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Camera Lens News No. 13
Spring 2001 |
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Teleconverters – Their Pros and Cons
When it comes to the use of teleconverters in photography, there are ardent believers as well as fierce opponents. The opponents insist that teleconverters lower the image quality unacceptably. However, the quality-concerned Carl Zeiss has been making Zeiss Mutar® teleconverters for decades. How does that fit?
We at Zeiss believe that teleconverters have their merits. However, extreme care in conception, manufacture and use must be observed to allow their successful use. As in every aspect of life, there is no simple, universally applicable solution to a wide range of problems. Thus, the use of one or several general-purpose teleconverters will never totally substitute for the judicious use of the best-suited prime lens in a given situation.
However, high-quality Zeiss Mutar® converters, used in conjunction with Zeiss prime lenses, can be an effective solution to many problems, when used with discretion. In the following article we will go over some important facts to enable discerning photographers to make a wise decision about the use of a teleconverter:
Pros:
- A teleconverter makes two focal lengths available with only one lens. This is very convenient e.g. in wildlife photography.
- A lens combined with a teleconverter is normally smaller and lighter than a ”direct” telephoto lens of the same speed and focal length. This can ease carrying, packing, and handling considerably.
- A lens combined with a teleconverter is normally cheaper than a ”direct” telephoto lens of the same speed and focal length, and much cheaper than the combined cost of the two telephoto lenses. This is certainly attractive especially for the amateur photographer.
- A teleconverter offers a practical solution, where even the longest focal length in a camera system’s optical arsenal is still too short. This is an essential aspect in wildlife photography. Paparazzi also appreciate this point.
- A teleconverter usually maintains the close focusing limit of the prime lens, thus increasing the magnification significantly.
Cons:
- All teleconverters lower the speed of the attached lens by at least 1 stop (1.4x teleconverter) or 2 stops (2x teleconverter). This is unavoidable due to the very working principle of the teleconverter.
- Almost all non-Zeiss teleconverters for 35 mm photography cause an additional light loss of 0.3 stops (1.4x teleconverters) to 0.7 stops (2x teleconverters) due to attempts at cost-saving in design and manufacturing. This is avoidable and is avoided with all Zeiss Mutar® converters.
- Teleconverters add from five to seven optical elements and several glass-to-air-surfaces to the optical system. This added amount of glass will lower the light transmission and increase the stray light level (flare) both of which reduce image brilliance.
- Since teleconverters magnify every detail in the image, they logically also magnify residual aberrations of the prime lens. It is therefore recommended to use a Mutar® with a fixed focal length lens except in a ”do-or-die” situation. Most zoom lenses introduce more compromises.
- Camera bayonets, as used to combine a lens with a converter, must allow for far greater mechanical positioning tolerances than the mechanics used inside a Zeiss lens. Therefore a combination of lens and converter, with a lens/mount interface inbetween optical components, usually leads to a lower level of centering accuracy of optical elements than a well-made telephoto lens of the same focal length would have.
- Most camera bayonets show a certain amount of rotational play, which is practically unavoidable in the mass manufacturing of bayonet locking mechanisms. With a big lens directly mounted to a tripod, the attached camera body rotates by a few degrees upon handling. This is perceived as inconvenience by some, as annoyance by others. A teleconverter adds to this rotational play.
- Most camera bayonets are subject to angular inaccuracy due to springs used in the coupling mechanics. So, any combination of lens and teleconverter can allow a ”bent optical axis”, which even varies during focusing and any manipulation with the lens and camera in picture taking. As a result, the sharpness of the images can vary considerably when teleconverters are in use. The technical quality of the resulting pictures is far less predictable compared to the results from a longer telephoto lens.
The abovementioned information is provided to give a photographer the elements for making a wise decision about the use of teleconverters in his or her work. As we often do in these pages, we conclude with the simple rule of thumb that it is a good practice to choose excellent optics for combining them with a teleconverter. Apochromatic lenses in general and Zeiss Superachromats in particular are the best basis for obtaining satisfying results with teleconverters. | |
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