Using Photographic Filters

Filters 101 - What do they do?

 Filters have been around almost as long as cameras. As cameras got more sophisticated and photographers became more creative, filters were used to modify light to provide artistic or practical enhancements to the image.

Photographic filters in their simplest form consist of an optically flat sheet of glass or resin with a tinted coating. Some are made by adding coloring to the glass as it is manufactured or by sandwiching the color layer between two sheets of glass. However they are made, their purpose is to absorb (block) light of a particular wavelength and change the way the image is recorded by the camera.

In the early days when black and white was the only game in town, yellow, orange, red and green filters were used to alter the contrast of an image. Yellow filters markedly increased the contrast in scenery shots (like a pair of shooting glasses) with slightly darker skies and lighter foliage. Orange and red do the same, but with more pronounced results. A black and white photo with a red filter will have very light foliage and very dark skies. A green filter will produce a lesser effect since it is in the middle of the visible spectrum and was often used to even out and enhance skin tones in portraits.

With the advent of color film, filters were used to compensate for the color shift caused by using different lighting than the film was calibrated for. Since the chemicals in color film react very predictably to light, film was commonly made for the color temperature of “daylight” for outdoors and “tungsten” for use with the color temperature of incandescent studio lighting. If you shot with Daylight film outside, a white object would show up as white. The same would be true with the Tungsten/studio light combination. If you had tungsten film loaded in your camera and you wanted to shoot outdoors, you had to put a light brown(ish) 85a filter on to keep the pictures from coming out with a heavy bluish cast. If you had daylight film loaded and wanted to shoot under home or studio lights, you had to use a blue 80a or 80b filter to keep the pictures from having a yellow cast. There were variations of each to compensate for different colors of standard lighting or to add a slight warm cast while using daylight film in open shade or on a cloudy day. A pink FL filter was added later when fluorescent light became common to cut the green cast when daylight film was used. The goal of using the right film/light source/filter combination was to render a white object as white which would ensure that all other colors appeared as true as possible on the final print or slide. This was the only way to set white balance with film. Digital cameras automatically adjust white balance to match the light source. Most do very well with daylight, shade, flash and studio lights. Most have a hard time with tungsten since different wattages and bulb coatings produce different temperatures (compact fluorescents can be even worse). Many models allow you to over-ride the auto setting and some even let you adjust the color temperature manually or shoot a picture of a white or 18% grey card and use that to calibrate the camera.  

Note: Light “color” is measured in degrees Kelvin “K”. The Kelvin scale is similar to Celsius, but instead of 0° C being at the freezing point of water (32° F), 0° K is -459.67° F where all molecular motion stops. “Daylight” is standardized at 5500° K and “Tungsten” at 3200° K to 3400° K. (your basic 100W incandescent bulb usually has a color temperature of  around 2900° K) The temperatures relate to the color (wavelength) of light given off by an object heated to those temperatures. An object heated to 1000° K would only glow a dull red but the 5000° K flame of a carbon-arc searchlight appears as bluish-white. Another odd note is that as the color temperature rises, it is considered “cooler” or bluer and as it lowers, it is considered “warmer” or more yellow/red. Hence, an 81a warming filter actually causes the camera to record a lower color temperature. An odd conceptual twist that likely has its roots in the fact that this all started back when cameras only recorded a negative image.  

Other types of filters were used for various effects on either color or black and white film.  

A Neutral Density filter (ND) is colored with a neutral grey pigment that absorbs all visible wavelengths as evenly as possible. ND filters are used to artificially reduce the light level (remember EV?) so a slower shutter speed can be used. Back when you were locked into a single ISO film speed by the film you were using, an ND filter would allow you to shoot in bright sunlight with a fast film when your camera didn’t have a fast enough shutter or your lens wouldn’t stop down far enough. It was (and still is) also used to allow for a very long exposure without waiting for dark. ND filters come in various levels of tint depending on how much light you want to shut out. Graduated ND filters are used primarily for landscapes where the sky may be a lot brighter than the land. They are only ND on half of the filter with the middle fading from ND to clear. You line up the middle on the horizon and it darkens the sky so the sensor or film can expose correctly for both in one shot. Pretty neat, huh?  

