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Could this work? Adapter w/ lower crop factor.

Discussion in 'Adapted Lenses' started by carpandean, Jan 3, 2011.

  1. carpandean

    carpandean Mu-43 Top Veteran

    Oct 29, 2010
    Western NY
    I'm relatively new to photography, but learning a lot fast. I'm also trained as a mechanical engineer (though I have not practiced in many years), so I tend to think a lot about how things work. That said, I haven't had time to sit down and think about the true geometry of what I'm proposing or made any serious calculation. This is a more of a back-of-the-envelope type design.

    We all know that (in theory) the adapters for legacy lenses place the mount at its original distance from the film or sensor. If done right, the lens focuses to infinity, but not beyond. As most of us also know, cheaper adapters often don't get this measurement correct, leaving the lens either too far away (can't focus to infinity; in essence an adapter plus a small macro extension tube) or too close (focuses beyond infinity, but not on anything close.)

    Additionally, we all know that the :43: system has a crop factor (a.k.a., focal multiplier or magnification) of 2.0, which means that the field of view seen on a :43: camera is roughly that as seen on a 35mm (or full-frame digital) camera with a lens that has twice the focal length. For example, a 50mm adapted lens on a :43: camera has the same field of view as a 100mm lens on a full-frame 35mm camera. However, the equivalence is limited to just the field of view. With the smaller sensor on the same plane as the original 35mm film or full-frame digital sensor, what you are actually getting is a center crop of the image that actually covers the full 35mm frame. In some sense, with lenses designed for a 35mm camera, you are wasting a lot of the image that is passed through lens; the opposite of vignetting seen with CCTV lenses. Moving the lens closer to the sensor actually decreases this crop factor (picture an image cone that diverges from the back of the lens; the further away the plane is, the larger the projected image), but you run into the focus problem described above with a short adapter.

    With these facts in mind, I started thinking about whether these effects could be used/corrected in order to create a different type of adapter. There are two ways to get the close-focusing needed for macro photography: (1) move the lens out using an extension tube, or (2) use a diopter ("macro") lens/filter with an optical element. The former doesn't help, as it is just the first problem (long adapter) listed above, but the second could be useful. So, to test my theory, I hand-held an MD 85mm lens right at my GF1 body mount. As suspected, the field of view increased relative to the same lens when adapter mounted, but close focusing was not possible (could focus well beyond infinity, though.) Then, I attached a Century Optics +2 Achromatic Diopter lens to the end of the lens and, viola, close focus was possible again.

    With this loose test in mind, I had an idea for a different type of adapter for legacy 35mm camera lenses. The length of the adapter would be substantially shorter, but an optical element (like my Achromatic Diopter) lens would be added in the middle. I picturing something that looks like this adapter. What this would (theoretically) do is allow legacy 35mm lenses to be used with their originally intended field of views or at least closer to them This would be particularly useful for wide-angle lenses, which currently become normal lenses. Obviously, the quality of the adapter and especially the glass would be critical and, thus, the cost would likely be high. As a nice ancillary benefit, it would also make a more compact package.

  2. apicius9

    apicius9 Mu-43 Veteran

    Feb 1, 2010
    Philadelphia, PA
    Just wondering, from a perspective of total ignorance - physics almost cost me my high school diploma :rolleyes:  - isn't what you are describing similar to a focal reducer? There have been discussions about those on the boards with the general conclusion that it would be nice but next to impossible.

  3. carpandean

    carpandean Mu-43 Top Veteran

    Oct 29, 2010
    Western NY
    I'm not sure. Would a focal reducer use the same flange distance, but add a lens to - for lack of a better term - narrow or focus the image coming out the back of the lens, such that it does not spread as much by the time it hits the sensor? If so, then it would be a similar idea with a different execution.
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