Quick question: In same scenario, fixed subject, fixed background. Which should give more subject 'isolation' (not sure of the best word - i mean a more blurred background): 135mm f/2 and crop to give same size subject as... 200mm f/2.8 and don't crop Actual lenses not really so important -- I'm interested in how you work it out. (I would test it myself but have lent the 200mm lens to my sister). Thanks! Jonathan.