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Sigma 24-70mm F2.8
Its a good lens. Fairly heavy and for the first 6 months its worked quite well.

Until Just a couple of weeks ago it has started locking up. I am not able to focus or zoom in or out. Its as if the insides of it has broken. I dont feel confident to use Sigma now in the future. Which is now why I will not purchase another sigma lens again. It is cheep, but you get what you pay for.

Great lens for the money.
This has now become my main lens for my canon 50d. It has fantastic build quality and feels solid. I would not change it for the world BUT I do have 2 little issues that I must mention. I think these problems are a reflection of budget and I would not expect to see them on a L series lens, but then I would not expect to pay the associated price tag either!

Firstly there is some zoom-creep when the lens is angled steeply up or down. I personally do not take many photos in this way and tend to prefer either horizontal or gentle angles below or above a subject. I am not sure if this is a problem experienced by ALL users of the 24-70mm lens; if it is not then hopefully a good Sigma service may sort the issue.

Secondly mine is not quite pin-sharp at 2.8 so I tend to stop down a bit to great some truly lovely results. But again using a lens at mostly 4+ raises the question of buying such a fast lens in the first place!

I have had to list these two niggles with the lens as I think they are worth considering as a consumer. However I still really like this lens and would not hesitate in buying another Sigma in the future(probably 70-200 2.8). Well until my budget can stretch to the L series glass anyway!

Ok lens, looks fantastic but not quite up to specification.
This lens reports itself as being f2.8 across all focal lengths but in reality its really not. Fully open at 2.8 my images have often suffered from terrible vignetting, really not at all subtle but really quite drastic darkening covering most of the image with huge great black corners.

Also the lens barrel on mine (maybe not others?) falls forwards, rolling from 70mm to 24mm whenever the camera is facing downwards with the lens barrel at the ground. This surely shouldn't happen! It should have enough grip to hold its focal length! I have a feeling that this issue has worsened since using.

Also I got some dirt particles inside the elements of mine within the first month of use. I've tried to keep it as clean as possible although have been on a fairly hectic 5 week trip with the camera and lens always together in a camera bag. There are quite big gaps between the barrel elements so its easy to see how this could happen.

Its a real shame as the lens looks at first sight to be really quite good quality. Its big, heavy and aesthetically very pleasing but upon finer inspection mine fails to meet the grade. Sure its a lot cheaper than any canon lens but its not actually that cheap an item so annoyed its not better.

Pretty good considering
I recently bought this lens for my Canon 5D as a walk around lens and for using on my upcoming trip to Africa. I would have liked to have gone for the L series canon made lens but I couldn't really afford it so I opted for this one instead. On a test shoot I was very surprised with the image quality especially on close up shots and the sharpness is pretty good as long as your shooting about f8 or higher which with the iso quality in the 5D isn't a problem for me.
The auto focus is a bit slow especially focusing from one extreme to the other and I have to say the lens hood provided is pretty useless and in bright light gives no protection from lens flare so be prepared to make one yourself out of bog roll if bright lights are your chosen subject.
I love the matt black finish and also the weight which in my opinion gives more stability when shooting but thats just my personal preference.
All in all pretty damn good considering you would be paying over £600 for an L series of the same focal length.

Good value robust lense
I have owned this lense since 2006, originaly it purchased it to replace the 18-55 that came with my 350D. The 350D has been replaced with a 7D now. I find the Sigma lens to be of very good build and image quality. I use this lense for about 90% of my photograpy form landscapes , close ups using extension tubes to snapshots. I have used it in climates from Arizona deserts in high Summer +40C to minus 23C Idaho mountain Winters and anything the British weather can throw at it.I found the auto focus stops working at around -10C and after -15C it give up the game all togeather but was still OK after it has thawed out. This is a robust lense as it still works despite it not having an easy "pampered" life. The only niggle I have is the satin black coating on the lens body gets dirty/dusty and is hard to clean.
I Have use this lens with a ring flash after I got a 82 -77 MM stepdown ring and still get good results. Overall I am very happy with it as it has served me well. Focus could be a little quicker and it gets confused by close up twigs and grass when taking landscape shots. If/when it stops working I will replace it with another Sigma 24-70 F2.8 I can offer no higher praise than that.