The 100mm f/2.8L Macro Lens is Canon's mid-telephoto macro lens to include Canon's sophisticated Image Stabilization. With the highest quality optics available, combined with near-silent Ultrasonic focusing and life-size close-up capabilities without an adapter, the EF 100mm f/2.8L Macro IS USM is simply unrivalled.
This is the first lens on the market to incorporate Canon's Hybrid Image Stabilization Technology. Hybrid IS effectively compensates for both angular and shift camera shake during close-up shooting. The lens was developed to expand users' photographing range and allow a wider range of users to easily enjoy macro photography.
I love Canon. Although there were times when I shot with Nikons (D700, D300) and were pretty impressed with the result, I always came back to Canon.
This is the first is macro lens for Canon and they got it right. I have used the 60mm, 100mm, & 180mm macro before and by far this is the best!
Now, if you already own a 100mm macro you should try it before upgrading because the IQ of the lens are identical. I usually use this lens for portraits (yes, I know the 85mm & 135mm is a better portrait lens.) of my daughter and the IS is awesome. Hand holding 1/40 I can still get a sharp picture.
The thing that I really hated about the non-IS 100mm macro was the distribution of weight - it was the most awkward thing to shoot with. This lens feels lighter because of the even distribution of weight and size (gradual taper) and it includes a deep hood.
I know $1K is a hefty sum of cash, but considering what you get and how long it can last you - I don't know why you would settle for the non-IS.
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I found that for portrait, the bokeh on this thing is incredibly smooth. Instead of the angular blurs (lights, flowers, etc), you get a smooth circular blurs due to its spherical diaphragm. But I guess that's in the eye of the beholder...