Duclos PL Extension Tubes achieve extremely close focus and macro-like effects, similar to the effect of a diopter close up filter. Extension tubes allow for the optical characteristics to remain unaltered while shifting the close focus to nearly touching the front element of the lens. They are more effective when used with wider lenses than with telephoto lenses. A loss of light is incurred with the use of extension tubes.
• Decrease the close focus of any PL mount lens
• Maintains the optical characteristics of the lens
The exact decrease in close focus, and subsequent light loss (decrease of maximum aperture) depends on the length of extension and the focal length of the lens used: the magnification ratio of this combination.
Extension tube length in mm / focal length in mm = magnification ratio
Magnification ratios and light loss (a rough guide):
0.1- 0.2x = 1/2 stop
0.25-0.33x = 1 stop
0.5x = 1.5 stop
1x = 2 stop
Example: 85mm Lens with a 20mm extension tube:
35/85 = 0.24x magnification = about 1 stop light loss