I have a question about understanding the model names of lenses.
For example, comparing Tamron AF 17-50mm F/2.8 XR Di II LD , and Sigma17-70mm f/2.8-4.5 DC Macro.
The Tamron model refers to F2.8, does that mean that whether i am at 17mm or 50mm, i will still be able to use F2.8, and for the Sigma one, the F stop number will automatically increase from 2.8 to 4.5 as I zoom from 17mm to 70mm, meaning i am forced to use a smaller aperture at the longer zoom.
I just got my DSLR not long and i notice that with my kit lens. So just like to clarify, so that i know what I am buying when i get other lenses in the future.
Pls kindly correct me if i am wrong.
And if i am right, then the Tamron in this case should be a better lens compared to the Sigma, except for the shorter zoom?
Thank you for you input.