Hi James, Thanks for the formula. I guess we need to go back to glass plates ;-) Art
James L. Sims wrote: >Art, > >There was a depth of focus formula in the American Cinematographer >Handbook that was gospel until proven wrong. The depth of focus, given >a specific blur circle size, is a trig function of the cone angle Tan >½Angle = .5 x f# ÷ Lens Focal Length. Without special pressure plates >or vacuum plates, the film bow in 35mm cameras is typically .003". 2¼ >square format cameras have film sag that ranges from about .006" to >.010". At large apertures, these dimensions can make a significant >difference in image sharpness. > >The flatbed scanners that I'm familiar with have great depth of field, >suggesting the lenses have very small apertures. However, image >sharpness degrades as the lens aperture is reduced. I'm not sure what >this effect is with flatbed scanners, because each lens is recording one >element of the image per increment. > >Jim > >Arthur Entlich wrote: > > >>There seems to be two main issues with depth of focus with film. One, >>when the image is captured within the camera, and two, when it is then >>reproduced, either as a print, or made into a digital file. >> >>With 35mm frames, in my experience, the second one is not that >>significant as long as the digital scanner has a decent depth of focus, >>which is determined by the aperture of the lens within the scanner. On >>standard optical CCD film scanners, at least with 35mm frames, if the >>light source is sufficient, it isn't a great issue, and is easy to test >>for... either the grain (dye clouds) are evenly in focus or they >>aren't. The places I have seen a real problem are with larger format >>films, which may require special mounting, glass carriers, or some other >>method of maintaining flatness and with film scanners that have >>inadequate light sources which lead to needing to use a rather wide >>open lens to capture the image, causing limited depth of focus. >> >>The CCD flat bed scanners I have used seem to have substantial depth of >>focus. I have scanned 3d objects with very reasonable resolution and >>sharpness. >> >>The in camera issue is another matter. I don't know the actual depth of >>focus at film plane different apertures allow for in camera. Perhaps >>someone has a chart that indicates the depth of focus relative to >>aperture. It would be interesting to know. 35mm film is physically >>small enough that I expect the deviation is of less significance, but I >>can see how larger roll films or sheet film could end up problematical. >> >>Does anyone know if there is a chart which shows depth of focus at the >>film plan versus aperture of lens used? That could be valuable to know. >> >>Art >> >> >> >> >> >>James L. Sims wrote: >> >> >> >> >>>All other arguments aside, flatness is much more important that some >>>realize. Back in the eighties, I had a lengthy dialog with a well known >>>research lab about depth of focus - it ain't exactly what the American >>>Cinematographer's Handbook says it is. Film bows and sags. That's hard >>>to control. >>> >>>Jim >>> >>>gary wrote: >>> >>> >>> >>> >>> >>>>One last point here. Film will probably never be as flat as a piece of >>>>silicon. >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>> >>> >>> >>---------------------------------------------------------------------------------------- >>Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' >>or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or >>body >> >> >> >> >> > >---------------------------------------------------------------------------------------- >Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' >or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or >body > > > ---------------------------------------------------------------------------------------- Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
