Dario wrote: "So, when you use a MF lens on 35mm, you usually get smaller DOF that when using a lens of the same focal length designed for 35mm. This can also happen when using lenses designed for 35mm on APS cameras (either film or digi). This does not happen when using lenses designed for that format (compact digicams, Olympus 4/3, Pentax DA, Nikon DX, Canon EF-S)."
REPLY: I don't think so. All other things equal DOF is dependent on focal lenght. Hence, say, a Pentax 150mm lens for the 645 will give exactly the same DOF regardless of the format of the camera it is mounted on (angle of view though depend on the format the lens is used on). As DOF is dependent on the laws of physics it isn't really an issue for optic design. The circle of confusion isn't something engineered into the lenses but a wholly subjective value calculated for DOF tabels for marking those DOF scales on lenses (BTW Pentax 645 lenses are way off. At least 2 stops). Demands to set THESE scales may be different from format to format but again the DOF on a 150mm lens on a 35mm camera is the same regardless of what format the lens was designed for. The only thing that is off may be the DOF scale on the lens but DOF on film will be the same. Only the plane of focus is truly in focus. Dario: For this reason, a 50mm lens used on APS format should show less DOF than a 75mm on 35mm format (provided that both lenses are designed for 35mm and are set on the same relative aperture). REPLY: No. a 50mm lens will show more DOF from a certain shooting position, subject and subject distance than a 75mm lens used from the same shooting position with the same subject camera distance regardless of format. Pål
