Don't buy from any shop on Nathan Road in Tsim Sha Tsui. They are all tourist traps. Or any generally place that has a "tax free" sign, because everything is tax free anyway! The only reputable place I can think of at the moment in that area would be Tin Cheung Camera on Hanoi Road.
There are a row of good shops on Stanley Street in Central, but I cant remember the name of the shop that a lot of people recommend there. There's a real cheap place that devs slide film for about a pound in a few hours! Also worth looking around the Mong Kok area. Wing Shing Photo and Man Shing Photo are places I check out first, these are pretty close to the MTR station.
Generally you'll find prices are about 20% cheaper. Anything significantly cheaper is probably a grey import, this is where the real bargains are but of course its down to whether you are willing to risk the absence of warranty.
Generally everything is supplied with a HK/China warranty, but I think its worth asking if the product has international warranty or if they can put in an international warranty card
There are also some really small shops to be found with huge savings but I really wouldn't recommend these to someone stopping over for a day, unless you have a local to take you around and argue for you!
If you search camera things on
http://www.price.com.hk/ and decipher it, you will be able to gauge the general cost. A 行 next to the price means its official HK stock. 水 means that its a grey import.
You can also try haggling for discounts at most shops and they will be willing to do something especially if you buy multiple items, not sure of the chains like Fortress or Broadway would be willing.
Hope this helps a bit.
