Re: What truely defines good glass?
When I read a post on this site, where somebody has posted "great glass", I'm thinking exceptional optical clarity and quality. Yet, like the saying: "beauty is in the eye of the beholder", it is somewhat difficult to compare lens systems because each user has different vision characteristics and much of our vision is selective. Another factor I believe to be a wildcard is contrast and other color balance factors that are engineered into the optic.
Theoretically, the best optic would be the lens with greatest clarity, and color neutrality, showing no change at any magnification level, but are riflescopes or any terra observation lenses actually engineered to deliver those parameters? Probably not. I'm not an optical engineer so can't answer definitively, but along with cost and manufacturing design considerations, there is the matter of what the consumer expects from the product.
Doing a google on telescope design and other key phrases, I found a 6mmBR.com article (
http://www.6mmbr.com/optics.html). The author states, "Image Contrast: Take two scopes with equal optical resolution (sharpness), and give one better image contrast and it will be better for target use. More contrast helps you resolve fine lines and pick out bullet holes better. Some scopes have excellent light transmission, but they would appear much sharper if they were tuned for better contrast. Image quality can also be improved with lens coatings that filter out UV and specific blue wavelengths that degrade perceived image sharpness."
Recently saw that Hensoldt Optronics or maybe a Zeiss spotting scope I looked at has optional Yellow filter available.
I have wondered for a while why screw in lenses weren't ever used on a rifle scope. Why not get the color balance YOU need and want? Skylight and other color filters make a lot of difference on film. A Red or Green filter will heighten variations in the color warmth of various subjects. Might show camouflaged persons or gear more clearly as well. A UV filter can cut the haze and for sure would keep your objective lens free from dirt or having its coatings scratched.
Hell, quality SLR cameras have interchangeable viewfinders and one in particular, a sports-finder Nikon/Canon could be adapted to scopesights. Why not offer a way to use huge objective lenses but with adjustable height occular lens?
Basically, what I am saying is "Best Glass" is much a matter of color balance and contrast; for the perception of the user. If you can alter your scopes contrast and color balance with filters, many "lesser scopes" might not be lesser at all...
I find that most "great glass" scopes are too damn heavy and have ungainly long ocular lens groups, long tubes, long objective lens, and sometimes even have turret housings that are several inches long. Did I mention that over 3lbs with rings is "too damn heavy"?
Needless to say, a "great glass" or "good glass" scope will not show any distortion anywhere in the sight picture at any magnification.
Any decent scope manufacturer/importer with even a pretense of customer service will have a fully staffed warranty & repair/service facility here in the USA. If not, what good is your "great glass" should the scope need adjustment, repair or you want it updated or changed?
I think all manufacturers could do better on inspections. New product that ships with easily spotted defect(s) is a disgrace.
Turret design and range of movement, reticle design and integration, illuminated reticle, parallax adjustment and ocular lens adjustment are all further factors that help define the scopesight, but aren't really related to the lens system clarity and performance.
I guess if a scoped rifle were left on the dash of a truck in the direct South Texas Sun, inside temps would be approaching 175F or more. Pretty good test of scope toughness. Would your warranty cover it were failure to occur? Some might not as they don't warrant problems arising from customer negligence.
There are many variable size filter housings in the photo industry. A pro photographer couldn't have a set of filters for each lens objective size and every format he used. So there are clamp-on frames and adapters... Might be very interesting to see how Leupold mk4, Burris XTR and other 2nd tier; even NightForce compare with filter adjustments for the user.
Fascinating topic.