Hi Wayne and all,
From: "Wayne Harridge" <wayne.harridge@xxxxxxxxxxxxxxxxxxx>
Note that the light loss is dependent on magnification (however you might
achieve it: close-up lenses, extension tubes, teleconverters, etc.) !
Actually, close-up lenses are the only way to increase magnification
_without_ light loss -- becasue they reduce the actual focal length of the
lens, while keeping the diameter of the aperture, thus reducing f-number to
compensate for the higher magnification.
For example: let's suppose a 100mm f/2 lens. The entrance pupil has a
diameter D=50mm (100/2). This kind of lens is basically a highly corrected
+10 diopters lens (1 m / 0.1 m)
If the film (or sensor!) plane is at the second focus point (100mm from the
theoretical, infinitely thin lens) the lens is focused to infinity -- no
extension. But if we place, say, a +5 diopter, the resulting combo (assuming
again infinitely thin, close elements) will add their powers: +15 diopters
grand total means the 'actual' focal length of the lens is no longer 100mm,
but about 67mm (1000/15). But the film is still placed 100mm behind the
'lens', just like it had a 33mm extension. The 'effective' focal length
(actual+extension) is what computes _speed_; it hasn't changed, and neither
did the aperture, thus we're still at a real, effective f/2 -- no light
loss!
By the way, in these conditions the lens would be focused at about 1:2 macro
ratio (33/67). The subject should be located at about 30cm from the image
plane. _Real_ lenses with thick lens elements will differ somewhat from
these figures, but you get the idea.
On the other hand, if we want to get 1:2 macro with our 100/2 lens, we need
to add 50mm of extension -- be it from the helicoid, bellows, tubes or
whatever. The _effective_ focal lenght is now 150mm, increasing the f-number
to f/3 (150/50) -- more than one stop loss! But we get a longer working
distance, about 45cm.
Anyway, there are other problems with close-up lenses, like aberration
correction, etc. They were not really intended for "extreme" macro but to
give a mild close-up capability to simple RF/VF cameras with limited
focusing range.
Cheers,
--
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
--
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