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08-08-2010, 06:49 PM   #1
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RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
Insane Diopter / Fast Lens Project

OK, another hare-brained idea. I've joked that a cheap way to reach f/1 is to get a meniscus or other simple lens from a supplier such as Edmund Scientific, a lens that is 40mm (diameter and focal length), and jam it onto a focusing helicoid or bellows. And I thought, hmmm, is there a cheap way to do this with off-the-shelf glass? And I read a little about closeup diopter lenses, and I found what the diopter value means -- it's the reciprocal of the focal length in meters. Or,
d = (1 / fl) * 1000
Plug in the 40mm I mentioned above, and we get +25 diopters. (Or dioptres, as our Brit and Euro cousins might insist.) So, are any cheap +25 diopter lenses available? I haven't found one yet. Ah, and I found that the model numbers of Raynox lenses specify their focal length. So my DCR-250 is 250mm and thus +4 diopters, and a DCR-150 is 150mm and +6.66 diopters. I think.

(Used as close-up supplements, more diopters means more magnification, but that depends on the focal length of the host lens. Longer hosts get magnified more, and wide-angles are handly worth the bother.)

Anyway, the glass can be stacked, with the diopter values adding up. Two +10's and a +5 stacked would give me +25 diopters which is 40mm, my f/1 goal. This assumes they're about 40-42mm diameter. Y'know, like the M42 standard? Alas, some +10 diopter lenses are sold, but they ain't cheap. And I'm not sure of their diameters. And except for some expensive two-element lenses, they're not corrected for aberrations. But at f/1, who cares about CA?

So, the question: Am I on a fool's mission? Should I just save up my pesos for an A50/1.2, which I'll maybe be able to afford in about 5 years? Is f/1 unobtainable for the non-wealthy and -lucky? RSVP.
"There's more than one way to skin a cat," she thought,
as she pinned its little feet to the dissecting board.

08-09-2010, 01:31 AM   #2
Digitalis's Avatar

Join Date: Mar 2009
Location: Adelaide.
Posts: 8,704
the more glass you add to an optical system the less light will be transmitted,(this includes filters) ensuring that the lenses you are using are multicoated will help against this light loss but the number and quality of the coatings will play an important part in this process. Older uncoated 8X10 lenses from the 1930s lost nearly 60% of the light that went through them due to internal reflections. At that point 35mm format was still in it's infancy, and the 35mm lenses were just as bad when it came to light loss.

interesting experiment, worst case scenario will mean you might end up with a lens with the aperture of an f/1 lens but the light transmission of an f/4.5 lens. I would go for the 50mm f/1.2 in either it's K or A version they are both Brilliant (excuse the pun) lenses and will serve you well.

Last edited by Digitalis; 08-09-2010 at 01:37 AM.

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