Chapter 5
Filters
The basic filters you should consider purchasing for your kit are the Circular Polarizer, UV/Skylight and Fixed Neutral Density filter. I keep an assortment of these filters in my kit at all times.
The Circular Polarizing Filter.
Another useful filter to carry in your kit is the Circular Polarizer filter. This filter will do something that can't be emulated in post-processing at your computer. It will filter out polarized light entering the lens. If you have ever used polarized sunglasses, we're talking the same general idea here, and that is to eliminate glare and polarized components of sunlight. This can be especially useful in scenes where the sky may be intense or where there are a lot of reflections and glare from glass, water or from other bright reflective surfaces in your scene. Think of it as a variably polarized sunglass lens you attach to the front of your camera's lens.
Most commonly, this filter is round and consists of two elements. The front element rotates and the rear element is fixed. By rotating the front element you control the amount of light and polarization elements of that light entering the lens and striking the sensor. The net effect is to allow you to take a bright blue sky and darken it, or reduce the reflected light from bright surfaces. This filter is most effective when used between a 45 and 90 degree angle from the sun (or other bright source of light) and in some situations and angles the filter will have very little effect. I find this filter to be quite useful when photographing scenes near water where glare needs to be reduced from the surface of the water, drawing out details of objects underwater and reducing the specular highlights reflected from the water's surface.
The two images above were taken near San Pedro, Belize in the Caribbean from the same vantage point using a circular polarizer. The polarizer was adjusted between photographs. As you can see, the color and reflectivity of the water and the clouds in the sky have changed between the shots.
These filters are normally measured by diameter in millimeters and can be purchased to fit almost any lens diameter. It simply screws on to the front of your lens. My standard circular polarizer is 77mm in diameter, which matches the diameter of my largest lens. I can adapt it for use on smaller diameter lenses by using step rings. Dimensions up to 95mm can be purchased and a top grade circular polarizer can cost over $1,000. Most commonly though, a decent grade 77mm filter can be found for less than $200, and budget filters can be purchased for less than $100. Keep in mind though, the larger and the better the grade of the filter, the more it will cost. The better grade filters are made of glass and the budget grade filters are made of plastic.
In typical use, you attach the filter to the front of the lens and then rotate the front filter element to achieve the light effect that best suits the scene. Every situation will be different and the best way to get a feel for it is trial and error. When using a circular polarizer you'll find that it will attenuate the light entering the lens
Most lenses can take a circular polarizing filter, but you'll find that using a lens hood with the filter attached may pose a problem, since the rotating element of the filter will be under the mounted lens hood and you'll be unable to rotate it. A few lenses will allow you to use this filter with a lens hood attached, so keep this in mind when selecting a lens.
The UV/Skylight Filter.
Walk in to a camera store to buy a lens and you'll probably hear the salesperson try to convince you that you need a UV filter for that lens. They'll whip one of the house brand, cheap-o filters out and tack another 30-50 bucks onto your purchase. It is my personal opinion that you don't normally need to keep a UV filter attached to your lens. Many things in digital photography seem to be carried over from the "old days" of film photography. UV filters are one of these hold-overs. But it doesn't hurt to have a few in your kit, and for a couple of reasons.
The basic premise of the UV or Skylight filter is to reduce the amount of ultraviolet light striking the sensor, reduce blue casts created in certain conditions under bright skies and to reduce the amount of visible haze in some scenes. Secondary consideration is to use the UV filter as protection for the front lens element against debris and scratches.
In their basic forms, UV filters are normally grayish in color and do not generally have a tint and Skylight filters are most often found with a slight pinkish tint. Different brands of filters have different levels of effectiveness as well. In practice, I've found that the cheaper filters are made of plastic and are mostly useless when it comes to making the claimed optical purifications and in some cases will actually degrade the quality of your images. The more expensive filters are generally more likely to be made of glass, multi-coated to reduce flare and more likely to produce the claimed optical results. You get what you pay for. My personal preference is the Tiffen and Hoya brands. Modern lenses already have UV protection built into the lens elements, so unless you have deep pockets and can afford the more expensive filters I can see no reason to buy them for controlling blue-casts and haze in your photographs. What I find to be the most useful aspect of the cheap UV filters is for protecting my lens from being damaged when I'm working outdoors in muddy, rainy, at the beach or other conditions where debris may be easily accumulated on the front lens element. You are most likely to scratch a front lens element when cleaning it. By keeping debris off the front element, you will reduce the amount of cleanings and thus reduce the likelihood of scratching it as a result of cleaning. The filter will also prevent (to some degree) projectile damage to the front lens element. If you've ever experienced a broken windshield as a result of flying rocks hitting your car, you can appreciate having this extra protection if you're doing photography in an area where dirt, rocks and mud may be flying.
The bottom line though, when shooting digital in RAW format is that almost any color correction can be easily made in post processing, so there is no true optical need for UV filters if you are shooting raw and have the proper photo editing software. In my mind, using a UV filter is simply placing another piece of glass between your scene and your lens thus potentially reducing the quality of the image captured. I'd recommend only using them when you need protection from the elements. If you do elect to place UV/Skylight filters on your lens you'll find them totally useless indoors as well.
The Fixed Neutral Density Filter.
Have you ever seen a photograph of a waterfall or stream where the water movement was blurred and looks like cotton with a smooth misty flow? The technique is called veiling and one of the primary tools for generating this effect photographically is the Neutral Density Filter.
The Neutral Density filter is quite simply a filter that attenuates the amount of light entering the lens and thus reduces the amount of light striking your camera's sensor. The fixed ND filter behaves similarly to the graduated ND filter discussed earlier, but the fixed ND filter will attenuate the light evenly across the face of the lens. It will allow you to select a larger (more open) aperture with your lens than would otherwise be possible due to the brightness of available light. It will also allow you to make exposures with longer shutter speeds. It is this longer shutter speed that produces the veiled effect.Fixed Neutral Density Filters are usually grey in color and do not introduce a color cast into your image. The most common ND filters are the ND2 which provides a 1 f/stop light reduction, ND4 which provides a 2 f/stop light reduction, ND8 which provides a 3 f/stop light reduction and the ND64 which provides a 6 f/stop light reduction. I typically keep a ND2 or ND4 with me.
The filters are quite simple to use. You'll find them in either circular format or square format, with the circular filters being the most common. They simply screw on to the front of your lens or as in the case with the square filters, they fit into an adapter that screws on to the front of your lens. As I've stated before, I would suggest buying a set of filters with the same diameter of your largest lens and using a step ring to convert the mount to your smaller diameter lenses. This will keep the cost of your kit and the clutter to a minimum.
To understand the math (and I've been trying to avoid math in my writings), a typical ND4 filter will allow you to slow the shutter speed down by 2 f/stops. So, lets say you are on location trying to photograph a waterfall during daylight hours. You'll want to slow your shutter speed down to at least something between .5 and 2 seconds of exposure time. Available light may bee too bright to allow you to achieve this slow of a shutter speed without overexposing your shot. Imagine you've got your ISO set to 100, aperture to f/16 but the best you can meter for a shutter speed is 1/4 of a second. Attaching a ND4 filter to your lens will allow you to achieve a shutter speed of 1 second (2 stops slower than 1/4th of a second.) The alternative to the ND filter is to wait until a time of day when the light is less, which isn't always an option.