| |
Achieving top sharpness with red filters on telephoto lenses
When using telephoto lenses in black & white landscape photography, red filters are often used to cut through haze and produce far reaching vistas with stunning clarity.
Red filters used for this purpose, absorb significant amounts of light, requiring exposure compensations of three to four stops, which lead to rather slow shutter speeds. They also darken the viewfinder image considerably. Therefore, some photographers prefer to compose and focus their viewfinder image at full brightness and attach the red filter only just prior to releasing the shutter.
With the exposed films back from the lab they are sometimes disappointed with the sharpness they have achieved, although they used a sharp fine grain film of 100 ISO or even lower, and they made sure they locked up the mirror before tripping the shutter. Has something gone wrong? Was the tripod not good enough to avoid vibration at shutter speeds around 1/15 of a second? Did the wind shake the tripod and camera? Was the red filter of insufficient optical quality? Was it scratched or somehow dirty? Is something wrong with the individual telephoto lens? With the b & w film development? With the atmospheric situation on that day? Instable air? Was the exposure grossly incorrect?
How to identify the cause and what to do in order to prevent it from happening again next time becomes a key to further success. Almost any of the abovementioned reasons could have caused the problem of insufficient sharpness, or could have contributed to it. But there is another suspect, not yet mentioned:
In theory, photographic lenses ideally focus all details of an object plane into one corresponding image plane – regardless of color. In real world optics, however, color does make a difference. The usual ”achromatic” lenses cannot focus more than two different colors of an object in the same image plane, simultaneously. If only two colors can be perfectly in focus at the same time, all the others must then be slightly off. We are used to finding an ”infrared index” on almost any lens. This in itself is a hint, that focus settings have to be corrected when dealing with the long wavelength end of the spectrum, in the red colors. This need for compensation is usually small with wideangle lenses; but it drastically increases with longer focal lengths, especially telephoto lenses.
To state it clearly: Focusing an achromatic telephoto lens of 200 mm or more in white light, then taking the photo through a red filter without correcting the focus, will unavoidably lead to less than convincing sharpness. Stopping the lens down is not sufficient to cure this problem. These statements even apply to many ”Apo” lenses on the market.
There are three ways to get the sharpness really right when taking photos with a red filter:
1. Make sure the red filter which will be attached for taking the photo will also be attached in the same way during focusing. Then the focusing result, which is found visually, is also correct for the exposure on film.
2. Use an estimated correction index. If using the red filter during focusing is not convenient, an index half way between the normal index marked on the lens and the infrared index could be used for this purpose. While not as precise as method #1, it will provide better results than no correction at all.
3. Use a Zeiss Superachromat lens. Because of their unique optical properties, these remarkable lenses do not need any of these considerations or corrections or work-arounds. Focusing can be done without filter and the focus position found can be used without correction, leading to perfect results.
One test with such a lens is sufficient to demonstrate the real difference between an ordinary achromat and a fully corrected Superachromat. If the difference is so readily visible under these specific conditions, just imagine what the difference could be in other applications. We invite you to see for yourself. | |
|