Nishant, Let me try to answer this...Landscape
- Daytime landscape (15mm canon / 17mm Tamron), Tamron is much sharper when stopped down to (f4 onwards). Assuming good day light. Canon starts @ f3.5
- Poor day light / Evening / Night, Tamron @ 2.8 will help a lot. But using f2.8 for landscape is a matter of personal preference and photography style.Potrait
- Indoor, single person / outdoor 'poor' light: Tamron @ f2.8 beats Canon @ f3.5, assuming you are not using flash.
- Indoor, groups: You will have to stop down, where both of them will produce similar results. I personally felt Tamron is sharper @ all ranges compared to Canon 15-85mm, given enough light is available.
- Outdoor 'good' light - Both of them are good. But Canon @ 85mm is a lovely focal range for potrait. Not from a reach perspective only, but also from qualitative aspects of picture @ FL.Street photography
Again when light conditions are poor, Tamron has advantage. However Canon additional 35mm is also very helpful for street photography.
Overall I felt Tamron's fixed apperture is more important to me, than additional 35mm.
From a reach perspective, I can take three large steps to cover the additional 35mm (FOV) with Tamron f2.8, but I can't crank up ISO to 1600 (in 550D) or above to shoot potrait @ 50mm. I personally felt @ ISO 1600 on 550D, does not render 'clean' pictures.
Autofocus in Tamron @ low light is slow but atleast you can take 'clean' pictures in low light, without flash. Apart from 'poorly lit indoors', focusing speed is 'decent' in Tamron. I'm comparing against Canon 85 f1.8 and similar. I dont think Canon 15-85mm focuses 'drastically' faster than the Tamron, in good light.Moolah
: A new lens of both these are quite different in price points. Thats very subjective.
Hope this helps