RF triangulation
RF triangulation
SolaresLarrave said:
...Sure, a larger baseline (concept I still haven't grasped)...
Francisco, it comes from how a rangefinder basically works... But if you already know all this please forgive the following!
Envision a right triangle, one with its 90 degree right angle at the lower left. We'll call this the Origin point. There's a horizontal line running off to the right from the Origin that we'll call the Baseline. There's a vertical line running straight up from the Origin, this represents Distance from the Baseline. These lines are two sides of the triangle, with the third side diagonally connecting their ends. Visualize the Distance side of the triangle as longer than the Baseline side. Because of that, the angle at the right end of the Baseline is greater than 45 degrees.
Let's say we double the Distance side of the triangle and leave the Baseline alone. Now the angle at the right end of the Baseline changes, becomes less acute, a larger angle. For instance, it might have changed from 75 deg to 78 degrees. If we had instead shortened the Distance side of the triangle, that angle might have changed from 75 to 72 degrees.
Let's imagine we're looking down at the top of an RF camera. The Baseline fits between the centers of the viewfinder window and the smaller rangefinder spot window, but back within the body of the camera. The viewfinder eyepiece looks through the Origin and out along the Distance side of our imaginary triangle.
At the Origin point within the viewfinder is a half-silvered mirror or prism that splits our view so that a spot in the center looks partly, ghostly, to the right along the Baseline. At the right end of the Baseline is a small mirror that can pivot, changing the angle at that corner of the triangle. As it pivots, it "looks" at different points along the Distance side of the triangle.
So in the viewfinder we see a doubled image in the center spot; two images of the same object at that one distance from the Origin. We pivot the little mirror on the right until the two overlapping images coincide. Bingo! We know the distance through a method called "triangulation"!
As you may appreciate, as the Distance side of the triangle gets very large in comparison to the Baseline length, the angle at the right corner doesn't change much for a given Distance change. It's just in the nature of trigonometry that this is so. Therefore, with such smaller angular changes, our measurement accuracy drops off as Distance increases.
We can even invent a mechanism to connect the focusing of the lens to the pivoting of the mirror and calibrate it so the lens is in focus at the triangulated distance.
I think that covers the pedantic preliminaries... Now to the Baseline length. I don't know about you, but throughout this I've been imagining a longish camera RF Baseline such as in a pre-war Contax II. Naval gunnery rangefinders have much longer Baselines measured in yards, while you'll have noticed that some small RF cameras have their RF spot window only an inch or so from the viewfinder window.
Having a long Baseline effectively scales up the whole triangle, extending a given level of measurement accuracy out to a longer Distance line. Artillery ballistic computers need range measurements accurate out to several miles. Or you can consider that at the shorter distances, the long-baseline RF's mirror angle changes more for a given distance change, providing more accurate measurement.
What has puzzled me is why camera makers seem so stingy with RF baseline length. A longer baseline doesn't inherently cost more, though of course the designers need to arrange other parts so there's a clear optical path along the baseline. Heck, they could even use a fiber optic bundle to snake around other components... Why couldn't the Bronica have another precious inch of baseline? But then we'd have to learn to keep our dratted fingers away from the RF spot window, like Kiev/Contax users!
