The Sigma 100-300mm F4 EX HSM lens is a uniquely sized portrait, sport and wildlife lens aimed at professional photographers that seeks a longer lens than a 70-200 F2.8 yet still weighing roughly the same.

Despite the focal length span of 100-300mm the lens does not feature a focus limiter like the F2.8´s of both Canon and Nikon. Quite frankly I don´t think its even necessary since the lens focuses incredibly fast perhaps the fastest Sigma I´ve tried certainly as fast as the F2.8´s. The focus is almost instant. Adding teleconverters slows pickup speed a little but on a pro body its barely noticeable.

Sigma 100-300mm F4 EX HSM + 2x TC (600mm) on a 1D mark II body wide open on a very overcast day

The lens itself balanced perfectly on a pro size body or a smaller body with a grip. The zoom ring sits a little to close to the tripod foot and I highly recommend you´ll remove the foot to ensure full control and grip on the lens. Otherwise it might be in the way. The zoom ring is oddly small close to a third the size of the focus ring. I certainly would liked it to be the other way around. The zoom ring is smooth to use and you can zoom from 100-300 in one movement – thankfully! The ring features a rubberized surface in a open pattern design. I would have liked if they used the same pattern as on the 300-800mm F5.6.

Sigma 100-300mm F4 EX HSM (126mm) wide open on a 500D

The focus ring is much larger and although smooth a bit more tight to turn. It features close to the same pattern as the zoom ring but since its much larger it functions that much better. The focus ring is a joy to use. Although its quite tight you´ll have to move your hand in order to focus from one end of the scale to another. Its sits completely free from any obstructions and would have been perfect for the zoom ring and visa versa.

Sigma 100-300mm F4 EX HSM (126mm) wide open on a 1D Mark II body. Even in difficult situations I find the bokeh beautiful.

Compared to most 70-200mm F2.8´s is does not features any IS/VR/OS. Certainly it would help in quite a lot of situations and I suspect Sigma will release a OS version in a near future as I´m hoping they´ll do with the 300mm F2.8, 500mm F4.5 and 800mm F5.6 / 300-800mm F5.6.

Sigma 100-300mm F4 EX HSM + 1.4x TC (420mm)
wide open on a 1d Mark II body – Not so wildlife creature.

Another very important thing and perhaps the most important is the image quality. Luckily the Sigma 100-300mm F4 EX HSM doesn´t disappoint here either. Going into this review I was quite sceptical since I´ve read and heard both good and bad things about this lens. It seems as there could be sample varieties.
Wide open the lens is very sharp especially in the center. It does benefit stopping down but overall impression is very promising. The colors are a bit to the warm side which I for one love but does especially in harsh light suffer a bit from the lack of contrast. Easily fixable in a raw converter. Compared to the Canon 70-200mm F4 USM the image quality are very alike both very sharp but the Canon with slightly better contrast. – Either way its definitely impressive and I´ll rate the Sigma 100-300mm F4 EX HSM as sharp as the 300-800mm F5.6 EX HSM at 300mm both wide open.

Samples (Straight from RAW no processing) – Right click and choose “save as..”
SAMPLE 1
SAMPLE 2

Conclusion
The Sigma 100-300mm F4 EX HSM is a great lens. Not weighing a ton you´ll be able to shoot this handheld all day long. It would be the perfect action lens alternative to the 70-200´s especially for those not seeking the heavier F2.8´s. It focuses lightning fast, displays great image quality even wide open and has a very usable focus range. For nature photographers it might be a bit short. Using converters certainly help but then you´ll need more light depending on what teleconverter you might choose which isn´t always the greatest thing since the lens isn´t stabilized. Sharpness suffers only so slightly so I recon a 1.4x TC would be glued to the lens all of the time. A 100-400mm Canon or similar size might be a better choice for close to the same money.
So its definitely highly recommendable especially for action shooters.