In Oregon where I live, there is an immense amount of avian fauna - aka birds - both outside of my old farmhouse - and in the multitude of local and state parks, mountains, rivers and coastal areas. I've succumbed to the siren song of taking bird pictures, and my compact and eminently affordable small Olympus 75-300ii zoom lens covers 90 per cent of what I want to do. For the other 10 per cent though, I've occasionally thought about the superb Olympus 300mm f/4 prime (which seems to have become the weapon of choice among serious mu4/3 birders, but its stratospheric pricing (over $2K new) leaves it beyond my reach. Enter my budget alternative, freshly acquired from a thoughtful eBay seller in London: the (relatively ancient) Sigma 300mm Tele-Macro APO prime. Like the newer Zuiko, it has a fast f/4 maximum aperture; and like the Olympus, it's a relatively large and heavy lens, though definitely hand-holdable.
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It's also beautifully built, a solid bit of engineering. My version has the relatively rarer HSM focusing (most of the used ones for sale don't) and came in a Canon EF mount. I was lucky enough to score a Metabones EF-MFT 'smart' adapter at a stupidly reasonable price. The 'smart' designation means it preserves automatic aperture functions (as in most Canon EF lenses, there is no actual aperture ring, it's all electronic), and though the non-HSM versions have been faulted for (glacially) slow AF-focusing, this one seems to focus relatively quickly. Theoretically it might focus even more rapidly on the newer Olympus bodies, but (so far at least) it works nicely on my older E-M1.1.
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Though you can't see it in these photos, it has a relatively nifty slide-out lens hood, which retracts over the front element of the lens. There's a usable tripod adapter ring but I can already tell that there's going to be a significant learning curve for handheld photography.
The funny thing is, size is relative: I used to think my 75-300mm Olympus zoom was large but in comparison, it feels svelte, almost 'petite'. Go figure.
The Sigma is definitely an impressive chunk of glass, though.
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Total cost of lens + (necessary) smart adapter = more or less what a used copy of the nifty Olympus zoom sets you back. In other words, if I can figure out how to get some decent images with it, it will be a good addition to the wildlife lens quiver. The downside? I'm going to have to start going to the gym, working out, or lifting weights, in order to lug it around on longer hikes. Hmmmmm....