, 58, etc. That is your filter size. The other common type is typically part of a filter system where you attach a mount to the front of your lens and the filter is often a square of colored plastic which slides into a slot in the mount. Cokin is probably the best known filter system of this type.
UV, Skylight, and so-called protective filters
Anyone who has ever purchased a lens has had the salesmen offer to sell them a protective filter. The pros and cons of using a protective filter will not be discussed here, but I will mention that any filter adds one more obstacle to the clear path of light from the object to the film (or sensor). As such, it degrades the image, even if only slightly and also increases the likelihood of flare. My personal advice on this issue is only use a filter if there is a specific need to use a filter although I am sure that many others will disagree on this point.
A UV filter is used to reduce UV rays from entering the lens and striking the film. Modern lenses have UV protection built into them as do digital sensors. This lens is probably the most frequently used lens for protection.
A skylight filter (also known as a 1A filter) has a very pale magenta tint which is designed to slightly cut down on blue light and UV light. The effect is negligible with B/W film and is mainly an issue when using color film in situations where light is slightly bluish such as that light found in hazy or cloudy weather. If the light is a stronger blue, then one would use a stronger magenta filter such as the 81A or 85C. The problem with using these filters is that you really don’t know which one you need until after you develop your film. A good rule of thumb would be that the skylight filter works well for overcast or hazy conditions, but would fall short in open shade or at higher altitudes. While digital cameras can surely get the same benefits from these filters, it is a simple matter to correct color casts using post editing software.
Neutral Density (ND) filters are filters that are designed to reduce the amount of light entering through the lens. They come in handy when you’re in a bright light situation and you want to shoot at wider apertures to minimize depth of field or slower shutter speeds to show the motion of water. The come in various densities to reduce light by 1, 2, 3, or more stops. It is possible to stack 2 filters together to increase the density as needed. Similar to the ND filter is a Graduated ND Filter (GND). GND’s have a gradual change from clear to gray, again in varying degrees of density. They are used when there are significant tonal differences between the foreground and the background. Taking a picture of a beautiful scene can be often spoiled when the camera’s meter properly exposes the sky but leaves the foreground in deep shadow. By using a GND, one can decrease the light from the brighter skies by several stops and properly expose the foreground. GND’s come in hard or soft versions. This refers to the abruptness of the change from gray to clear. Hard GND’s are good when there is a clear delineation at the horizon, such as oceans or flatlands. When trees and mountains are in the scene, a soft GND will help blend at the horizon level. GND’s are generally square and fit in a special holder, such as Cokin, and can be slid up and down inside the slot to align the horizon line to the transition line. ND and GND filters work well with film and digital cameras.
No camera bag should be without a Polarizer Filter, whether you shoot digital, color film, color slide, or black and white. A polarizer is different than most filters in that it is composed of two pieces of glass, mounted independently inside a round frame so that one can be rotated relative to the other. As it is rotated, it increasingly cuts out glare and haze. With an SLR, you can actually observe this change through the viewfinder. Polarizers reduce glare from water, glass, or any non-metallic surface. It will not reduce glare from metal. You can also use a polarizer filter to darken blue skies. The blue of the sunlit sky is partially composed of polarized light. If you cut down on the amount of this light, you darken the image, so the sky appears darker. Polarizers come in two versions, linear and circular. Without going into the mechanics and physics involved, all you really need to know is that autofocus cameras will perform best with a circular polarizer.
Fluorescent lighting provides another problem for color film/slides. While they come in a variety of different types, all fluorescent bulbs lack sufficient warm reds, yellows, and orange light while having excess green. For digital camera users, you simply adjust your white balance for the correct color temperature. For color film users, a filter is required. If you’re shooting with daylight film, you’ll need a FLD filter. A FLB filter is useful when shooting type B tungsten film under fluorescent lighting. Unfortunately, these filters are only half the battle and professionals generally resort to CC filters (color compensating). Which filters to use depends upon the color mix of that particular fluorescent bulb.
Filters for BW film are used to make one gray tone stand out more from the others. Since colors show up as shades of gray on BW film, one must have a way to differentiate between the blue dress of the model and the green background. A filter transmits light of its own color and holds back light of other colors. Let me repeat that… A filter transmits light of its own color and holds back light of other colors. A yellow filter transmits light of yellow objects and holds back light of other colored objects. The same holds true for a green filter and a red filter and any other color filter you choose to use. Yellow filters: The darker the yellow of the filter, the darker-gray the sky will appear. For an even darker sky, you can use a green filter and the white clouds will stand out even more. A red filter will produce a sky that is almost black. Of course, filters are used for more than just changing the color of the sky. If you shoot BW film, you should own several different color filters for altering the film.
In addition to the above filters, there are many specialized filters for producing special effects. Starlight filters, multiple image filters, starburst, softening filters…the list goes on and depending on the type of photography you’re doing, you may want to explore some of the possibilities available.
One final note that must be covered is that filters reduce the amount of light available to some degree. For instance, a polarizing filter will reduce light anywhere from 1 to 2 stops. Unless you’re using your camera’s built in meter to determine exposure, you must compensate for this light loss. Since most people here are probably using an automatic camera with TTL metering, this shouldn’t be an exposure issue, but is worth mentioning as you’ll find yourself in situations where the light loss is significant enough to force you into using slower shutter speeds or wider apertures.
While I didn’t cover every filter available, I think I highlighted the major ones that we come across frequently. While Photoshop and other editors do a great job of altering images and reproducing the effects of filters, there is no substitute for a polarizer. Even a ND filter will come in handy for those photographers that like the silky, ribbon effect of flowing water.