Hi APers!

I'm getting into more dental photography at work. Right now I've got a 17-55mm 2.8 with an on camera flash that I've been using. I need to crop all my photos but it is sufficient for now.

I'm thinking of getting some extension tubes to let me get in closer. Anybody know how much it will crop the photo when I add a set of kenko tubes at 50mm? I'd rather not get another lens if I can help it. Was thinking something cheaper like a sigma 70mm if that was the case.

I also need a ring flash. I'd like an actual ring so I can use it for portrait fill - not just one of those 'close up' photography ones. Suggestions and user reviews needed please.

Thanks