Originally posted by wildman What I don't get is wasn't all that obvious to designers from the beginning?
A 300mm lens requires an actual physical light path from the objective to the focal plane of 300mm no matter what body is used?
Or to put it simplistically another way - a mirrorless 300mm lens is essentially a 300mm DSLR lens with an additional permanent extension tube added to the back end all else being equal. So what else is new?
That is not strictly true, lens designers have tricks to shorten or lengthen the lens relative to its focal length. My Sigma 150-500 is nowhere near 500mm (20 inches) even fully extended. My Sigma 8-16 is much more than 16mm (about 2/3 of an inch) long. For these lenses, though, the conclusion is still valid, the short register distance of a mirrorless camera is wasted.
Quote: The question that comes to my mind is what essential innate advantage/disadvantage does/does not the two systems have over one another?
The big advantage for mirrorless is for lenses with a focal length larger than the registration distance of the mirrorless camera, but less than the registration distance of a DSLR. IIRC this is typically around 20mm to 40-something mm. This covers the traditional 135 film format wide angle range. A mirrorless lens in this range would not need to use the more complex "retro focus" formula, and it should be easier to produce a fully corrected lens with fewer elements, making it smaller and lighter.
But in reading that article, what struck me was not so much that FF 135 format lenses are necessarily big and heavy, but that high quality, fully corrected, fast "pro" lenses are necessarily big and heavy (for any format). That still makes the point that a FF 135 "pro" level mirrorless camera is not particularly attractive from a compactness perspective. Maybe for a back-packing landscape pro using 20-40mm lenses where speed and correction wide open is not as important and size/weight is critical.