Range of the field angle
The formula shows that for
a given base a narrower field angle increases the stereoscopic
effect and a wider field angle lessens the stereoscopic
By the formula we compare
the difference of projection of the subject on the background
considering a field angle equivalent to our vision,
in which M = 1.
When using a tele lens the
field angle is smaller which enlarge a part of the scene.
Thus we can see smaller details kike if the keenness
would be smaller. This is [a . M] and M < 1.
When using a wide angle lens
it's reverse. En utilisant un grand-angle, c'est l'inverse.
We see a wider part of the scene making out less
details; keenness is [a . M] avec M > 1.
the 3D pictures at the bottom of the page.
From what has been said about
making out details, for the same base:D, the stereoscopic
perception, or relief obviously:
- lessens with wider field
angle (smaller focal length)
- increases with smaller
field angle (greater focal length)
Of course, once understood
the magnitude process, the stereophotograph is free
to choose either keep or not the effect and to force
it further up or down. The main thing is to know the
conventional limits and evaluate their transgression
Effect of lack of proportion
With a tele lens the apparent
sizes of subjects are not varying much with the distance.
A person very far appears just a little smaller than
another one in the first plan.
The impression is that these
persons are close because their sizes are almost the
same. The plans look flat
With a wide angle lens it's
the opposite, the nearest person will appear much greater
and the far person much smaller and will appear much
further. The plans look stretched.
On a 2D image, the lack of
proportion, larger difference of sizes or not, are interpreted
as bringing together or apart the plans of the scene,
making the plans flat or stretched.
In 3D pictures, these effects
are partially vanished because the subjects and the
plans already look separate from each other in the space.
The true distances are better felt. The subjects themselves,
like trees or group of people have depth.
The effect of lack of proportion
in 2D is counterweighted by the stereoscopic effect.
Example by image
pairs hereunder are shown the most significant stereoscopic
ratios SR, calculated at each set of plans.
pair the base is 65 mm close to the eyes and binocular
cameras. Between these views, where only the focal length
changes, the variation of relief is very clear.
from wide angle to tele lens makes out the relief more
in the far than near. Enlarge the base modify in the
same proportion the relief in all plans.
easier to use simple numbers to quantify the relief
than to have literary words for relief appreciation