Resolution Charts and Testing

Can someone please describe in layman's terms how to conduct and decipher results from a resolution test for a spotting scope? What chart, if any, would be involved? Thanks.

I've never done this before, so this is kinda schematic, but here goes.

Take a test target like this one and print it out: http://www.pacifier.com/~tpope/NBS_Circ428_Test_Target.jpg. Better yet, buy a real test target.

Then set up your scopes a fixed distance from the target, and look through each to see the finest block of lines you can see as lines instead of a blob. For head-to-head comparisons, that should already nail down which scope has the best resolution.

If you want to estimate the resolution quantitatively: the printout isn't to scale, so you'll need to find out the number of millimeters per line with a ruler. Measure the width of each block of lines, count the number of white/black cycles, and divide the number of millimeters by the number of cycles. Also, measure the distance from the scopes to the target. The angle subtended, in radians, is the distance per cycle divided by the distance to the target.

Example: Scope X can barely resolve a block with 1.4 millimeters per cycle from a distance of 25 meters. So the angle resolved is (0.0014 m) / (25 m) = 5.6E-5 radians. There are 57.29 degrees in a radian and 3600 arc seconds in a degree, giving 5.6E-5 rad * 57.29 deg/rad * 3600 arcsec/deg = 11.5 seconds of arc. So you could say that scope X has a resolution of about 12 seconds of arc.

Thanks much.

You're most welcome! Do post results if you manage to do such measurements.

Hi

Keep in mind that it takes some experience to use these charts and there is still a judgement call as to what grid you can or cannot resolve. Furthermore, unless you do all tests at exactly the same distance and lighting conditions, the test means nothing. Even when tests are properly controlled, you have to allow for daily differences in your vision. Yes, your vision can change on a daily basis, especially if you have a cold, allergies or your eyes are tired. By far the most reliable tests are side-by-side comparisons.

Lastly, numbers should be used as a basis for comparison, not as something literal. Numbers are always relative in these tests and be aware that small differences may not be statistically significant. In other words, avoid the tendency in all tests to give numbers a life of their own. We sometimes tend to give results an objectivity they don't deserve when we deal with numbers. Never forget that the numbers you get may not be the same numbers someone else gets, even in the same tests.

This is based on hundreds of tests I have conducted on binos and spotting scopes. Hope it helps.

Great points Joanie, and thanks to you also. What got me interested in this was that I purchased a Nikon 82ED based on yours and some other folks recommendations. I do not have the fixed wide angle eyepiece (30x or 38x, havent' decided). In using the 25-75x zoom I was simply amazed at the resolution, contrast, and overall fantastic views I got, especially at 25-50x....spectacular colors and detail! It is even very usable @ 75x early in the mornings right now. I was able to watch a group of domestic goats at over 2 miles and could clearly make them out. I will admit, and I knew this going into it, that the zoom could have a wider field, but I found the eye relief fine. The more I fiddled with the helical focus ring the more I like it. So really the resolution thing was curiosity, as I don't see how a spotter can get a whole lot better than this one, granted I haven't looked through a Kowa or Swaro HD. Thanks again.

Hi JG

That's why I have always rated the 82ED to be worthy of inclusion with the Kowa and Euro brands - really is right in there. The Fieldscope 82Ed gets you into that premium spotting scope arean, no doubt about it. T owned for a couple years and I also got decent images right up to 75x when conditions were right. Only reason I got rid of mine was the eye releif - I wear glasses - and the weight. Just don't care to carry that big a scope over my shoulder, anymore.

Enjoy