A few thoughts, as this is a good discussion for hunters, and I mean no disrespect to any posters:
LGP is a theoretical measure of the amount of light that will pass through an objective lens. The term "gathering" is a bit of a misnomer, as the lens doesn't gather anything. It simply allows light to pass through it, so a bigger lens naturally has more area for light to pass through. LGP is typically used in astronomy as sort of a "this telescope could be this good" way.
In sport optics for terrestrial viewing (binoculars, spotting scopes, and riflescopes), exit pupil is much more important than objective diameter, although they are linked. Exit pupil is a simple calculation of objective lens diameter (in mm) divided by magnification. You could also use a micrometer and measure the exit pupil and come up with the same result. For instance a 50mm scope at 10x has a 5mm exit pupil. The human eye when it is healthy and young will dilate to a maximum of about 7mm in low light. As you get older, this number gets a little smaller.
So, in low light your 7mm pupil is only making use of 5mm of exit pupil, because that is what your scope is showing it. If you go to either a bigger objective or lower magnification, the exit pupil will enlarge, providing a brighter image.
That is why when light is low, and you turn your magnification down, you see a brighter image: exit pupil.
During bright to normal viewing conditions, your eye's pupil will be constricted to a size that is smaller than the exit pupil of most scopes regardless of whether they are 40mm or 50mm (and aren't really high magnification), so the images will appear similarly bright. When light gets low, the bigger objective will provide a larger exit pupil, and apparently brighter image.
As far as shooting a marmot at 600 yards at 12x, I would feel quite confident at 12x. I would feel confident at 6x. Think of it this way: The marmot would be magnified twelve times, so it would appear 12 times closer. 600 divided by 12 is 50. Could you shoot a marmot at 50 yards with open sights?
Thats a very well sorted post Bitterroot. However Marmots vs Idaho whistlepigs are two different buggers. Idaho whistle pigs are goffers. They sit about 6 inches tall vs the 18in Rock Marmot most other Western States call whistlepigs.
I should have clarified the content of the nickname better, :)
That and I also should have stated the LGP percentages are based on a 1x comparison of the Obj Lens alone, not the entire calculation of the internal lenses and the resistance they produce from Obj to occular.
However I believe you clarified what I left out rather well, so thank you on that.
I am responding to your edit:
It is true that reading mirage is helpful in long range shooting. I actually have quite a bit of training in this area. However, mirage is typically used to dope the wind, not a heat related drop adjustment. Heat will affect trajectory over long distances, but it has to do with atmospheric density, and mirage will not help you evaluate that, at least I've never been through a training like that. You can evaluate mirage through a parallax adjustable riflescope, although it is much more effective with a high-magnification spotter.
If you can get a good cheek weld with a 50mm objective scope, great! You will always be able to mount a 40mm scope lower, however. My 4-12 is actually a 50mm objective on a Remington 700 Sendero, and I get a good weld with it.
Sorry, I always called marmots whistlepigs.
Although I still think I could resolve a gofer at 600 yards through my scope at 12x, I will say that when your targets get super-tiny, a little extra magnification is nice (ask benchrest shooters), as long as you realize the tradeoffs.
Still, for a big game rifle, even for longer range shots, a person usually doesn't need too much magnification.
Yes you can read the waves in the lateral movement angle to dope wind speeds when you dont have rangeflags(ie in the field settings). You can also use them vertically by comparing how compact they get. With parallax you can compare them at say 100 meter range point vs the 400 vs the 1200. Increase the magnification equally each step to keep the picture relatively the same. If your doing it right and say there is a hot spot farther down range, it can "lift" the bullet if you will. Use the mirage for both heat rise(atmospheric density), as well as the dope,(windage changes).
Yes your spotter should do this for you, however if your solo, you may not have the luxury of movement from a HP spotter to your weapon, so a single scope can do it for you.
So you are saying that you can read a different "rate" of mirage between you and the target, and that will tell you that there are "spots" of warmer air that are going to be so different from the surrounding atmosphere that it is going to affect your drops to a noticeable degree? I have not heard that.
I don't think mirage is great at helping to dope wind speed, either. It is most useful to me in doping wind direction. I estimate wind speed by evaluating movement of the surrounding vegetation, and I use a portable wind meter to get the wind speed at my location.
If you have no vegitation, and no fancy wind meter, and no fancy balistic calculator, you have to work your art of doping to a more refined degree.
Here is an article you might find a great read. This guy puts into words the best what it is I do.
Bitterroot Bools, that is a good post, but you failed to mention the exit pupil as pertaining to age. My longest shot on a mule deer was 467 yards using a VX 2 3x9 set on 5 power in the closing light. Now I have much better glass, but at my age I am not able to use the extra exit pupil of my VX 7L or Z5. I do sometimes use the larger magnification, 4.4x18x56 for the VX7 and 5x25x52 for the Z5, both are mounted low. I no longer hunt elk, if I did weight would be a concern and a really good 3x9x 40 would be my choice
I thought mentioned age and exit pupil in the post, but I think it is good to emphasize the point. I also agree with your thoughts on magnification. I think getting a scope with high magnification is one of the biggest and most common mistakes new shooters make. A 3-9X40 with the best glass you can afford is a great way to go for an elk rifle. You have a nice large exit pupil, don't have to worry about parallax, and have sufficient magnification for even extended range shots.
Hey didn't know I was capable of starting such a intellectual conversation. You guys are going to have to dumb it up alitttle for me to understand! Lol