Quote:
Originally Posted by Bart B.
What's the difference between a SLR camera lens focusing a subject on the screen or film and a scope focusing a target on the reticle?
What dimension changes in scope lenses when you adjust parallax with the side knob?
What direction does the scope adjustable objective lens system move when focusing a closer target on the reticle, forward closer to the target or backward further from the target?
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I think what Bart is asking is why we describe one process on rifle scopes as parallax adjustment and describe a very similar (if not identical) process on cameras as simply focusing.
The difference is that rifle scopes are sighting systems and cameras are image recording systems. So the importance of parallax is greater in a sighting system, and image clarity is more important in an image recording system. The terminology used to describe what are two very similar processes reflect the primary goals of each system.
Cameras can have issues with parallax; twin lens reflex (TLR) and rangefinder (RF) cameras have a great deal of parallax and users of those cameras have difficulty composing an image of nearby objects. The single lens reflex (SLR) camera was developed to minimize the effects of parallax by making the imaging axis coincident with the viewing axis. As a result, parallax in a SLR is essentially inconsequential and the discussion of parallax has largely been eliminated from photography.
In a rifle scope, reducing sighting error is important along with a high quality image of the target. So we have a two step process (with adjustable objective/parallax adjustment) to focus the eyepiece on the reticle and then bring the target image into focus on the same plane as the reticle. This gives a sharp image and reduces/eliminates parallax.
On a SLR camera where image quality is paramount, we have a similar two step process. The viewfinder diopter adjustment is used to focus the eye on the focus screen and then the lens is adjusted to bring the image into sharp focus.
The adjustment process for the rifle scope and camera are similar, but the primary goal of the adjustment is different for each system. So we call one "parallax adjustment" and the other "focusing", even though they are essentially optically identical. Adjustable objectives were added to rifle scopes specifically to reduce parallax error, so that's why they were described as parallax adjustment.