A polarizing filter is also neutral in color, but rather than just darkening the image, it has a layer of material that forms a molecule-sized grid (picture a picket fence, but really tiny) that blocks all light that isn’t vibrating in the same direction as the orientation of the filter’s grid. Let’s clarify that a bit. The light falling on any object is generally not polarized and vibrates in all directions as it travels. However, when it is reflected by an object’s surface, most of the little waves end up vibrating in the same direction. Since your eyes aren’t polarized, you see a bright glare from the object’s surface or a reflection. A polarizing filter is mounted in a two-part frame that can be rotated until the polarizing grid is at right angles to the reflected light, blocking most of it (which is why you lose 1½-2 stops). The effect eliminates nearly all the glare from shiny surfaces, makes water transparent and removes reflections from windows. In scenics, it can remove glare from shiny or wet leaves making them more colorful. It will block scattered light from the sky making it a deeper blue. Due to the nature of sunlight reflected by the atmosphere, the sky-darkening effect is greatest at a 90° angle to the sun and fades to nearly nothing away from that. If you are using an extreme wide-angle lens, this can lead to odd effects in the sky. The original photographic polarizer filters were linear polarizers like the one I described above. When sophisticated metering and autofocus started showing up in SLRs, It was discovered that since the metering and focus sensors relied on reflected light passing through a partial mirror, polarizing the light entering the camera blocked the meter and focus sensors unless it was rotated just right. Enter the circular polarizer. A circular polarizer adds another layer of polarizing grid behind the initial grid that re-scatters the light, allowing the camera’s electronics to function properly. They are harder to make, having the second layer, so they tend to be more expensive. Both are still available since digicams, P&S cameras and most camcorders rely on readings taken from the sensor for focus and exposure and aren’t hampered by a linear polarizer. Both types function the same way via rotating the two-part frame and produce the same effect.

Some filters have no color or correction at all, but are used for special effects. Star filters, diffusers, center spots, vignettes and prism filters are used for in-camera special effects (many of which are done with software now). Some of these like the star filter can be very useful and create effects that are very difficult to re-create with software. As for the rest here are catalogs full of color, graduated color, split-image and other esoteric items that seem to have been added to the catalog just to make the number of pictures on the page come out even.


What to use and when

The basics

There is a long-standing argument as to “when” where it relates to protective filters. Many people leave a protective filter on the lens all the time to protect against grit, spray and accidental impact. Some don’t. The “don’t” group argues that adding another layer of glass between the subject and the sensor reduces image quality and can contribute to flare and ghosting if you don’t use an expensive multi-coated model, so why do it unless you have to?  They maintain that the rigid, protruding hood on modern lenses protects against the bumps and unless you’re caught in a sandstorm or on the open sea, simple cleaning will take care of the rest. The “do” group maintains that a protective filter only has a small impact on image quality and that they would rather get a scratch or crack on a $40 filter than a $600 lens. Personally, I’m a fence-sitter on the subject. If I’m out shooting for maximum image quality and the conditions aren’t dusty or subject to salt spray, I skip the filter. If the conditions warrant protection, I use the best filter I can afford to minimize the impact on image quality. Your mileage may vary.

UV and Skylight 1a: Both are used to reduce haze and can be left on the lens all of the time for protection. The UV filter is clear and blocks ultraviolet rays that you can’t see but film and digital sensors can show as distance haze in a photo. The skylight filter does the same but has a slight color cast to further reduce the bluish cast to distance haze.

Color-correction 80a, 80b, 81a, 85a, FLD: Auto white-balance has rendered these all but obsolete. If you shoot film, you will need to know how to correct for color temperature differences and if you only shoot digital it may be worthwhile to understand how they work anyway. You can get some neat simulated moonlight scenes in full daylight by adjusting your exposure and setting your camera to “tungsten” to give everything a bluish cast. It also helps to understand color temperature when you are correcting problem photos in an image editor.

Color filters Red, Orange, Yellow and Green: If you shoot black and white digital, using these filters will have the same effect on you B&W images as they did on B&W film. Some things never change and the coolness of black and white photography is one of them.

