A red dot, or sometimes known as a reflex, has the illuminated
reticle reflected off the rear side of the objective lens that
is pointed towards the targeting eye in which the source is a
low powered light emitting diode. The lens will have a special
coating that only reflects the light wavelength of the illumination
and allow all other light waves to pass through.
Although many red dots advertise that they are "parallax free"...
they are referring mainly to a specific distance using perfect eye
alignment. Even then, there still a noticeable amount of parallax
distortion. These type of sights also would require that you aim
with the lit reticle towards the center of your sight's lens to obtain
the best accuracy. The further the target, the larger the parallax
distortion. Longer tube red dots would generally give you better
accuracy over shorter tubes or open lens reflexes.
Red dots are more ideal for closer range shooting in most lighting
conditions. Quality red dots would've reduced much parallax but
never all of it. Many lower brands like to jump on this bandwagon
by making simple inexpensive sights which are virtually useless for
shooters seeking fast acquisition accuracy. Most people would
normally use one eye to shoot with a red dot.
A holographic sight floats your reticle in the target viewing area
from a laser emitting pattern. The lens do not filter any light
waves passing through it, your target view is unaltered except for
the floating holographic reticle that is superimposed in front of
it. Holographic apertures requires it to be mounted slightly
higher also giving you a larger field of view (FOV). Being more
advanced using laser imaging, holographic sights will be slightly
bulkier and possibly heavier.
The holographic sight is virtually parallax free because of its
ability to produce the "virtual" floating reticle to proper positioning
regardless of the angle of your eye's view. To obtain the best
holographic reticle you should use both eyes ignoring the frame
of the sight itself... since many holographic sights have the
ability to create 3 dimensional reticles.
The battery life on a holographic is very short compared to a
red dot/reflex. Also, holographic sights tend to not work as
well in very low or hot temperatures whereas red dots/reflexes
are not affected at all.
A shooter's eye will tend to place a red dot's reticle downrange
on top of their target whereas holographs appear to float in front
of their targets. This alone can be a deciding factor for people
that prefer one or the other regardless of the technologies.
Depending on your budget, decent red dots/reflexes & holographics
would be the Aimpoint, Trijicon, Vortex, Burris, Nikon & Leupolds
and of course... EoTech.
Hope this helps.
Peace,
GH-07