It's been almost five months, but better late than never. I just stumbled upon the answer in the rule book.
Rangefinder Binoculars provide a +4 to Perception rolls at Medium, Long, and Extreme Range.
Range modifiers for the Perception skill are in the general Action Check Modifiers table on page 41 of AToW (as opposed to being in the description of the Perception Skill).
Medium Range is 301 to 600 meters, Long Range is 601 to 900 meters, and Extreme Range is over 900 meters.
So Rangefinder Binoculars would give you a total Perception modifier of +2/0/-2 at those three ranges, as opposed to the normal -2/-4/-6.
I guess they don't focus well at short range, since you would have a +1 or 0 bonus at less than 300 meters.