The
main imager (LORRI) is a 208mm diameter telescope with a 2630mm focal length, or f/12.6. The spider and secondary obscure 11% of the area, so that's equivalent to f/13.4 in terms of light gathering for photographic purposes. Exposure times are 50-200 ms, or 1/20 to 1/5 sec.
On Earth, the
sunny 16 rule says on a sunny day the proper exposure at f/16 is when your shutter speed is 1/ISO. So f/16, 100 ISO, 1/100 sec. The atmosphere absorbs roughly half the sunlight, so in earth orbit that would become f/16, 100 ISO, and 1/200 sec.
Pluto is about 32.6 AU from from the sun right now, so the sun's brightness there is 1/32.6^2 = 1/1063 what it is at Earth.
Going from f/16 to f/13.4 gets you about 1.43x more light.
Increasing exposure time from 1/200 sec to 1/10 sec gets you 20x more light.
That leaves a deficit of 37.2x, which you can get by increasing CCD sensitivity to ISO to 3,720.
ISO 3200 was easily attainable by high-end consumer digital camera sensors 10 years ago, much less a commercial one specially designed for scientific purposes.