Re: Viewing Distance for Prints

----- Original Message -----
From: "Jonathan Bailey" <jon@jonathan-bailey.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Sunday, May 01, 2005 8:29 AM
Subject: RE: Viewing Distance for Prints

> Greetings,
>
>> Sandy King wrote:
>> This may be more than many of you will want to know about
>> the subject so
> be forewarned.
>
> ...critical standard assumes a maximum resolution of 16
> lppm at the standard
> observation distance and the COF can be calculated with
> the formula d/5150.
> The final figure is based on a fixed viewing distance of
> 250 cm, or 10
> inches, regardless of print size, and is called the super
> critical...
>
> <snip>
>
> Just wondering: At what point do we loose the forest for
> the trees?
>
> Wink,
>
> Jon
>
> www.jonathan-bailey.com
> Tenants Harbor, Maine
>

   I must reiterate that the "standard" viewing distance is
based on duplicating "natural" perspective, it has nothing
whatever to do with circle of confusion. Circle of confusion
is a measure used to calculate depth of field. DOF must take
into account the size of the image at the retina of the eye
since it is usually based on the perceived sharpness of the
image. This is completely independant of perspective other
than magnification is also a factor in calculating viewing
distance for "correct" or "standard" perspective.
   The perspecitive is controlled entirely by the location
of the camera when it takes the picture. In order for the
eye to see the same perspective it must be located at the
right distance from the print or transparency. That distance
is the focal length of the lens times whatever magnification
is used in printing. To be rigorous the distance should
really be calculated from the first principle point of the
lens rather than its focal length but this is of no
concequence in normal pictorial photgraphy. It does make a
difference in macro or micro photography.
   As an example, if one uses a 12" lens on an 8x10 camera
and contract prints the negative the "correct" viewing
distance will be 12". If one uses a 45mm lens on a 35mm
camera and enlarges the resulting negative to 8x10 the
magnification will be 8 times and the correct distance will
be 45 x 8 mm or about 14 inches.
   This factor may become rediculous in practice as when a
telephoto or long FL lens is used. However, incorrect
viewing distance is why pictures taken with wide angle
lenses (but not fisheye type) look "distorted". When viewed
at the correct distance they will be rectilinear and look
normal.
   Camera clubs may decide to set standards for viewing
distance or for sizes of prints but no art gallery or museum
would. Viewing distance should be left to the viewer but its
interesting to know that there _is_ an optimum distance as
far as duplicating original perspective is concerned (if
thats what you want to do).

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com