What Makes A Good Focus-Rangefinding Scope

[TwiceHorn]

What technical or dimensional features make a scope a good rangefinder?

Fundamentally, I believe it is an issue of depth-of-field, which goes along with focal length and lens aperture diameter:  longer and larger diameter are better.  Yet, these things do not seem to "define" scopes that have good focus rangefinding.

[Scotchmo]

You need a lens system with minimal aberration, that moves precisely and consistently when focusing. So all rays coincide precisely at a single focal point, for near perfect focus. A larger diameter lens spreads out the rays (greater dispersion of rays) when out-of-focus. And then the higher the magnification, the quicker the dispersion when you adjust the focus.

So, large diameter/high quality lenses, high magnification, and precise mechanicals. In other words - a big, high quality, high mag scope. ($$$)

Before laser range finders, there were cheap optical range finders that used two relatively small lenses, spread apart to increase the dispersion (parallax) of the two images. The distance estimates were marked to indicate when the two images coincided at a single point. Similar to the AO markings that indicate when all rays coincide at a single focal point in a scope.

In either case, you'll also need good eyes to detect the point of maximum clarity.

Someone could produce a dual objective rifle scope that could range find easier and maybe more precisely. But outside of Field Target, there is no real market for a such a devise. Economical electronics has made the laser range finder the preferred method of ranging. Once an electronic solution is found, older methods fall out of favor. Hardly anybody uses a slide rule anymore and hardly anybody outside of Field Target uses optical range finders.

[rsterne]

So, large diameter/high quality lenses, high magnification, and precise mechanicals. In other words - a big, high quality, high mag scope. ($$$)

^  100% agree....

Bob