Every now and then someone asks why I shoot micro four thirds (a smaller sensor size) now that Sony has full frame (bigger sensor) mirrorless cameras. The answer is the lenses. Because a micro four thirds sensor is one quarter the area of a full frame sensor, the lens only needs to cast light onto one quarter the area. This allows for much smaller and light lens designs. When we start talking about really expensive glass elements in the lens, it means less expensive too.
Sony and Panasonic have both recently announced 400mm equivalent f2.8 prime lenses. Both are serious sports and wildlife lenses capable of ultra fast auto focus, super shallow depth of field, and working in low light. While the Sony works better in really low light, in the majority of cases (day light or stadium lighting) most people would be hard pressed to tell the difference between the final images. Yet the Panasonic lens costs $3,000US and weighs 1.2kg while the Sony costs $12,000 and weighs in at 2.9kg.
If I was shooting the Olympics or the NFL, I might consider the Sony, but for everything else, it’s micro four thirds for me.