[filmscanners] Re: film and scanning vs digital photography

["Arthur Entlich" <artistik@shaw.ca>]

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.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>      
>>>>        
>>>>
>>>    
>>>      
>>>
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>>
>>  
>>    
>>
>
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