If you're in Central between now and Wednesday evening, this is well worth a visit.
I went along this morning, and there is a great selection of old (1860s-1930s) photographs and maps of Hong Kong to enjoy. They're well lit and clearly displayed, so it is as good as anything you'll find in the local museums.
The exhibition is at the Rotunda in Exchange Square, and is open from 9am - 7pm today, tomorrow and Wednesday. Free admission, no cameras.
Regards, David