RE: [Rollei] Re: slight OT DOF in MF vs. 35mm

["John A. Lind" <jlind >]

At 08:36 5/5/02, Austin Franklin wrote:
>Here's an on-line depth of field calculator:
>
>http://www.darkroom.com/MiscDocs/DOFCalc.html
>
>It's a Unix server, so the URL is case sensitive.

Yes, there are a number of these around.  Look at the CoC values 
though.  These should be adjustable and my "complaint" with them is most do 
not allow setting them to desired values.  A 3.5x5 print made from as much 
film frame as possible (minimum cropping) at 12" viewing distance and 1 
minute arc visual acuity requires approximate CoC values (on film) for each 
film format as follows (rounded to nearest 0.005mm):
  3.5 x 5 Inch Print CoC
    35mm  0.025mm
    645   0.041mm
    6x6   0.041mm
    6x7   0.051mm

If 0.025mm CoC is used for 35mm film with a 11x14 print, the values for 
other film formats shift because the aspect ratio of the print is different 
and requires comparatively different enlargement ratios:
  11 x 14 Inch Print CoC
    35mm  0.025mm
    645   0.043mm
    6x6   0.046mm
    6x7   0.057mm

IOW, the CoC values for larger formats tend to be too large compared to 
that used for 35mm.  Why?  They don't take into account the true 
magnification required to make a print for each common print size.  They 
are based more on the length of the film frame diagonal and the ratios of 
them between film formats.

These CoC values are also presumptive that larger prints will be viewed at 
greater distances, in direct proportion to linear size dimensions (e.g. a 
16x20 is viewed from twice as far as an 8x10).  In practice this isn't 
true.  Large prints (8x10 to 16x20) made for juried exhibition will be 
examined at about 12-14 inches regardless of size.  A casual viewer 
browsing through the gallery may back off to about 18 inches.  If 
"accepted" CoC values are used blindly for critical DoF control, most 
notably for using hyperfocal focusing or with close macro work, the 
apparent DoF in the print will be noticeably shallower than was 
"calculated."  I have seen larger prints in which it became obvious the 
photographer used hyperfocal focusing for maximum DoF without considering 
the size of print that would be made (using.  The very distant background 
looks slightly fuzzy as a result.  For 35mm and 645 destined for large 
prints I tighten up the CoC to bring it more in line with print size and 
anticipated viewing distance based on 1 minute arc of visual acuity.

If the model in the link is used for different film formats at the same 
distance, even a one stop stop-down from 35mm to the medium format sizes 
produces a larger depth of field.  This is the first clue that the CoC 
values for the various medium format film sizes are too large.

- -- John

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