Differences in lenses become vary apparent when you're shooting with two of the same camera bodies. Now, I've done many test shoots to determine the limits and sweet spots of my lenses. The problem with test shoots is that I've done them serially and not in parallel. After the shoot is finished the various images are compared side by side and the differences are analyzed. The lab is very different than the field. In the field, controlled lighting and stationary subjects aren't available. You're lucky if you're able to get two of very similar shots with similar autofocus (AF) points in the field. It's these live test shoots where one learns to truly appreciate the difference between a fixed f1.2 lens, a fixed f2.8 lens, a variable f3.5-5.6 lens, and a variable f4-5.6 lens. (As an aside, the difference between a full-frame sensor and a sensor with a 1.6 magnification factor also is apparent.)
This past Saturday - in a low-light environment, I shot with a fixed f1.2 lens (85mm with the 1.6 magnification factor) and a variable f3.5-5.6 lens. I had an external flash unit attached to the variable f3.5-5.6 lens. Adding to the inequality of this particular field shoot was the fact that one lens was an L series and the other wasn't - both had the same Hoya Pro Digital filters.
After comparing the images across the lenses and the various apertures - I'm sold on getting the best possible lenses you can afford. In the field, having to remember which lenses have an aperture limitation and switching bodies takes too long. I don't want to have to focus on the technical details - I want to focus on the artistic presentation of an editorial composition. For my gigs this coming weekend, I've traded out my variable f3.5-5.6 lenses for a fixed f2.8L. The three gigs are all low-light environments and the clients need the ability to crop and enlarge the resulting cropped files. Rather than focusing on switching from the wide/zoom lenses at 5.0/5.6 to another zoom at 3.5/4.0, I can now focus on depth of field, composition, etc.
