I\\\\\\\'ve been using the Sigmonster (Sigma 300-800 DG, Canon mount) as my main birding lens since since April this year, and I\\\\\\\'m very, very impressed with its optical performance.
THE GOOD POINTS:
It beats my beloved 400 5.6L in sharpness and contrast at any length (with TC on the 400 prime) and aperture.
Here are unprocessed 100% crops straight from the camera (jpeg large fine, parameter -1/0/+1/0) from my copies of the 400 5.6L and the Sigmonster:
http://www.pbase.com/liquidstone/image/52379202/original
This beast is prime-like in sharpness (on my 20D/350D/300D) from 300-700 mm wide open, and I stop down in those focal lengths only for more DOF. At 800 mm, it\\\\\\\'s still sharp wide open, but improves back to super-sharpness at f/8 - f/11.
Here\\\\\\\'s a shot at 800 mm, f/5.6 , 1/60 sec, ISO 200:
http://www.pbase.com/liquidstone/image/51039579/original
100% crop:
http://www.pbase.com/liquidstone/image/51039585/original
Here\\\\\\\'s one at 800 mm, stopped down to f/9, 1/250 sec, ISO 200
http://www.pbase.com/liquidstone/image/52733557
100% crop:
http://www.pbase.com/liquidstone/image/52733454/original
A test shot at 1000 meters:
http://www.pbase.com/liquidstone/image/52021274
100% crops
http://www.pbase.com/liquidstone/image/52021338/original
http://www.pbase.com/liquidstone/image/52045420
Wide open shots at 700 mm or shorter:
648 mm, 1/60 sec, ISO 400
http://www.pbase.com/liquidstone/image/48133059/original
687 mm, 1/80 sec, ISO 400
http://www.pbase.com/liquidstone/image/52847449
With a 2x TC, there\\\\\\\'s some IQ degradation but it\\\\\\\'s still better than upressing. Here\\\\\\\'s a shot at 1600 mm with a Sigma 2x TC (2560 mm equivalent AOV with the 350D), f/18, 1/50 sec, ISO 400, manual focus and exposure:
http://www.pbase.com/liquidstone/image/50832927
100% crop
http://www.pbase.com/liquidstone/image/50832965/original
The Sigmonster (with 350D) is now my main long gun and it allowed me to get close to a lot of shy Philippine endemics. For flight shots, my 20D + 400 5.6L is of course the better tool.
THE NOT-SO-GOOD POINTS
Sigma has increased the price of the DG version to circa USD 6200 (B&H). From the outpout I\\\\\\\'ve seen, I can\\\\\\\'t see a difference in IQ between the older non-DG and the newer DG one. So if I were to buy this lens all over again, I\\\\\\\'d go for the non-DG version if it\\\\\\\'s cheaper by more than USD 1,000.
This lens would benefit greatly with the addition of Optical Stabilization (tripod-ready OS) and a focus limiter switch. The latter, a standard feature of Canon long Ls, works wonders in speeding up AF at fliers. The Sigmonster\\\\\\\'s HSM is fast, but Canon\\\\\\\'s USM is a bit faster (just a field use impression, not based on objective test).
The Sigmonster is a heavy beast, and I\\\\\\\'ve not been able to hand hold it in any manner. But I knew about the weight and bulk before I bought it, and had accepted these trade-offs for extreme reach, near-prime optics and zoom flexibility.
It needs a good support and long lens technique (LLT) to do its job well. For support, I use a Manfrotto 3421 gimbal head and 475B tripod.
The classic LLT of \\\\\\\"left hand on the lens, face against the VF and gentle shutter press\\\\\\\" work for many people. However, I prefer to use a remote switch, hands off the lens and wait for the vibration to die down (evident in the VF) before tripping the shutter. With this LLT, I have gone as slow as 0.8 sec (with MLU) and 1/40 - 1/50 sec (without MLU) at 800 mm and got decent results. In strong wind though, I revert back to the classic LLT and just use faster shutter speeds of 1/160 or faster.
MY CONCLUSION:
If you\\\\\\\'re looking for 500 mm to 1000 mm and beyond reach, the 500 f/4 L IS is lighter (hence can be hand held) and more compact, 1 stop brighter and most probably a bit sharper at 500 mm, has IS, faster AF and weather-sealing.
But if you value the framing flexibility of zoom while still retaining near-prime sharpness, and can live without IS, the Sigmonster is a worthy alternative to the similarly weighing 600 f/4 L IS.
Most of the recent bird pics in my galleries are taken with the Sigmonster:
http://www.pbase.com/liquidstone
Warmest regards from our islands,
Romy Ocon
Manila, Philippines
UPDATE (April 14, 2007):
I purchased the 500 f4 IS in April 2006, about a year after using the Sigmonster, to reinforce my mid-range as far as hand holdability and AF speed are concerned.
After tests and much field use, I wish to correct my earlier statement, made before I got the 500 f4 IS, that the Canon prime is \\\"most probably a bit sharper\\\" than the Sigmonster.
To my eyes, both lenses are equivalent in sharpness at 500 and 700 mm, at any common aperture. Here\\\'s a link to a shootout between the two:
http://www.pbase.com/liquidstone/image/58879465/original
The 500 is lighter, has IS and autofocuses faster, while the Sigmonster is longer and has framing flexibility. I consider each as complementary to the other, and I always bring both in my birding trips.
Romy