I'm not sure if this is obvious to you or not, but I'm an intuitive (read: touchy-feely) photographer. I make most of my photographic decisions based on the feel of a scene, the light, my mood... I can read a light meter, and adjust my exposure, but that's about as technical as I get while shooting.
So, when it comes to choosing a new lens, I'm often at a loss. Focal length? Barrel distortion? I know academically what they mean, but frankly my brain shuts down when faced with all that data. It would be an expensive mistake to get the wrong lens, but I just can't make the decision based on the technical specs.
I know how I want a new lens to feel - I want a nice narrow depth of field and beautiful bokeh, with macro capability being a plus. But even after reading countless reviews, consulting photographer friends, and looking for example images taken with various lenses, I still wasn't ready to commit. I did, however, narrow it down to two fixed focal length Nikon lenses - a 50mm or an 85mm.
That's when Alicia Bock (one of my favorite photographers) suggested I rent the lenses to test drive them first. Sheer genius. I spent a fraction of the cost of a new lens and can now try them both and make an educated decision about which to purchase.
So far I'm leaning towards the 85mm, I love the macro capability, and it does have beautiful soft blurring in the background. It's also incredibly sharp, much more so than anything I currently have in my kit. Just look at the lovely crisp focus on the curling waves, and the silky smooth blur in the foreground - heaven!
The verdict? Next time I'm in the market for a new lens, or heck, any other photo equipment, I'm going to look to rent it first. No amount of number crunching or schematic reading can substitute for holding the real thing in my hands and trying it out.