Filters
When you buy a lens, or walk into a photo shop with one, you'll be questioned, almost instinctively: "do you want a skylight filter to protect that lens?" This sort of question ignites one of my pet peeves. You may well want a filter to protect the lens, but if you know what filters do, you probably don't want a skylight filter if your main subject is trains.
For about as long as there have been lenses, there's been a debate over whether one should have a filter in place all the time or not. We won't settle this long-standing debate here, but we can state the two sides:
- Against: if the designer wanted the front element of your lens to be a flat, blank piece of glass, he would have designed it that way. Filters get in the way of the lens design and cause your pictures to be less sharp. Furthermore, most filters are cheaply made and cause distortion.
- For: a filter is an inexpensive way to protect your lens. If something hits it, causing breakage or a mark, it's a lot cheaper to replace the filter than the lens element.
Both arguments have some value, and as the debate twists and turns, both sides imitate politicians, stirring up hot air, comparing apples and oranges, and avoiding logic. It often seems that neither side knows the basics, and both overlook that there are many other uses for filters.
For a start, many store personnel, hired off the street with minimal technical training have no idea that the skylight (#1A) filter is not the same as the UV filter. They have simply been told to sell one with a lens (because the store typically makes more on the filter than the lens). If you take one of each and put them on top of a piece of white paper, you'll see the difference. The skylight is a little pinkish; if you see any color at all in the UV it will be a twinge of blue. Skylight filters are intended to make Caucasian people look good, especially in shade or on overcast days (without the filter the pink-white skin would have an icy-blue color). Skylight filters are also the first correction step if an electronic flash unit's color balance is off, which can cause the same blue tint. The skylight is part of a series of color balancing filters (you can buy color meters that will suggest the proper adjustment); the next step is a #81B.
The UV filter removes ultraviolet radiation--it's a weaker version of your sunglasses. The reason for a UV filter is that ultraviolet light will record as a vague bluish cast on color film or sensors. On black and white film, it will record as a hazy overcast, thus you may also hear it called a "haze" filter. It is preferred for scenic shots (which trains are) or aerial photography. There are several strengths of UV filters; the typical filter is a UV-1.
When you look at the instructions for a filter, they will often mention a filter factor. Because a filter absorbs some light, an adjustment in exposure is required. The good news is that through-the-lens metering, which nearly every camera uses, will do the adjustment automatically (so don't panic when your exposure is different). If you're using a UV or skylight filter, the adjustment is negligible. If you are using a hand-held meter or a flash with a stronger filter, you'll need to change the exposure.
Filters generally attach to the front of your lens by a screw-on thread. Use some caution to avoid cross-threading. On some extreme telephotos and wide angles, they are built in or drop into a slot on the rear. To purchase the proper filter, you'll need to know the filter thread size, which is not the same as the focal length. It's best to take the lens(es) with you. If you have different size filter threads, and want to use the same filters on all, you can buy the largest one and some step-up rings (using step-down is not recommended, as they'll cut into the picture area). You can also buy square filters which fit into a holder which holds a threaded ring. Cokin is the most well-known of this type (and is the topic of another column).
One of the arguments against using filters, as noted above, is that many are cheap and will distort the photo. When you are looking for a filter, don't jump for the cheapest one: the shoe-string budget photographer knows that while quality costs, buying cheap and then replacing it with quality costs more. Look for one that is mounted firmly, that doesn't show ripples or chips in the surface, and is clean when you buy it. At the risk (and likelihood) of leaving some out, B+W, Nikon, and Hoya make good ones.
One reason we use filters is that films or digital sensors do not "see" color the way our eyes do. In black and white (whether film or digital), there is a similar problem: shades of grey are not rendered the way we'd expect. Blue, whether visible or not, is again a problem--it fades away into a haze, and clouds disappear. It has long been a practice among black and white photographers to use a medium yellow (#8) filter to absorb blue light and make the sky darker. On trains, not only do you get the clouds, the colors are better balanced. As an example, on the left is Amtrak Turbo Train #66 at Porter IN. This is from a black and white negative, and taken without a filter. Note how the red and blue bands are almost the same.
On the right is Amtrak F40 #298 at Niles MI. This is, again, a photo from a black and white negative, but with a #8 yellow filter. The filter has restored the clouds, and the red and blue show up as diferent tones. It's worth noting here that a red filter will make the clouds stand out even more dramatically--but some caution is called for, because it will also make grass and vegetation look odd, and may alter the color scheme of some equipment so much that you won't recognize it.
Along those lines, here is a similar example with a more far-reaching effect. Grand Trunk Western #4136 at South Bend IN, is a "Battle Creek Boo-Boo." This railfan's slang-name does not refer to a mistake, but to a conscious decision of Grand Trunk Western to use colors other than those of its owner, Canadian National. The body is blue and the nose is CN red. But in a black and white photo, the two colors appear to be the same.
Notice how the yellow filter has restored the difference of the two colors on GTW #6216 at Dolton IL. The #8 yellow filter is considered by many to be a "standard" for black and white photographs, just as the UV or sometimes skylight is for color. It darkens the blue, so clouds stand out (notice that there's a change here similar to that of the Amtrak equipment), and in general restores to photos the sense of shading that we see with our eyes--for trains and other subjects. If you try some other photographic subjects, consider using the yellow filter there, too.
We should also note that the yellow filter is for black and white only. If you use it with color, you will have a yellow tint to everything in the photo. This is also the case with the other black and white filters, such as green and red. Their use, along with balancing filters for color, will be a later topic.
As mentioned before, filters absorb some light, so the instructions for one often mention a filter factor. When you use a yellow filter, this factor is noticeable (one to one and a half stops). Again, if you have through the lens metering (which nearly every camera does), you don't have to make an exposure adjustment--but you need to be aware that you may end up with a slower shutter speed if you aren't watching.
21 October 2003