My new 800mm lens is pretty enormous - enormous enough to struggle with budget airlines. It also precludes full-on birding, as you're simply not as mobile. I've discovered the hard way that if I hang the lens via its strap off my shoulder, then the strap with all that weight attached can catch on the buttons of the top plate of the camera. It happened a few months ago with the the Flash Exposure Compensation button, no great disaster, but last week it also pinged off the ISO button, and that I couldn't live without. In to Canon it went, and seemingly buttons are not replaceable so I ended up with a whole new top-plate at a cost of £200. I don't fancy doing that again so the new method is to have the lens over my shoulder, holding it via the monopod - think of a hod-carrier and you'll get the approximate idea. The camera is safe, but I only have one hand for my bins, and that isn't enough. With a trip to Shetland beckoning, which requires both budget airlines and full-on birding, the 800mm seems not ideal, but focal length is still key. So instead I'm turning to the 300mm f2.8 IS, the only other longish telephoto I have, and both the 1.4x and 2x converters - both of which are the latest mark III versions, though the lens is the older model. If it works as a viable combination I may upgrade to the newer model which is lighter, works more efficiently with the teleconverters, and has a better image stabilisation system.
I took a walk around lunchtime to see if I could find a bird - this needed to be a real life example, not a test chart. All the shots are hand-held, and the body was a 5D Mark III - lots of lovely pixels! The first photo is at 420mm, 1/5000s at f4, and the second one below is at 600mm, 1/3200 at f5.6. Neither photo is cropped, so this is the actual difference in pixels on the bird that comes with the 2x converter. Both hand-held at ISO 400, so if slower shutter speeds are required on Shetland, as seems highly likely, my monopod is bound to come in very useful.