I have one HD spotting scope and one lesser quality.
When I compare them side by side at 100 yards, the HD scope is a bit clear because it has more light coming in.
Is it fair to assume that the comparison at 100 yards will result the same at 1000?
Or the HD scope is much better at longer range than the lesser quality.
Am I making any sense?
Now that you have said which optics you are comparing, let's examine them closer.
The Gosky 20-60X80 is something with which I am unfamiliar. The internet shows it to be the Gosky Horizon 20-60X80 with an MSRP of $219 at the Gosky website. The one technical detail that is useful is the fact they say it's fully multi-coated. This means that all lenses in the spotter are coated, and have more than one coat. In reality this is the strictest minimum for any optics in the last 40 years.
The Vortex Razor HD 27-60X85 is MSRP @ $1,999 at the Vortex website, where it says that the spotting scope has ED glass and is also fully multi-coated.
So, what do you get for 1X the price? Well, for one thing you get ED glass in the Vortex. ED glass is Extra-low Dispersion glass that reduces color fringing, or chromatic aberration (CA). This will make for a better image quality (IQ) without adding more elements. I also believe that the multi-coating of the Vortex will be superior to the one in the Gosky. I did notice that Gosky has a spotter with ED glass for $799, and in the specs for that spotter they talk about how their "multi-coating" transmits up to an "incredible 98% of light per glass surface". That was "incredible" 30 years ago. These days, this is sad.
Why am I harping so much about coatings? Because of their effect on light transmission in particular and IQ in general. Uncoated glass loses around 5% of the light to reflection at each air to glass surface. So light going through one lens loses 5% coming into the lens and another 5% of what's left, leaving the lens, ie 95% of 95%. In other words, only 90.25% of light goes through one single uncoated lens. In an optic, where there are multiple lenses, this loss rapidly affects the amount of light going through it.
What coating does is it greatly reduces the amount of light being reflected at the air-glass surface. The problem is that one coat only works for one wavelength of light. If you only have one coat, the color fidelity of the image will be affected. Let's say the coat is for green light, this means that the color green will not be reflected like the other colors so more of it will come through compared to the other colors and you picture will have a greenish hue. To address color fidelity and overall transmission, lensmakers use multiple coatings, each coat addressing a specific wavelength. The state of the art in multi-coating results in near 100% light transmission at each air-glass surface so that even complex optics with double digit lens elements have an overall (all elements) light transmission north of 95%.
If Gosky's finest optics boasts of a 98% transmission at each surface, this is pretty pathetic compared to better glass such as the Vortex Razor HD.
The difference between 80mm and 85mm in objective size may account for some of the additional light coming through the optics, but the coatings will have a lot more to do with the brighter IQ and the better color rendition. This is why smaller objective lenses have replaced the genormous objective of the past in binoculars. What larger objectives do is allow for greater resolution at the higher magnifications.
The other differences between cheap optics and more expensive ones are things like construction, fit and finish and the smoothness of the controls. I notice that the Vortex Razor HD is argon-filled instead of nitrogen-filled; that's more expensive and is better for longevity of the unit.
I think you will find that the Vortex presents a better IQ overall, and on cloudy days, or dawn or dusk, the difference will be markedly superior.
