Originally posted by bjsmith What dSLR system can give you 300mm equivalent f/2.8 in 5" long? ... And Pentax's product (DA* 200 f/2.8) has so many of the critics continuing to rave.
Just for correctness, a 300mm F2.8 equivalent would need to be something like 200mm F2. The DA* 200mm F2.8 is more like a 300mm F4 equivalent in terms of pure image output. You can't make the same image using 200mm @ F2.8 on APS-C as you can using a 300mm @ F2.8 lens on FF. Depth of field will be smaller and background blur great when using the 300mm @F2.8 on full frame while shooting the same scene. Whether or not this matters to you (or could even perhaps be an advantage to APS-C given extra DOF) is a totally different topic.
Also, I am convinced that FF will always produce better output (by at least a stop) than APS-C, given that the sensors are manufactured using the same technology, and the image processor is the same. This also goes along with the above, that F2.8 on APS-C is not equal to F2.8 on FF. The APS-C will lag behind at the same ISO.
When the noise levels in sensors get so good that noise is not visible at regular ISO values even on APS-C sensors, then I would concede that the FF noise advantage would diminish. But there is still the first point about equal image output that will always hold true, not matter how good the sensors are.