Thanks for that Carlos, I missed the point about the aperture (f no.)
decreasing when the CU lens is added.
...Wayne
>
> 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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