Spotting Scope Lens Size Comparison

Think area, rather than diameter. Remember, 3.1417 x diam^2. Consider ~ 20,000 mm^2 vs ~11,000 mm^2. You have a lot more light coming through the larger glass.

Seperately, quality matters as much (if not more than) as size. An 80 mm crappy glass will do a lot worse than a much smaller good glass.

I am blessed, as my wife is a pro photographer. This allows me to have good glass, too. I bought a 80mm Swarovski spotting scope through Cabela's black hole. Getting it home, I started looking at birds on the wire behind the house. The bird was OK, but I read the embossing on the wire. WOW - I was reading the black on black molded embossing of wire.

If possible, test both side by side. Ideally in the light you expect to use it. You are much better buying quality, rather than size. Buy once, cry once.

Something to consider, use a resolution chart to compare (or something more approariate - like an existing target) at the range you expect to view. It will provide a good comparison.

https://en.m.wikipedia.org/wiki/1951_USAF_resolution_test_chart

Yep, pi x*r^2. I screwed that up, but you figured that out - sorry. Please remember glass quality. Nothing better than what you see - test together if you can!

By the way DB, you should be able to afford what you want. If not, the book rights should be top shelf

That link in post #3 is for photographic lens images on sensors, not for telescopes and the human eye.

The Dawe's limit is for telescopes: https://en.m.wikipedia.org/wiki/Dawes'_limit

R = 116/D

R in arcseconds (60 per MOA) where D is the diameter of the clear objective lens in millimeters.

A 50mm lens highest resolving is about 1/26th MOA.

Lens numbers, glass quality and coatings reduce the resolution a little.

Lens size is only part of the equation. Coatings and quality of the glass are the other. Give me a great lens and great coatings over a larger lens any day.

I think that there is a big difference as well. I would go with the larger one. Unless you're packing in miles and miles to hunt, where every ounce and inch counts, you won't regret it. I love my 80mm and I backpack hunt with it. Worth it IMO.

D.B. Cooper said:

Regarding spotting scope objective lens diameter, is there much practical difference in image quality (at dawn/dusk)

Click to expand...

There's a formula for comparing light gathering power between two scopes based on objective diameter.

LGP= (D1/D2)^

(80/60)^ = 1.78

The 80mm gathers 1.78 times as much light as the 60mm. How much is lost through the lenses is another story as pointed out above.