[Icelander]

The simplified RAW is that optics reduce -1 in range Vision penalties per doubling of magnification and with an Aim maneuver, the user can focus on a target and reduce -2 in range Vision penalties per doubling of magnification for seeing details on that specific target.

As a practical matter, I'd rule that as magnification goes up, scanning the horizon with binoculars takes longer. As such, I don't think I'd allow increasing magnification to reduce range penalties to Vision for all purposes, with a practical maximum for general scouting probably around removing -3 in range penalties, only getting greater penalty reduction when scanning a smaller area than the entire forward facing of the character.

This would mean that typical mid-power binoculars, like the 10x42, were more or less optimal for scanning for threats or hunting for deer, with higher magnifications being primarily useful for more specific tasks requiring greater detail resolution. That's not to say that I'd never allow -4 or more in reduced Vision penalties from high magnification binoculars without an Aim maneuver on one specific target, just that getting a higher bonus requires that the character accepts that he's watching a somewhat smaller area, even if larger than one specific target.

Also, the more you go over 10x, the more stability becomes an issue, so to really get the maximum benefit from the highest magnification, you need a tripod or another way to stabilise.

Rifle scopes give +1 Acc per doubling of magnification and can be used in the focused role above. Personally, I'd not allow a rifle scope to reduce range penalties to Vision when scanning the entire forward arc for targets, ruling that the limited field of vision counteracted the magnification unless focusing on something in specific.

Greater magnification granularity

I was considering giving magnification levels close to breakpoints a slight boost, at least if the optics are high quality. In the real world, plenty of good optics are 6x and 7x, which GURPS treats identically to 4x. I think there is room for greater granularity.

My current idea for modifiers would be something like this:

(Magnification level = Vision penalty reduction / Vision penalty reduction with Aim)

1.5x = -0 / -1
1.75x = -1 / -1
2x = -1 / -2
3x = -1 / -3
3.5x = -2 / -3
4x = -2 / -4
6x = -2 / -5
7x = -3 / -5
8x = -3 / -6
10x = -3 / -7
14x = -4 / -7
16x = -4 / -8
20x = -4 / -9
25x = -5 / -9
32x = -5 / -10
40x = -5 / -11
50x = -6 / -11
64x = -6 / -12
80x = -6 / -13
100x = -7 / -13
128x = -7 / -14
160x = -7 / -15
200x = -8 / -15

Any suggestions for different breakpoints?

And, yes, I know that makes 10x kind of the ultimate sweet spot. I figure that as it's heavily used both in binoculars and riflescopes, we may as well elect to put the benchmark where it matches real world selection.

Low light performance

High-Tech notes that all but the cheapest TL7-8 telescopic sights reduce darkness penalties by 1, due to the ability of the scope to gather light. It is not noted that a similar rule applies for binoculars, but I'd personally rule that such light amplification applies generally to any kind of optics, assuming they are of a suitable design and quality.

As I noted above, I like Anthony's suggestion that some optics actually reduce the amount of light picked up compared to the unaided Mark 1 Mod 0 eyeball. Given the impact of good lens coatings and glass quality on real world performance at twilight, I hesitate to use only the ratio between objective lens and magnification to determine whether given optics give an additional darkness penalty or reduce it, but for optics of otherwise equal quality, the ratio is a good guideline.

Perhaps the simplest solution would be to say that some late TL8 optics are available at expensive quality levels that reduce darkness penalties by 1 more than the objective lens / magnification ratio otherwise suggests. This applies most often to optics that are otherwise close to ratio breakpoints, like 6x30, 7x28, 8x30, 8x32 and 10x50.

Then the modifier for objective lens / magnification ratio would be:

2:1 or lower= -2 additional darkness penalty.
2.01:1 to 4.19:1= -1 additional darkness penalty.
4.20:1 to 5.59:1= no effect on darkness penalty.
5.60:1 or higher= reduces darkness penalty by 1.

Would anyone suggest different ratios, especially based on either physics or real world experiences?