When the object is 50 meters away, how do you know that the camera is focused at a distance of 50 meters? Is it that we can only focus at about 50 meters?
When the object is 50 meters away, how do you know that the camera is focused at a distance of 50 meters?
That object will be the sharpest. Focus aids like autofocus systems can help, and there are various ways to ensure accurate manual focus.
Is it possible to be laser-measurement precise with a camera designed for photography? No. That's not what they are for.
Is it that we can only focus at about 50 meters?
If you mean "is it only possible to be approximate", yes. If you mean "is 50 meters the only possible focus distance", obviously not.
Perhaps a restatement of the question is: How accurate are focus markings on lenses?
For all but very long lenses, focusing at 50 meters is very close to focusing at infinity. The actual difference from infinity, on a 50mm lens, will be just a degree or two different from the infinity stop on the lens.
So practically, a lens setting of 50 meters would have a large error, to begin with.
The second issue is how accurate are the focus settings on a lens? For most consumer lenses, the focus settings are not spot on. Close, but again, there is a resolution issue on the scale. If the difference between being in focus and out of focus is less than a degree of rotation of the focus ring, how fine can one print the scale? And is the focus ring position established by normal manufacturing tolerances, or is it individually calibrated? Most likely by manufacturing tolerances.
Having said all of this, it is possible to measure distance optically, and a common method is using two perspectives, and using the angle differential of a point on the object, as seen through the two optical points (lens placement). However, the more accurate and less costly method today is to use the time of flight of an emitted pulse of light and it's return (eg LIDAR).