However that said, occasionally I have found the need to get a little more reach or to be able to get some close up shots that the Canon 100-400L is not great at, but where the Sigma 150-500 was better.
|Sigma 150-500 - View on Black|
My reasons to go for the Canon over the Sigma wasn't so much image quality but was the fact that it was slower to focus, it didn't have focus limiters, larger than normal front filter ring (86mm), zoom creep (there's only one lock position while the 100-400 has a tension ring), it was noisier than the 100-400L and it was rather large and impossible to pack into any of my camera bags.
It does have some other advantages over the Canon which is the extra 100mm range, excellent close-up range, better image stabilizing, better tripod collar (that doubles as a carrying handle), less conspicuous black matte colouring, and better lens hood design.
|Canon 100-400L with pocketable accessories|
These accessories don't have to be specific to the brand, but these are handy because they are very pocketable. For me, if I can reduce the amount of gear I carry at any given time the more enjoyable my hikes are. A 1.4x teleconverter and extension tubes are just the thing that works best.
Now word of warning about teleconverters. The Canon teleconverter doesn't work with all lenses because it has an element that sticks out the front of the converter but it also reports the proper aperture compensation to the camera which then disables autofocus on these slower lenses. To combat this there's a simple little mod that takes very little effort. But before you think of doing this, you do compromise some minor image quality and AF speed (a lot!). This works best on a non-canon teleconverter. For some reason when it's done with a Canon version it freaks out seeking and seeking until it fails more than half the time (this was tested with the mark II and not the forthcoming mark III).
|Modified Tamron 1.4X Tele-converter|
The modification is relatively simple. It's a matter of just taping the first three contacts on the lens side of the teleconverter (look at the example). There are some issues with this though. The pins are sprung, so be aware that you need to have some really strong tape to keep those pins down. Aside from that, you can tape the last three pins on the lens' contacts (the clockwise most side of the contacts). This fools the lens in thinking there's no tele-converter attached and enabling a limited auto focus function.
Why is this limited? Phase detect focus is designed to work best with F/5.6 lenses or faster. Because we added a tele-converter, the actual aperture is now F/8. This means you need a lot of light for the auto focus sensors to work. It's not ideal, but if you have a subject which isn't moving a lot and you want a little more reach, this is where this mod comes in handy. This is actually why the Sigma lens at 500mm is slower to focus because at it's full extension it's a F/6.3 which although close to the F/5.6 still requires a little more light to keep the AF working well.
Canon 100-400L with Modified Tamron T-Con
I find this is a great emergency accessory and I try not to use it if necessary, but knowing that I can reach into my pocket for the t-con is always reassuring.
The next accessory is to carry a set of extension tubes. I recommend the Kenko ones with electronic contacts on them. Not only are they much cheaper than the Canon branded one, but it comes in a set of three. You won't really need to use all three of them, in fact I really only recommend the smallest 12mm one (as displayed on the earlier example). Just to clarify a point, this doesn't actually make it so you can use your zoom closer, it just makes it so you can multiply the size of your image thus getting a closer crop of your subject.
|Canon 100-400L with 12mm Extension Tube|
Although it does seem like a futile exercise to carry these two accessories and I'm sure some would question why I didn't just stay with the Sigma, this comes down to a personal choice. 99% of the time I use my Canon unmodified and I am very happy with the size, the range, and the functions, but in that 1% of the time when I wished I still had the Sigma, these accessories more than makes up for the lack of that without compromising for the reason's why I did change over to it.