The effect you see and like by using +1 or +2 can be mitigated/altered depending on what aperture you use and additionally whether you select these settings on your DC control ring prior to or after focusing. Since setting the DC control will ultimately cause a shift in the depth of field ( and your subject placement within this zone), the sequence used for setting DC ring and focusing will have quite different results, as will aperture selected.
Dave (D&A)
Dave, the other Sigma that is worth mentioning is the 120-300 f2.8! It is clearly bigger still and not a travel lens, but a lot of glass for the money. IIRC, the 120-300/4 is not OS/VR, correct?
That's correct Jack, the 100-300 f4 was never offered with OS/VR. Both lenses you mentioned are a lot of lens for the money, considering their competive pricing and their performance. The reason I believe the 100-300 f4,wasn't offered with OS/VR at this time last year (or approx 12-18 months ago) when Sigma was updating many of their popular lenses with OS/VR, was that they were trying to promote sales of their 70-200 f2.8 which wasn't a very big seller and therefore updating the 70-200 f2.8 lens with OS/VR and along with the accompanied sale of their 1.4x with that lens, they would have something close to the equivalent of a 100-300 f4 and also a combination to directly compete with Nikon's 70-200 f2.8VRII.
I can tell you from personal experience that the 100-300 f4 is far superior than their updated 70-200 f2.8, especially when that lens is used with their 1.4x. I tested the 100-300 f4 along side their older non OS/VR 120-300 f2.8 and I'd give the edge to the faster zoom but not by much. When their 1.4x is used with both lenses, centrally it was almost a dead heat but the edges sides were distinctively better in the faster lens, but again not dramatically. When you consider the 100-300 f4 even at that time was $2000 less expensive than it's faster brother and much smaller and lighter by a significant margin, the 100-300 f4 was such a good buy, that most believe that Sigma was worried by keeping the 100-300 f4 lens in production, that it might take sales away from both their new 70-200 f2.8 and new 120-300 f2.8, especially if they (Sigma) updated the 100-300 f4 with OS/VR.
One last thing to note. The older 120-300 f2.8 non OS/VR had major issues with correct focusing on Nikon bodies and rectifying it with a trip back to Sigma, wasn't always successful. The 100-300 f4 basically was issue free except for the usual Sigma sample to sample variation. I haven't shot with the newly released 120-300 f2.8 OS/VR lens yet (hope to soon), but in speaking with a couple that already shot with this newly updated lens, optically it's close to the original but appears to be better with regards to correct focusing. Optically comapred to it's older non OS/VR version, nothing definitive yet from what I've heard so far. They definitely improved the mechanical and handling design of the lens though. Unfotunately sample to sample variation in both older lenses was often onsiderable and finding a good optical sample takes some doing.
After owning and using both (with and without their 1.4x), I kept the 100-300 f4, it being an invauable lens for concert work, much like Nikon's 70-200 f2.8 VRII, not only due to manageable size/weight but also that the images it produces has always been impressive.
Dave (D&A)
***(This post has been substantially corrected and updated on 4/6/12)