Feb. 2000
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Article: Photographic Lenses - A Follow-up
Photographic Lenses - A Follow-up
- Eric Kissa
During Erik Kissa’s lecture on photographic
lenses, several questions were asked, most of which were of general interest:
1. What precautions should be taken to prevent damage to a lens?
The lens is a sensitive optical instrument vulnerable to mechanical shocks. Avoid walking around with the zoom lens extended, an unexpected blow on the side of the barrel may end its useful life.
Always use a filter (UV, skylight, polarizer, etc.) to protect the front element. Yes, a filter degrades the optical performance, but with multicoated filters the loss is not really significant.
Protect the lens against water! All autofocus lenses contain electronics sensitive to moisture. When shooting in rain, wrap the lens in plastic or use a storm shield, such as the EWA Marine.
When mounting a lens, place the lens into the mount and feel gently that the lens has been seated properly, before rotating the lens. Do not force the lens if the fit seems to be tight.
A rubber hood on the lens absorbs blows from the front, in addition to reducing flare and reflections caused by sun rays. However, a hood on a zoom lens is ineffective as a lens shade. Wide angle lenses require a narrow hood which is entirely useless for tele shots. As an ingenious solution to the problem, a Konica zoom lens had a built in hood which automatically expanded while the lens was zoomed. Recent lenses do not have this feature. Telescoping hoods are useful but require an adjustment every time the lens is zoomed. Sometimes a baseball cap is the most effective tool.
2. Filters. Can two filters be used together?
My answer is yes, but with some caution. With two filters, the loss of contrast and resolution can be significant but not forbidding with multicoated filters. A polarizer combined with an 81A filter was very popular until warmer films such as E100SW appeared. A much more serious limitation of using two filters is vignetting with wide angle lenses. Special wide angle filters have a thin ring to reduce vignetting but do not have a thread for attaching a lens hood. I use oversized filters but one size larger is not adequate for using two filters on a 28 or 24 mm lens. A solution of this problem is to mount a round multicoated filter on the lens and hold a large square filter, like Cokin, Lee etc. in front of the lens. The square filters are made of polyester or a resin. They are not coated and two square filters may cause a noticeable loss of contrast.
3. Are expensive filters better than cheap ones?
The price of filters varies a lot but no decent filters are really cheap. A $50 Hoya or Tiffen circular polarizer is good but a $300 Helioplan Kaeseman is better. The question is really, how much better? I have been using multicoated Hoya, Tiffen, Nikon, and the more costly B+W filters. As far as image quality is concerned, I cannot tell any difference, although the expensive filters are said to be more planar. The main difference is in the durability of filters. After long use, a Hoya filter and a Tiffen filter came apart - the retaining ring popped off. Fortunately, I caught the glass before it hit the ground and remounted the retaining ring. The expensive filters have brass rings and are sturdier. The B+W filters cost about twice as much and are constructed very well but they do not have the filter type and size indicated on the side of the filter, a nuisance.
4. How do circular polarizers work?
The Maxwell theory visualizes light as a electromagnetic radiation consisting of waves which vibrate in all directions perpendicular to the direction of their travel. Light reflected from a non-metallic surface is linearly polarized, that means, the reflected light wave propagates within a single plane. The blue color of sky is caused by reflections on atmospheric particles and sky light is polarized. Reflected light from non-metallic surfaces can be controlled with a polarizing filter. A linear polarizer may be compared to a slot or a grid which allows polarized reflected light to pass through when the filter is oriented in the direction of the plane of reflected light. The filter blocks reflected light when it is turned crosswise to the plane of the reflected light.
Unfortunately, linearly polarized light interferes with the proper functioning of autofocus sensors. Therefore, the plane of the light passing through the linear polarizer must be scrambled again. This is done with a circular polarizer. Circularly polarized light wave does not propagate in a plane, but the direction of the vibration (a physicist would say the vector) is rotating in a circular fashion. The filters called circular polarizers consist really of two components: (a) a linear polarizer and (b) a circular polarizer mounted behind the linear polarizer.
Experiment: Screw two linear polarizers together. By rotating one of the filters light passing through the filters diminishes gradually. Replace the second linear polarizer with a circular polarizer. Again, light can be blocked by rotating one of the filters. Now make the circular polarizer the first filter and a linear polarizer the second filter, behind the circular polarizer. With this combination, filters can be rotated and light is not blocked. This observation, that light transmitted by the filter called a circular polarizer cannot be blocked by a linear polarizer, shows that polarized light entering the filter is not vibrating in one plane when leaving the filter.
Send questions concerning cameras, lenses, and photographic equipment to Erik Kissa. ekissa@aol.com or call 302 478-6737.