What is Averted Vision, Anyway?
often employ an observing technique called "averted vision",
the art of looking slightly to the side of a faint object
being studied. This works because, we are told, there are
more rods slightly off the optical axis of our eyes. But there
is a great deal more to it than that, and with some understanding
of the physiology of the eye, it will be seen that there are
right and wrong ways to use averted vision.
is true that the density of rods peaks well outside the center
of vision. Since the rods are the eye's faint light detectors,
it stands to reason that this peculiarity of physiology is
what makes averted vision work. The density of the rods at
a point 20 degrees off the center of vision reaches about
160,000 rod cells per square millimeter. This is a greater
density than the peak density of the cones - the eye's bright
light and color detectors - on the fovea (the center of vision),
where cones only reach about 140,000 cells per square millimeter.
point of greatest density of the rods does not correspond
to the point of greatest sensitivity, however. The area of
greatest sensitivity has been shown to vary considerably from
observer to observer, but it is never as far as 18 degrees
from the center of vision. The reason for this has to do with
the manner in which the retinal cells are "wired" to the brain.
the fovea, each cone is connected to a single ganglion cell,
which in turn is hooked up to a nerve fiber that eventually
joins the optic nerve. As we move away from the fovea, each
ganglion cell starts to service several cones or rods. Eighteen
degrees from the fovea, 100 rods might be connected to a single
ganglion cell. At some point on this line extending outward
from the fovea, the number of rods per ganglion cell is such
that the eye operates at peak sensitivity. For most people,
this point is somewhere between 8 and 16 degrees from the
so far we have only been considering the sensitivity of the
eye as a function of an image's angle from the fovea. One
might suppose that it makes a difference if we avert our vision
to the left or right, up or down, or at some angle. And it
does matter. The most effective direction to avert our eyes
is that required to place the object on the nasal side of
our vision. Simplified, this means if you are a right-eyed
observer, you shift your eyes to the right; if a left-eye
observer, you shift your gaze to the left. Whichever eye you
use, you avert your gaze in that direction.
using this most efficient portion of the retina, you will
experience a gain of some four magnitudes or more over your
direct vision! The effect of this is not insignificant. It
means the detection or not of many stars and most details
in deep sky objects.
is important not to avert your vision the opposite direction
- that is, if right eyed, you should not use averted vision
by shifting your gaze to the left. This will place the image
on the blind spot, right where the optic nerve connects to
the retina. Nothing will be seen in such a circumstance, no
matter how bright!
poses an interesting dilemma for binocular observers and for
those who use binocular viewing attachments on their telescopes.
Averting one eye to its optimal position puts the image on,
or nearly on, the blind spot on the other eye. This is counterproductive;
the advantage of the binocular system is its use of two eyes.
Inadvertently disabling one eye makes no sense. The solution
is simple, and astronomers have been saying it for centuries:
second most efficient direction to avert your gaze is upward
- look in the direction of the top of your head, so that the
image is below your center of vision. The area of the retina
in use here is somewhat less sensitive than the optimal horizontal
location, but only slightly so. Doing this does not put the
image in the blind spot of either eye, and considering the
gains to be had from binocular vision, this will likely prove
as efficient (or more so) under such conditions as using the
optimal monocular method.
you choose to avert your gaze downward, you will find your
averted vision slightly less sensitive again. In actuality,
the retina is every bit as sensitive here as it is if you
avert your vision upward, but it is sensitive over a much
smaller area. Thus, it is harder to consistently rest the
image on the "sweet spot".
observers will notice that their most sensitive areas are
slightly to the side and down, or in other ways not exactly
as eye physiology would suggest. In my case, I find averting
to the right and slightly up (I am right eyed) is best for
me. There are large variations in the way our eyes are made
up - in fact, our retinas are even more distinctive than our
fingerprints. Almost nothing can be said categorically about
vision, but we can say what will apply in the majority of
cases. It is well known that experienced observers see much
more detail, and many fainter objects, than beginners. I believe
that this is caused in part by the observer learning about
the individual characteristics of his or her eyes over the
course of many nights of observations.
time you are out with your binoculars or telescope, take some
time to explore these different areas of your vision. It might
be quite apparent what is the most promising averted vision
method for you. And if it happens to be something other than
what medical science predicted, don't let that stop you from
doing it your way. They are, after all, your eyes, and only
you know what you can see with them.
ABOUT THE AUTHOR
Jeff Medkeff has been an amateur astronomer for over 20 years.
He is an enthusiastic and prolific observer, especially of
solar system objects, and has been writing about amateur astronomy
sporadically for 14 years. He operates the private Rockland
Observatory of Hereford, Arizona, which is dedicated to astronomy
education and journalism. In 1997, Medkeff was appointed an
assistant coordinator within the Association of Lunar and
Planetary Observers' Solar Section.
of Jeff's articles, which he freely provides to the astronomical
community, may be found on the web at his home