Sigma 24-70 f2.8 EX DG Macro & Tamron 28-75 f2.8 XR Di Macro
June 22 2006: If there are 3 lenses I'm VERY familiar with, they are the Canon 2470L, Sigma 2470EX, and Tamron 2875Di. Over the past 3 or so years, I've used and/or owned several copies of each, sometimes seeing slight QC/IQ variations between them but for the most part all good copies (i.e., all falling within normal factory tolerances). I recently reacquired another 2470EX and found it superb on the wide-side of things but a bit disappointing at the tele-end. So I had Sigma NY make some adjustments (it went to them several times) and they apparently did all they could for me. The short of it is that the 24mm end still outperforms the 70mm end, but that isn't unheard of with most zoom lenses (i.e., usually one side will be better performing than the other). In the meantime, I opted to try out another (brand new) 2875Di and see how it fared with the 2470EX. Suffice it to say that while I'm not a fan of the Tamron's build-feel at all, optically it's very good.
In this particular test gallery, I have put together MANY 100% crops taken from a VARIETY OF FOCAL LENGTHS AND SUBJECT DISTANCES and MOSTLY WIDE OPEN (f2.8). You'll see mostly center crops, but there are also border crops posted too. They are crops of images of all sorts of subjects and under all sorts of lighting conditions, many of them harsh/daytime conditions. For most of the captures, I shot in Av mode with my 20D and often found that the Sigma, under identical and constant lighting, would UNDEREXPOSE quite a bit; histogram information would almost always be shifted toward the LEFT, whereas the Tamron would tend to "nail" exposure and be more correct. So in some cases, I shot in manual and "forced" identical exposures. But there are images where I left the 20D in Av and hence there are some crops where the EX is darker. Also, for whatever reason, the Tamron tended to focus more correctly at the wide end, and so if you view a crop of the Sigma and think it's OOF, it just might be. But I still posted everything because whatever you see is the BEST capture I was able to get out of 5 attempts for each lens and setting. So if the image is OOF, that is actually the BEST capture available. From the crops you can not only see differences in sharpness and contrast, but also sometimes color fringing performance. I've also got some flare shots (simply resized for web) for you to view. It's also important to note that lenses vary in their optical performance at different focal lengths and subject distances. So keep this in mind as you view the various crops (which feature differing focal lengths and subject distances). Be sure to read my notes at each image, especially that the Sigma is always the LEFT or UPPER crop while the Tamron is the RIGHT or LOWER crop. As always, view in "original" size.
With regards to my conclusion about these 2 lenses, let me break it down to specifics. In terms of walkaround weight, the EX is 27oz and the Di 19oz, so we're talking about 1/2lb difference. For me personally, this is a non-issue. I'm used to walking around with my 70200IS and 100400IS (and prior to the 100400IS I had the Bigma), so these smaller zooms don't bother me one bit. I actually prefer the more robust build and "in-hand" feel of the Sigma over the Tamron, so the 8oz difference is not a meaningful savings to me. I could see it being meaningful, however, to someone else, so this is clearly a "preference" thing. But for me, I like the added weight/feel of the EX.
As to AF speed and noise, both make noise, albeit the EX has a higher pitch while the Di has more of a low-grind drone to it. Speed-wise, the EX is a tad faster. It should be noted also that during AF, the EX has no externally-rotating barrel but it does physically extend forward and backward; conversely, the Di does have an externally-rotating barrel but does not move forward or backward. Again, I prefer the EX here since the slight movement forward and backward doesn't affect shooting at all, while the rotating focus barrel on the Tamron can be damaged if you grab and turn it accidentally. All in all more nitpicking than anything though. Both lack any kind of HSM and FTM. The Tamron requires one switch to go to MF while the Sigma requires the two-step mechanism. Also, the Sigma focuses better in low-light; the Tamron tends to "hunt" more.
As to image quality, in most of my captures the Tamron Di is both sharper and more colorful at f2.8 (which is where I really wanted to test them out; both lenses of course get sharper stopping down). Optically-speaking, the Di truly SHINES. It's sharpness across the focal range (wide open), contrast reproduction, color fringing performance, distortion control, etcetera, is fantastic. It's definitely the best copy I've ever used. As for this particular EX copy, I think it's the second best copy I've used (I've had 4 copies over the years: 1 older DG DF, 3 newer DG Macro). The one previous to this copy was tremendous and was sold to a good man in Missouri. You can see just how good that copy was in this test against the excellent 2470L http://www.pbase.com/lightrules/image/45240719/original . But this current copy is IMO (and Sigma NY's opinion also) within "spec" and a good-working copy. Optically it's inferior to this Di copy but with even basic PP you can make images "pop" with sharpness and color. As to flare performance, the EX and Di are about the same; I'd say they both perform "moderately well" in this regard.
At day's end, each lens has its "perks" over the other. For those that want the closest "specs" to the excellent 2470L, then the EX is probably your best bet. The 24mm wide end is truly a significant difference vis-a-vis the 28mm of the Tamron, but for pure image quality starting at the 28mm focal length, you're not going to beat this "plastic-feeling" and "slow-AF'ing" Tamron. It truly "stunned" me with its optics. It's a lens that I haven't liked in the past, and now one that I "hate to love." You could say that "I hate it's build, I love its image quality." With the Sigma EX costing $429 and the Tamron Di at $379 (B&H pricing), and with both being FF compatibile, they are truly excellent "bargain" standard zoom lenses worth all the "hoopla" they get.
FM User Reviews Comparison as of October 2007:
Sigma 24-70: Reviews 113, 86% Recommended, Build Quality 8.80, Price Rating 9.43, Overall Rating 8.3
Tamron 28-75: Reviews 163, 85% Recommended, Build Quality 7.97, Price Rating 9.18, Overall Rating 8.6
[Note that for the Sigma the data includes both the older non-DG Macro version and the new DG Macro version. Since the newer version is optically better, this should be taken into account.]