Polarizers: For outdoor shooting, a polarizer can be a photographer’s best friend. It adds pop to foliage and makes the contrast between sky and clouds magical. I personally use the Hoya “Moose Peterson” Warming Circular Polarizer. It adds an 81a warming layer to the polarizer and it has an enhancing effect on foliage and earth tones that can’t be easily added with software, if at all. Using a polarizer at sunset or sunrise doesn’t have much effect due to the nature of the light and you lose a couple of stops at a time when it’s already darker than usual. An exception would be if you were shooting a body of water that you wanted to make transparent of kick up the blue. Don’t over-use the polarizing effect. Rotate the ring and see if the scene looks better without the maximum effect. I mentioned earlier that an extreme wide angle lens can span more sky than is affected by polarization. This can cause one or both sides of the scene to have light blue sky fading to dark blue in the middle. Sometimes this is broken up by clouds and looks ok, but more often, it doesn’t. The sky is most affected at 90° from the sun. If you want to estimate where that will be, make an “L” with your thumb and index finger, then point your index finger at the sun. Your thumb will be pointing at the ring of maximum effect, If you can point your finger at the sun and your thumb to the piece of horizon you want to photograph, then pulling out the polarizer should be worth it.

Neutral Density: Ever see the forest or jungle scene with the waterfall that looks like spun glass? Chances are that a neutral density filter was used to get that 15 second exposure. Besides stretching time for cottony waterfalls or shorelines, you can use an ND filter to turn a Ferris wheel into a disc of colored flame or make crowds disappear at a mall. A 15 to 30 second exposure will remove all but the laziest mall-rats from view. ND filters commonly come in .3, .6 and .9 densities that reduce exposure by 1, 2 and 3 stops respectively. If the best you can do with camera settings is a 1 second exposure, a 3 stop ND filter will take you to 8 seconds. There are higher densities available, but unless you really need them, the cost rises quickly can be prohibitive. As alluded to by the name, the neutral grey color of the ND filter doesn’t change the color of the light passing through it.

Graduated Neutral Density: Yes, the properly exposed sky over the properly exposed earth can be done using multiple bracketed exposures blended in Photoshop or an HDR utility (High Dynamic Range). The issue is that this requires considerable skill and worst of all, effort! A well-chosen and properly used Graduated ND filter gets it right in the camera. End of job. As described earlier, a graduated ND filter (sometimes called a split-ND) is clear on half of the filter and neutral grey on the other half. Most, but not all ND filters aren’t mounted in a threaded ring. They are usually square pieces of glass or optical resin that fit in slotted holders that allow the transition to be moved up or down in relation to the lens to adjust for different compositions. They come in variations that have wide, gradual or narrow, sudden transitions between the light and dark. Some start getting lighter immediately and transition over the whole filter without a “horizon line”. Another factor is greater or lesser density of the dark side. Helpful hint: If you are faced with an opposite situation where the bottom of the scene is brighter than the top, you don’t need a new Grad ND filter….just flip it over! J

Special Effect Filters: WARNING! Many photographers have 10, 20, 30 or more of these “special situation” filters. The warning refers to the fact that after spending $15 - $50 for each of these gems, they are seldom used more than once! Save your money and buy a good image editor like Photoshop Elements or Paint Shop Pro. In addition to their line of excellent resin filters, the Cokin filter company was very successful in promoting a vast array of special effect filters, including one that had a little transparent rainbow painted on it so you could add a rainbow to any scene! Wow! (Rolling eyes…) There are a few exceptions. Some of the less silly special effect filters are graduated color filters that are used like Grad ND filters except that they add a color cast to half of the image. Blue, magenta, tobacco (brownish) orange and yellow are common and can add impact if used in the right scene. In the truly useful category, a star filter can add radiating rays of light from any small, bright light source in a scene. This is often used on candles in wedding photography and provides a beautiful effect if not overdone. Jewelry can also benefit from the star effect. They are available in 4, 6, 8 and as many as 16 ray versions. Another useful item is the diffusion or softening filter. These were probably invented on the day that the first movie star hit 40. A diffusion filter is not perfectly clear, but allows the sharply focused edges to show through while smoothing the small details (facial imperfections are a prime target) and giving everything a soft glow. As with the star filter, a diffusion filter can really add to the right kind of photo when used properly. The star and diffusion effects can be added with software nowadays, but doing it in-camera can save a lot of processing time and effort.


Some other considerations

Coatings

 Plain glass doesn’t make a good filter regardless of how flat or how clear it is. It reflects light that creates flare and ghost images when there is a bright source of light in the frame like a streetlight or, especially, the sun. Higher quality filters are coated with special materials that allow light to pass through the filter with little or no reflection. Most filters have coatings on the front surface, but the better ones are coated on the back as well so light reflecting from the front element will pass out of the filter or be absorbed instead of reflecting back in and causing flare and ghost images. Resin filters are subject to the same problems as glass filters and the better ones are available with similar coatings to reduce their effects. My advice is to buy the best coatings you can afford, especially if you plan on leaving it on all the time. Be reasonable, though. Buying a $200 Heliopan UV filter to protect an $190 zoom lens sort of defies common sense.

