Wrote John Gaasland:
>I was wondering what size extension ring you need for 1:1 closeups?
>Thanks, John.
That depends on the lens you are using. To reach 1:1, you need 'double
extension'. When a lens is focused at infinity, its extension (the distance
from the main optical plane of the lens to the film plane) equals the
focal length. So for a 50 mm lens, it is 50 mm.
Double extension, then, is 2 x 50 = 100 mm, of which the lens mount
and the camera body take care of 50 mm. So you have to add an extra
50 mm. This then is the depth of the extension ring you should use.
You can in practice use less, for normally, the lens has a focusing
helical which can be used too. A set of OM extension rings has three
rings of 25, 14 and 7 mm, adding up to 46 mm which is just right for
a 50 mm standard lens (the helical takes care of the missing 4 mm).
Other focal lenghts behave in a similar manner. A 1:1 80 mm Macro
has to have 160 mm available, which means a bellows or an adjustable
tube.
Vänliga hälsningar/Best regards
Lars Bergquist
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