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All you need to know about lenses,
Whilst the 2/3 optic is producing a much bigger image, the smaller sized 1/3 CCD will actually be missing all the edge of the image produced by the lens, and so the overall definition with the more expensive larger format optic, should be noticeably much better.
Now whether this obvious improvement is worth spending the extra money on, is obviously somewhat debateable, but in terms of other less expensive larger format lenses, such as the 8.5mm or 12.5mm, there is a distinct and obvious improvement in image quality, which is well worth the extra expense when looking at the relative costs of manual Iris optics.
Its obviously important to remember that whilst you can almost always use a larger format lens on a smaller format imager camera, the reverse is rarely if ever possible. Should you wish to try it, at best you may get almost the entire picture covered, but with very poor edge definition, or alternatively, the more common outcome will be a clipped or even disc like image, not dissimilar from the appearance of a fish eye lens (the technical term for this is 'vignetting').
Whenever you can, Doktor Jon would simply recommend that with the possible exception of ultra wide angle lenses, you should try and stick to using the lens designs intended for at least the next size up imager.
Before we go into any further detail, its worth first exploring how the concept of standard lenses first began. With the old 1 and 2/3 format tubed' cameras, the imager had a circular faceplate, so the measurement actually related to the diameter across the image forming (or working end) of the tube. With the development of solid state chip imagers (MOS, CID, CCD,) the shape of the imager area changed from circular to rectangular, so the measurement was taken across the diagonal corners (as is the case with film based photographic camera formats).
Now, in order to achieve a lens coverage closest to that of the human eye, a focal length was selected that approximates the diagonal measurement of the imager ... at least thats the theory.
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