Mount

 Threaded rings are the most common mount for a filter. There are different types of threaded mounts to address different needs. Most filter rings have a male thread to the rear to attach to the lens and a female thread to the front to accept another filter, if needed. Some mounts are made very thin so they won’t block the edges of the image when used with very wide angle lenses. Some of these are so thin that they eliminate the front female thread. The mount material is usually aluminum. This can be an issue if you leave a filter on a lens for a long time since the lens threads are usually aluminum as well and the two can bind, making the filter very hard to remove. Some (expensive) brands offer filters with a brass mount to prevent this from happening. There are also square resin filters that use an adapter ring and a slotted filter holder. One of the advantages of this method is that you can buy one filter of each type needed, a holder and use them on several different lens thread diameters by getting several inexpensive adapter rings. Personally, I use both. My UV, ND and polarizers are screw-on but I prefer the 100mm square "P" series from Cokin for the graduated ND filters.

Materials

 Glass types vary from manufacturer to manufacturer. Some use rolled optical flats and some slice and polish thin layers from a solid glass rod. Others use high-impact plastic resins similar to the type used in eyeglasses. Colorants can be mixed into the material, applied as a coating or sandwiched between two layers. While not always true, the better the optical quality, the more you will pay.


How to use them

 SLRs and DSLRs These are the easiest cameras to use filters on. Almost all interchangeable lenses have filter threads on the front. The thread sizes vary from about 49mm to 86mm and larger. Most kit lenses that come with the camera are 55mm to 62mm. If you look at the front of your lens, the filter thread size will be engraved there. The thread size will be indicated by the diameter symbol (lower case o with a stroke) followed by the size in millimeters: ø55mm. This differentiates between diameter and focal length. Using a filter is as easy as buying it and screwing it on the front.

Digicams and superzooms Many of these cameras have threaded lenses that accept filters, but many more require you to purchase an adapter from the camera maker to so filters can be used. If your camera has threads or you have fitted the adapter, the buy it and screw it on method works here too.

Compacts and Point & Shoot  Uh oh… No threads and no adapters. Some third-party companies offer clamp-on adapters that attach to the lens barrel when it’s extended and let you attach a filter. The problem is that if you retract the lens while the adapter is clamped in place, it could strip the mechanism in the lens. You are also out of luck if you have one of the ultra compacts with a non-extending lens. You can always hold a filter in front of the lens while taking the picture, but that sounds like a pain and I’m only writing about it! Cokin to the rescue! Cokin makes a couple of nifty little adapters that allow one of their “A” series filters to be held in front of the lens. The A300 holder screws into the tripod socket of a P&S camera and has a frame that holds the filter. It is widely adjustable and qualifies as “one-size-fits-most”. The B400AC-M has a frame with a round magnetic ring that literally sticks to the front of a metal framed compact, holding the filter in front of the lens.

The info part of this was long-winded again, but hopefully it covered the basics of filter what and why. It may also have kept you from buying that three filter starter kit that contains a cheap polarizer, an 81a and FLD filter, I always chuckle when I see these kits since two of the three are nearly useless on a digital camera. Still, kudos to the guy or girl that came up with that marketing plan…selling dead stock cheap in a kit is better than a scrap write-off!


Try it out This might cost you some money. Get a polarizer. Get a good one or a cheap one, but get one. Go out and find a mountain, desert, forest or ocean scene where you can use the effects described above in a before and after shot.

Experiment with reflections on water in a lake, koi pond, fountain or even a puddle. Shoot a shop window with all the reflections then shoot it again with the polarizer and watch the reflections disappear. Find that sweet spot 90° from the sun and render the sky in 15,000ft blue. Learn what a polarizer can do and how it can be one of the single greatest tools you can add to your kit for improving your travel photos.

Remember, take a before and after shot to see the effect. It will surprise you.

Happy Shooting!

 Dave


Links to filter manufacturers:
Hoya    Tiffen   Sunpak   Cokin   B&W   Heliopan

Links to filter distributors:
B&H Photo      Amazon     Adorama    2filter.com    Filter House

Further reading:
Color temperature     Polarization     Using Graduated Filters