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[filmscanners] Re: film and scanning vs digital photography



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 
Angle = .5 x f#   Lens Focal Length.  Without special pressure plates 
or vacuum plates, the film bow in 35mm cameras is typically .003".  2 
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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