----- Original Message ----- From: "Tom C" Subject: RE: M85mm f2.0 bokeh
> Certainly what you state regarding DOF is true. I believe though the > writer > was not making a dogmatic absolute statement of scientific fact. He was > generalizing. > >>As the camera: subject/background ratio wasn't altered, DOF should be >>SIMILAR for all four lenses. > > I'm pretty sure he knows that actual DOF is not changed by altering the > subjects distance from the focal plane. That's only moving subjects in > to, > out of, or within the range referred to as DOF. I think he means that the > *perceived* DOF will be *similar*, which is true for lenses close to the > same focal length used at close to the same aperture. Not the same, but > similar. Depth of field is not subject to changes in focal length if the ratio of camera to subject to background is not altered, presuming the aperture is a constant. What happens if you change focal length is that the angle of view changes, nothing else. If you take a picture with a 35mm lens , then change lenses to a 100mm lens, and take a picture using the same camera position and aperture, you will note that the depth of field is identical (presuming a precise aperture match), but you need to magnify the picture taken with the 35mm lens rather substantially to match the picture taken with the 100mm lens. Wikipedia has a rather extensive article regarding this subject at: http://en.wikipedia.org/wiki/Depth_of_field William Robb -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
