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
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
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