7887mm08 said:
I am looking to buy a weaver rimfire scope (4x) to place on a air gun. I do not want an AO scope. My question is: Will a scope with the parallax set at 50 yds from the factory. be clear and in focus from 10 thru 30 yds? I have never looked thru a scope with 50yd parallax and these are the ranges I expect to be shooting. I do not want to have to focus an AO scope every time I shoot because the distances will very each shot Thanks for any help
Gary
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Depth of Field (the area that appears clear and in reasonable focus) is predicated on 3 things; focus distance, magnification and aperture size. You can use an on-line calculator to help you figure out the depth of field from an optics. I like this one: http://www.dofmaster.com/dofjs.html which works well for me in photography. I don't see why it can't be used to answer your question.
The first datum it needs is the camera, format or Circle-Of-Confusion to use. Since this is not a camera, we will use the CoC. You can Bing that term, and I suggest a value of 0.300mm for our use here. You can select larger if you want, it all depends on the required visual acuity or what is acceptable focus to you.
Next datum is the focal length of the lens. That is not something that is provided by the riflescope manufacturer, of course, but since it's a 4X scope and it is for a human eye, we can use 50mm per X, so 200mm focal length.
Finally it wants the F/stop or F-ratio of the optics. Again, this is not provided by the manufacturer, so what I have been doing is use the exit pupil as the aperture and so I divide the focal length by the exit pupil to get the F-stop. So in this case, 200 divided by 7 gives f/4 or thereabouts.
Then put in 150 feet for the focus and voila, you have your answer, from 78 feet (26 yards) to infinity.
Of course, you should verify this before making a decision.
