More specifically, the LACK of a tripod mount/foot on these lenses. It used to be that lenses 300mm and longer had tripod mounts, because people could NOT hold them steady enough. But in my recent shopping, it seems that some X-300 or X-400mm zooms do not have tripod mounts. In looking at some of the lens, it seems that you can't even retrofit a tripod mount to it. So is it cost cutting to sell the lens cheaper or ??? What makes this difficult to figure out is that some of these are DX/crop lenses so there is that crop magnification that makes it worse. So a 300mm lens on a DX/crop body is like a 450mm lens on a FX/FF body. 9x is definitely a difficult hand holding magnification. And to make it worse, some of these lenses do NOT have VR/IS. The other factor is weight. I use a monopod under my D7200 + 75-300 (which has a tripod mount) just to support the gear, so my arms don't get so tired. We are not talking just a few minutes here. Yes some of the newer plastic lenses are much lighter than the older metal lenses, so there is less stress on the camera's lens mount, if you tripod mount on the body. But the balance point of the camera+lens is NOT under the body, so your tripod/monopod is trying to support a front heavy camera+lens. Example, Sigma 100-400 lens is 1160g, my D7200 is 675g. The Sigma is 1.6x the weight of my D7200. The balance point is NOT under the body, with that lens. Yet the lens has no tripod mount, or option for one.But then you have the shorter 70-200 lenses, which either comes with a tripod mount or has one as an option.