I decided to start a new thread on the subject of distortions after reading some recent posts on the subject. I'm not surprised to find that some people find distortion more visually disturbing than others, but I am surprised to find that we can't always agree about what kind of distortion we see or whether we see it at all. I've also seen the term "distortion" used in ways I think are not accurate.
First, I think the use of the term "distortion" should be confined to the rectilinear distortions which come from uneven magnification across the field. They have nothing to do with sharpness. Highly distorted fields can be quite sharp at the edge and fields with zero distortion can be blurred at the edge from astigmatism and/or field curvature. In any optical system there is inherently zero distortion at the center. The magnification of the center obviously can't vary from itself. If magnification increases toward the edge that causes pincushion distortion. If magnification decreases that causes barrel distortion. I don't think doublers will be of any help in analyzing distortion. The boosted image will only cover a small central area of the original field where distortion is inherently low and the doubler itself will add the distortion of its own eyepiece.
I've been using a dynamic test for distortion that simply consisted of moving vertical lines back and forth across the field and noting whether the line bowed in or out as it approached the field edge. Today I tried to come up with an additional static test that allows a line in the binocular field to be compared directly to a reference straight line. This requires finding a group of vertical lines that can be viewed in the binocular and with the unaided eye at the same time. I used a door and a nearby window which gave me all the verticals I needed. Place one barrel of the binocular to one eye and view the verticals unaided with the other eye. You should be able to move one of the verticals that you see through the binocular to the edge of the field and superimpose it over a vertical line that you see with your unaided eye. It's quite apparent (to me at least) whether the line in the binocular is straight like the reference line or bowed in one direction or the other. Using this method I found that the one binocular I own that I thought had considerable barrel distortion appears to be virtually distortion free (a WWII 6x30) and every other binocular I tried showed varying amounts of pincushion distortion.
I would be very interested to hear what others find using the same test or perhaps someone can suggest a better one.
Hello Henry;
I will be glad to get a better understanding on this subject. Basically, I just start with a vertical line in the center of the field (or horizontal) and work it towsards one edge and then the other and watch the direction and amount of curvature. I try to do this at about 100 yards because the shorter the focus the more pronounced the curvature. I will try your recommendation for test when I get home this evening if still light out.
Attached is something from some Leica literature.
BTW: I use the unaided eye method you describe for color checking also. I have red, yellow and blue construction paper sheets mounted on a box and overlay bin image over unaided eye image. Works fairly well for me. Try it indoors and in sunlight, interesting results sometimes.
Ron
The eye is a tricky instrument as well. We think we see all in front of us, but mostly we are focused on a very small area. If the information you are looking at is not text, then your eye wamders a bit to see what the object is.
I look through the middle in the binoculars and move them quite a bit, and the 30% around the edges is just there to locate more birds, or movement.