Archived posting to the Leica Users Group, 1997/02/27
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>> You should only focus in one direction with any camera especially an SLR. >> You begin with the image completely out of focus,then turn the lens in >> one direction until it is sharp and then stop immediately. The reason >> that this is important is that the eye will try to make a slightly out of >> focus image appear in focus if it is exposed to it long enough, this >> process is called accomodation. >> >> SO this is good advice but it shoud be applied with all focusable >> cameras. >> >> HTH >> John >The human eye can accomodate to different distances. With an SLR the distance >is constant, i.e. it is the (virtual) distance to the ground glass. You see in >the same distance that it sharp or not. > >christoph This is the understanding of accomodation I got from studying the books in years past. Simply put, it is the eye's ability to focus on subjects at various distances. The effects of accomodation are more apparent when focusing an imaging system like a telescope, microscope or systems making use of an aerial image (clear glass focus screen, some types of enlarger grain magnifiers, etc.). With these systems, the eye may focus on a plane of focus of the system in space, even when its in a slightly different place than desired. For ground glass (non-transparent) type screens, the eye's focus is on the image formed on the screen as stated by christoph above. It can't focus on image planes to the far side of the screen since the screen blocks the view. Nor do rays from the imaging system travel through the screen allowing the eye to focus sharply on other planes on the near side. For split-image wedge prisms, micro-prisms and rangefinders, the eye looks through clear, refracting componenets and must focus on the subject as presented through the viewing system. The degree of focus accuracy does not depend to any great extent on where the eye thinks it's focused like it would be in some grain magnifiers. If the eye has trouble focusing to the distances presented by the viewing system then diopter correction is usually recommended. This affects accuracy in that we may see available detail in the subject nearer to the limits of normal vision. In summary, I don't think the rocking versus single-movement method is driven by accomodation either. My admittedly incomplete study of rangefinder mechanisms suggests that there is nothing supporting one method or the other as being superior unless some gross wear, dirt or damage is present. I think that whatever works best for a particular person is the correct answer. It doesn't seem a requirement to abandon one method unless one prefers the results of another. Regards, Kevin kburke@iterated.com