Focus question.

[jpsmith1]

Without getting into too much detail, can anyone tell me why I have to focus a pair of binoculars or a spotting scope, but do not have to focus a rifle scope?

I understand that my eyes can be made to adjust for focus in binos and probably even in a spotting scope, but is seems to take a fraction of a second.  Is this what happens with a rifle scope?

[savageT]

Not sure I'm following your question, but here goes my best effort:
You do need to focus a riflescope, just not the same way you would focus binoculars or a camera.  OK, to focus a riflescope you want to point the scope at the sky (away from the sun) and focus the reticle (crosshairs) for best sharpness.  This will prevent your point of aim from shifting regardless of how near or far away you are aiming the scope at your target.  If done correctly, you should not see a shift in the target image as you move your eye from side to side or up and down.
Hope I haven't lost you there.......
Jim

[jpsmith1]

I knew about focusing the crosshairs, but was more asking about the image.  When using a pair of binoculars, you normally have to focus the image to see what you are looking at, but with a riflescope, the image is just there when I bring the gun up.  No knobs or rings to adjust.

[Graybeard]

Differences in the compromises made in manufacturing each and to a lesser extent the power range of them. Binoculars and spotting scopes are made with a very narrow depth of field where as rifle scopes are made with a very broad depth of field and the eye is expected to aid in this as needed. And it works out quite well as long as magnification doesn't exceed about 9X. Much above that you really then need a parallax adjustment.

The narrow depth of field of binculars is sorta planned in to allow you to remove the clutter of brush, weeds etc and in a manner of speakign to "see thru" it to the target by taking out of focus the brush or weeds closer to you and allow your focus to be on your chosen target. Spotting scopes have very high magnification and that always comes with the price of narrow depth of field.

With the rifle scope your eye is doing a LOT of the focusing and making the accomodation that in binoculars and spotting scopes you generally don't have to and because of the way most are made you cannot. Some binoculars depend on this same feature and it can work OK depending on your use of them.

[jpsmith1]

Thank you.  I was thinking about it the other day and I couldn't figure out why there was a difference.