Tele-Converters For Close Up Photography

Tele-Converters For Close Up Photography

  • A tele-converter resembles a lens extension tube but contains a lens assembly of negative power - a reducing lens. Objects viewed through it alone will appear smaller than to the naked eye. Placed between the camera body and the prime lens, however, the tele-converter's lens assembly expands the cone of rays emerging from the back of the prime lens, thereby increasing image magnification. The degree of enlargement is given by the power of the converter. A 2 x converter will double the image magnification and a 3 x will triple it. If the closest normal focusing distance of your 50 mm lens is about 2 feet (.6 meter), the image magnification will be about x .1 (x 1/20); adding a 2 x converter will double that magnification to x .2 (x 1/2); a 3x converter will bring it up to x .3 (almost x 1/3). Although most often used with telephoto lenses, tele-converters will increase the image magnification of any lens setup, whether telephoto or close-up.

  • Because tele-converters increase magnification by expanding the cone of light rays, they also spread the available light energy over a larger area, resulting in a dimmer image. This affects not only the brightness of the viewfinder but, more important, exposure. You must increase exposure by a number of stops equal to the power of the converter. Thus, a 2 x converter requires two stops compensation, a 3 x needs three. This correction can be achieved by opening the prime lens aperture either by slowing down the shutter speed or by increasing the amount of light available (as by moving a flash unit or other light source closer to the subject).

  • Most tele-converters introduce very few lens aberrations, but they do reduce the image resolution (the ability to record fine detail as measured by the ability to show closely-spaced lines as separated) in proportion to the amount of image expansion. Thus, a given prime lens used with a 2x tele-converter produces an image with only half the resolution of the image produced by the prime lens alone.

  • However, the good lenses in use today will resolve from two to eight times as much as the films in ordinary use; so you will find that there is little or no discernible loss in the image as recorded on the film.

  • The biggest difficulty of using tele-converters is loss of image sharpness due to camera motion during exposure. But this has nothing to do with tele-converter quality or design as it is equally a problem whenever magnification is increased. A simple rule is good quality enlarged image require careful camera handling.

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