I use a red dot for Spring training to help smooth panning and tracking. It is easier and develops good technique. I do not use it in the filed because it IS DISTANCE DEPENDENT. If I dial in and adjust the sight for 50 yards and then a Heron at 15 yards the dot will not be aligned with the camera focal point.
I have been meaning to test it to see what the variance is for every ten yards to see if it is material.
Here is something explaining the angle issue and I see someone else responded they use CONSTANT method. I can't imagine always aligning the dot 3 inches OVER the focal spot but like anything I guess it could be learned. Photography is easier as you can discard wind and gravity issues.
Unfortunately the accompanying images did not copy. The Zero is an angle that meets at the target. Constant offset is like a set of train tracks that never meet.
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What is “Zero?”

Generally, shooters will zero their weapon sights – whether it is a laser, an optical sight or something else – for a certain distance. What does this mean? Picture the trajectory of the projectile as it leaves the muzzle of a firearm. Disregarding bullet drop, wind, atmospheric conditions and things of that nature, the trajectory should be roughly along a line parallel to the center axis of the barrel. Ideally, to target where the projectile will impact, the shooter would want to sight along that trajectory.

The line of sight for a scope or iron sights is a straight line between the shooter’s eye and one or more points in the sight. That straight line is a bit easier to visualize for a laser sight. Since sights are typically placed above, below or to the side of a firearm’s barrel rather than looking down it, something called parallax is introduced. Due to this offset, the line of sight and the center axis of the barrel are almost – but not quite – parallel. At some point downrange, where the two lines intersect, is where the sights are zeroed.

Pistol Zero

The image above shows this concept. Zeroing a sight for a certain distance is easy enough to understand and accomplish but, as you can see, at ranges other than the distance for which the sight is zeroed, the point of impact (POI) will vary from the point of aim (POA). In this example at closer ranges the round would impact above the laser dot while rounds fired at longer distances would impact below the laser dot. The further the target is away from the zero distance, the further the POA will be from the POI. Greater distance between the center axis of the barrel and the sight will increase the severity of the POA vs POI difference because the angle between the two lines will be greater.

An Alternative – Constant Offset

There is an alternative configuration for laser sights that is popular with some shooters that eliminates the problem of the sights only being accurate at one distance. The downside is that they are not entirely accurate at any distance. The upside, however, is that the inaccuracy is the same at all distances (again, disregarding wind, bullet drop, etc.).

Pistol Parallel

The image above shows the constant offset scheme in which the line of sight is parallel to the barrel’s center axis. You can see that the line of sight never intersects the line of the bullet trajectory, so the projectile will never impact where the laser dot is. However, since the lines are parallel, regardless of the distance the point of impact will be the same distance, h, above the laser dot. Of course, in the real world, there is bullet drop so gravity will eventually pull the projectile down across the laser beam. According to this ballistic chart, assuming a 2″ distance between the barrel axis and the laser aperture, that will happen somewhere after 50 yards (150 feet) – much, much farther than the average defensive firearm situation which happens between 0 and 21 feet. At these distances, bullet drop is less than bullet diameter.

Concur about the parallax Gary but if the sight and the lens are calibrated at whatever distance, presumably within the preferred range you'd be shooting (~100 yds) and the reflex dot is placed anywhere within the focus area, the camera should acquire focus. Again we're not talking about 1 moa as we would for shooting sports, 3, 4 or greater moa would seem appropriate for cameras....seems for less than $100 I could get the A-S mount, the Picatinny Rail and a decent holographic sight and try it....Cheers