Soon after I picked up the Canon 1D M3 (which has a 1.3x crop sensor), I narrowed down on buying the Sigma 24-70mm f/2.8 EX DG Macro lens to cover the normal range. Prior to this, I was using a Sigma 17-70mm f/2.8-4.5 DC Macro, which is an excellent lens. I sold it to a very good friend, who, unfortunately, hasn’t been using it as much as I’d have wanted him to 
I would strongly recommend the 17-70 to anyone who has a camera with an APS-C sensor (any make).
About the lens : To anyone familiar with the terminology Sigma uses, this should be fairly straight forward. The lens is a zoom lens that covers 24mm to 70mm on a full frame camera. Multiply it by your camera’s focal length multiplier (1.6 for Canon APS-C, 1.3 for Canon APS-H, 1.5 for Nikon APS-C). It has a fixed aperture of f/2.8 for all the focal lengths. The front element is 82 mm in diameter, weighs around 700g (heavy) and has a minimum focussing distance of 40 cm. This lens works on a full frame camera too.
My reasons for narrowing down on the Sigma 24-70 are fairly straight forward:
1. I needed a lens that would work on a full frame camera (Though mountable, the 17-70 was vignetting on my 1D M3). The telephoto lenses that I had earlier were compatible with Full frame cameras.
2. Value for Money – I do not shoot much in this range. I did not need a really expensive lens that would cover this range – but I needed a lens that was good.
3. Chitra – though she probably isn’t aware of the fact that she influenced my decision to pick up this lens. 
4. Fixed f/2.8 which makes it more awesome.
What’s in the box:
1. Lens + cap
2. User Manual
3. Flower hood
4. Carrying case (black)
Initial feelings :
1. Anyone who’s familiar with the terminology of Sigma would straight away understand what the EX here stands for. For those who don’t, it denotes superior build/optical quality. The rating is justified!
2. The lens uses a reverse zoom mechanism (Just like the Canon 24-70 f/2.8) meaning it is smaller at 70mm and the front element protrudes at 24mm.
3. Switching from Auto Focus to Manual Focus, in my opinion, very poorly designed. This is a minor inconvenience that you HAVE to live with.
4. Heavy – exceeding 700g, it is not a light lens.
5. Big front element – meaning filters are going to be fairly expensive! [82 mm]
I picked this lens up with a Sigma UV filter, which was fairly expensive, BTW.
To date, I have not had a problem with any of the technical stuff that one looks at the lenses for – color reproduction, contrast, aberration, distortion, flare or any other detail.
Pros:
1. Range – 24 to 70 makes it ideal for use for portraits, landscapes, products, anything else you’d want to shoot in this range.
2. Fixed aperture of f/2.8 at all focal lengths.
3. Color reproduction is excellent. So is the sharpness.
4. Works well on a FF camera (optimized for digital, but I don’t see a reason why it wouldn’t work on a 35mm film camera – I’ve never used it with a film camera though)
5. Price – The biggest plus point is the price – it costs almost half of the price of the OEM lenses (from either Nikon or Canon)
Cons:
1. Already mentioned – auto focus to manual focus switching issues, which again, I will re-iterate, is a minor problem.
2. Weight – at 700 grams, this is not a light lens. It is OK to hold it for a while and be fooled by the weight. But remember that when on a shoot you’ll have to be carrying it for a while.
3. On an APS-C sensor (1.6x crop on Canon, 1.5x crop on Nikon) – is not ‘wide’ enough @ 24mm. [Get the Sigma 17-70 instead].
4. AF speed – not as fast as Canon’s USM lenses.
5. No weather sealing
6. As mentioned earlier, front element is 82 mm which makes filters difficult to get, and if available, fairly expensive.
This lens would fit into a wide range of uses:
1. Landscapes
2. Portraits
3. Street photography
4. Low light photography (f/2.8 IS an advantage here)
5. Product photography in a studio
6. Basic macro photographs (though if you are serious about macros, get a dedicated macro lens such as the Canon’s 100 mm f/2.8)
Other options:
1. Canon 24-70 f/2.8 – At more than twice the price of this lens, Canon’s 24-70 is in a league of its own. If you have the funds, go for this one. Canon’s lens is sharper, has faster AF, and is weather sealed.
2. Sigma 17-70 f/2.8 – 4.5 DC Macro – If you have an APS-C sensor, and need something wider, this is the lens for you. However, it does not have a fixed aperture.
For photos shot with this lens, please click here. Images I’ve shot with this lens have not undergone extensive post processing. Here’s a basic workflow that I use – the below are used only if needed, for most, only one or two of the below mentioned were used.
1. Set white balance
2. Adjust exposure
3. Curve based adjustment
4. Dust spot removal
5. Sharpness enhancement
I have not processed any of the images to enhance a specific area in the photograph. I used this lens extensively during my trip to China, and did a lot of low light photography (Caves, cityscapes in the night and stuff like that). I was surprised by the results as I didn’t expect the sharpness and color reproduction that I got (especially at f/2.8). I would very strongly recommend this lens to anyone who has similar requirements.
Catch is – the lens is discontinued now and may be a little hard to get. A gem of a lens!
