Legacy Lens Adapters' Effect on Field-of-View

[JimAscher]

I've just noticed something (obvious?) as I've acquired an additional camera or two.  I'm currently using three cameras, all of which have CMOS-sized sensors.  (I believe that the field-of-view of CMOS sensors is 1.6 that of Full Frame sensors.)  However, when I employ (for example) my old Nikonos 50mm film (legacy) lens on these three cameras, in two cases utilizing necessary adapters, the images produced all apparently have different fields-of-view.  My Sigma SD15 takes the lens directly (having been modified for Nikon lenses) without an adapter.  My Sony NEX 5n requires a Nikon to Sony NEX adapter, and my Ricoh GXR with the A12 M-Mount requires a Nikon to Leica M-Mount adapter.  The field-of-view seems to gradually increase (get wider) from the Sigma (with no adapter) to the Ricioh to the Sony.  Is this due to the differing distances from the respective camera sensors that these lenses are placed by use of these adapters, or no adapter?  What effect does this situation then have on the "accuracy" of the presumed 1.6 difference between the CMOS and Full Frame sensor calculations?  As I indicated initially, is all this too "obvious."  And does one just accept that the field-of-view of the resulting picture is just what "results" no matter what the mathematical ratio turns out to be?

[Telecaster]

I've just noticed something (obvious?) as I've acquired an additional camera or two.  I'm currently using three cameras, all of which have CMOS-sized sensors.  (I believe that the field-of-view of CMOS sensors is 1.6 that of Full Frame sensors.)  However, when I employ (for example) my old Nikonos 50mm film (legacy) lens on these three cameras, in two cases utilizing necessary adapters, the images produced all apparently have different fields-of-view.  My Sigma SD15 takes the lens directly (having been modified for Nikon lenses) without an adapter.  My Sony NEX 5n requires a Nikon to Sony NEX adapter, and my Ricoh GXR with the A12 M-Mount requires a Nikon to Leica M-Mount adapter.  The field-of-view seems to gradually increase (get wider) from the Sigma (with no adapter) to the Ricioh to the Sony.  Is this due to the differing distances from the respective camera sensors that these lenses are placed by use of these adapters, or no adapter?  What effect does this situation then have on the "accuracy" of the presumed 1.6 difference between the CMOS and Full Frame sensor calculations?  As I indicated initially, is all this too "obvious."  And does one just accept that the field-of-view of the resulting picture is just what "results" no matter what the mathematical ratio turns out to be?

By "CMOS" I assume you mean the APS-C format. (CMOS is a particular semiconductor design that happens to be light sensitive and has been widely employed in electronic cameras.) My suspicion is that the differences in coverage you're seeing are down either to 1) the actual sizes of the various sensors your cameras use or 2) the percentage of available photosites (pixels) on a particular sensor devoted to the actual images you get in RAW/JPEG form. Or both. If I remember right the SD15's sensor is smaller than APS-C...this would explain the narrower field-of-view with that camera. Don't know about the sensors in the Sony or Ricoh cameras, though my guess is they're the same size. I doubt the adapters have anything to do with what you're seeing.

-Dave-

[JimAscher]

DAve:  Thanks for pointing me in a further direction for the possible explanation.  You're correct as to sensor size.  The SD15 has a smaller sensor size than the NEX and Ricoh:  13.8 x 20.7 vis a vis 15.6 x 23.5 for the NEX and Ricoh.  Also, you're correct that I meant APS-C rather than CMOS.  Now I just have to compare the images more closely(!) between the NEX and Ricoh.  Thanks again,  Jim