Hi DLord,
Thanks for your input.
I did a quick search and read some articles on the Tamron 17-50mm f/2.8. Not an easy read for a beginner like me. The f/1.8 is certainly great for low light photography. I noticed I forgot to mention that I may from time to time need closeup shots of the exhibits. The Canon 50mm f/1.8 sounded like an excellent choice if they allow me to stand just in front of the exhibits. As soon as its about 5ft away, would the budget Tamron zoom at 50mm still allow a closeup shot under low light condition with ony f/2.8?
Hope to hear from you again.
At about $120 new (or just below $100 used), you may wish to buy the 50mm f/1.8 mkII and see if it meets your needs. The Tamron at 50mm will not be any different from the Canon 50mm in terms of how large your subject appears in your picture. The f/1.8 will allow you to take the shot at a slightly higher shutter speed than the f/2.8 though - thus more "suitable" for your low light photography.
But then again, the f/2.8 *might* be sufficient for you low-light needs. The f/1.8 will give you slightly more than twice the amount of light at full open apertures.