The ultimate general purpose birding lens (at least until the mark 2 finally arrives) we've lusted after this lens for years and now we've finally got one. After renting one for a long week in Norfolk we just had to have it (we found it very hard going back to the Sigma zoom).
Although it is quite an old design now (first appearing in 1999) it is still optically outstanding, even compared to much newer lenses. It is this sharpness, along with the fantastic colours and contrast that made it so desirable to us. Also the fast (for such a big lens) f/4.0 aperture would allow us to work in much lower light levels than our cheaper telephoto lenses and enable the use of a teleconverter to get even more reach.
We very quickly saw a big step in quality (and reach with a 1.4x TC) after using this lens. Although it does require good long lens technique we were reasonably experienced with this after a few years of using the Sigma 150-500. Although we're still mastering the lens it's already putting many of our older shots to shame.
To help things the lens has 2-stop image stabilisation and is thankfully tripod sensing. We always have it turned on, at these high magnifications anything to keep blur down is useful. It has a low grinding noise when on, perhaps slightly louder than the Sigma, but without the loud start/stop noises.
Also on the side are more buttons alongside the usual AF/MF and IS modes. The auto focus can be limited to 4.5-10m, 10m-infinity or 4.5m-infinity. Most of the time this is left on full unless we're photographing very specific things, where it can limit focus hunting.
There is also a focus preset; if you're positioned waiting for something it's useful to have the focus preset to where you expect the subject to be, then you can go focus on other things but snap right back at the touch of a button. Useful if you know where the subject is going to be but not something we use very often.
There are three buttons near the front of the lens that can be used to temporarily stop the autofocus. Never used them and I can't think of a situation I would ever want to. It's easy to overlook them completely.
The included lens hood is almost essential, to both protect the front elements and keep flare down. The only time I would take it off is in very high winds, where it can be a bit like a sail. Made from carbon fibre it's not very heavy given its size but it is a little awkward using the twist screw, rather than the bayonet fitting we're used to. The size can also be awkward; it's very easy to bump into things and some hides are too small to fit it through!
Although we knew the optics, focusing, image quality etc were very good we did try-before-you-buy by renting one for a week. This was mostly to see if we could handle the lens in the field effectively and if it was going to give us better results (the fear being that our existing kit was better than we were and we would need to improve ourselves before improving our lenses).
The lens, even compared to the Sigma 150-500mm, is big and heavy. So much so that some people would struggle to use and carry it all day. Our typical method of working involves mostly hand holding, hiking and using a monopod where appropriate so being able to handle the lens' weight and size were very important. I would class it at the limit of what I would like to carry around on a day's shot. The heavier 600mm and 800mm lenses would be too heavy for our style and fitness.
We can hand hold the lens for short periods (although we would prefer not too!) The lens balances nicely with a pro-sized body. This is mostly for subjects that suddenly appear and may disappear again as quickly. If we get some more time to work we tend to put the whole setup on a monopod with gimbal head. This takes all the weight and provides much better stability without the burden of a full tripod. We will use a tripod in specific cases, usually where we're not moving very far, but generally prefer to be more mobile. We also use bean bags a lot in hides and hanging out of windows, with good results.
The lens is weather-sealed, so with an appropriate weather-sealed body it can be used in wet conditions. We haven't used this feature much yet, partly because we don't want to risk it too much but also because if it's raining it's probably quite dim as well which makes life more awkward. It is good to not have to panic when the rain starts, though, and with a rain cover we're able to shoot in bad conditions when the occasion warrants it.
The only particularly limiting thing we've found so far is the close focusing distance. At 4.5m (giving a maximum magnification of 0.12) we quite often find ourselves having to step back to get things in focus and it's hard to get enough magnification for small birds or dragonflies. The addition of a 1.4x TC boosts the magnification to roughly 0.17 at the same 4.5m, the same as the Sigma. This is just about enough for larger dragonflies and butterflies but this isn't a lens that's easy to use for insect shots and why we will still carry a second smaller telephoto lens.
All together it's a very impressive package but one that comes at a heavy cost in price and weight. We look forward to using this over the years to come but we appreciate that we're lucky to be able to afford and handle one. The mark2 solves some of the problems with the weight and close focusing (that alone would be enough for us without any optical changes) but the price will need to come down a lot before we would consider upgrading.