Re: Viewing Distance for Prints

From: Sandy King ^lt;sanking@clemson.edu>
Date: 05/01/05-07:47:13 AM Z
Message-id: <a06020407be9a8e05aa11@[192.168.2.2]>

This may be more than many of you will want to
know about th subject so be forewarned.

The observation distance is established by the
circle of confusion, but the question is what
minimum observation distance should we adopt to
calculate the size of the Circle of Confusion.
One solution is to adopt a distance that would
place the point of observation at a distance
equal to the diagonal of the print, which is the
distance used by manufacturers of lens and
cameras to establish depth-of-field scales. It
is also a logical distance to use because in fact
prints are usually observed from a distance that
approximates the diagonal of the print.

There is a drawback, however, to this method.
Very critical evaluations of image quality are
made in most cases from the distance at which
vision is best. For most persons it turns out
that this distance is approximately 25
centimeters, or 10 inches. So, while the normal
viewing distance may be a distance approximately
equal to the diagonal of the print many people,
especially photographers, evaluate image quality
from a much closer distance.

In a recent study of this issue I decided to
include three different figures for the circle of
confusion. In two cases I use the diagonal of the
print as the observation distance, but establish
two figures for this distance. One is called the
Non-Critical observation figure and is based on
the minimum threshold of vision as a point object
that subtends 1 minute of arc (0 01' 00"), a
figure well within the resolving power of most
people in ordinary viewing conditions. The
Non-critical observation assumes a minimum
resolution of 5.5 lppm at the viewing distance
and the COF can be calculated approximately by
the formula d/1775, where d is the distance of
the diagonal of the print. The second figure is
for more critical observations and is based on a
threshold limit to human vision as a point object
that subtends twenty seconds of arc, ( 0 00'
20'). Objects of this size, although not clearly
and consistently visible to most people, may be
discernible from moment to moment and in this way
may contribute to the perception of some detail
and texture in a photograph. The 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
standard. At a viewing distance of 10 inches the
limit of resolution for most persons with good
vision is about 20 lppm, though some persons may
observe differences in image quality up to as
high as 25 lppm. The super critical standard of
22 lppm is provided for individuals who evaluate
image quality by very close inspection. Providing
more resolution in lppm beyond this standard will
not result in any appreciable improvement in
image quality. The COF for the super critical
standard is calculated with the formula d/5500.
Because the assumption is a fixed viewing
distance the COF and minimum negative resolution
is the same for 8X10 and all larger formats. For
smaller formats the assumption is that they will
be enlarged to 8X10 size, which reduces the size
of the COF and increases the minimum negative
resolution in proportion to the degree of
enlargement. The minimum negative resolution
will of course be proportionally larger for any
print size beyond 8X10.

Sandy

>i'm speaking from the gut, so pardon me in advance
>i've always understood there to be an 'ideal'
>distance from which to view a print, which
>depends both upon the size and the content of
>the print;
>i've also always understood there to be
>negatives that *must* be printed one size--and
>one size /only/, and others that could be
>printed at a variety of sizes. the former are
>absolute, the latter relative, so to speak.
>it seems to me that people will position
>themselves in whatever is the "correct" viewing
>distance for them: if they're interested in
>technique, this will probably be very close to
>the print; if they're interested in aesthetics,
>it will be at a comfortable viewing distance; if
>they're not interested, it will be very far away
>indeed.
>i'm not sure that it's something that can be
>controllable, however; i've always thought it's
>something very subjective.
>
>cheers
>k
>
>
>
>R E Redman wrote:
>>The question of what is the "correct" viewing distance for prints has been
>>raised recently in my local camera club. Some suggest that it is wrong to
>>examine prints closely and to get a proper impression of the print it should
>>be viewed from several feet away (depending on its size). I think,
>>particularly with alternative prints, a close examination is desirable so
>>that the workmanship and technique can be fully enjoyed. Does anybody have
>>any views on this ?
>>
>>Bob (UK)
Received on Sun May 1 07:47:31 2005

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