That is a very strange thing for any competent reviewer to write. DOF is not dependent on an individual lens and would be the same for any lens at the same focal length and aperture.
Besides, narrow DOF makes focusing easier because there is a greater difference between focused and unfocused. That is one of the reasons SLR's focus with the aperture at its widest.
I actually wonder about that review. Seems a bit rushed to me.
Summary of the review:
Talk about the range on a FF and crop.
Talk about it being difficult to handle because slight movements can make a difference.
Therefore need OS (IS)
Talk about the OS
Say it's big and heavy - need to make sure you really need this length.
Talk about the mechanics of the lens (zoom, tripod collar)
Autofocus = fast/smooth.
HSM = full time manual overide.
Then...
"Accurate focusing remains a bit of a knack, as with all long lenses, not just this one. At these focal lengths there's almost no DOF at all, and the slightest movement of the part of your subject can send it out of focus. It's often better to 'prefocus' rather than try to follow a moving subject."
"The Sigma felt very good in use... disappointing, then, to find that the pictures didn't look particularly crisp, particularly those taken at longer focal lengths. The focusing was spot-on and the shutter speeds were usually high enough to prevent camera shake with or without the OS
The lens tests confirmed this impression at 150mm the Singma is not a bad performer, producing photos with fair to good levels of detail. But by 250mm sharpness has dropped to a point where images look visibly soft, and at 500mm they're softer still.
A second set of tests confirmed the figures gained in the first = disappointing in a lens costing 750 (british pounds) whatever it's specs and abilities."
They then go on to say the 50-500 is sharper but a bit more expensive.\ (but with no OS).
This is their own example at 500mm (posted before):
400D
1/250
f/6.3
500mm
Looks like they didn't focus properly (sure it looks "soft" if you look at the eyes... but the NOSE is in focus....) oh and perhaps stopping down a bit might help even a higher ISO to get it stopped down a bit more for more DOF?
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