Video and text from my review of the Canon 24 to something lenses.
Part I - My opinion of each lens and some summarizing thoughtsPart II - Some of the sample images and video that helped me form my opinionSome select thoughts taken from the two videos
I have the 24-105 f/4 IS, 24-70 f/2.8 mark II, and the new 24-70 f/4 IS
I think the most telling comment is that for the last week I have had my pick of these lenses to shoot with and every single time I picked up the 24-70 f/4 IS. I love the size and weight of this lens, the IS and the Macro mode. A quick disclaimer on that comment- I didn't shoot professionally over the last week, which for me is usually shooting a wedding. If I had I would have picked the f/2.8 mark II - in my opinion that extra stop of light is critical for the lower light situations. The IS of both the 24-70 and 24-105 is good and the 24-70 f/4 offers the newer hybrid IS but IS doesn't freeze people in motion. And freezing people in motion is important, especially during the low light receptions where all that dancing is happening.
If I had to summarize each lens in a sound bite -
Canon 24-105 f/4 IS older work horse or a lens that offers excellent value, is relatively sharp(just don't look at the edges) and provides a useful range.Canon 24-70 f/2.8 Mark II - Razor sharp at all apertures and from corner to corner. Bright too- at f/4 it is noticeably (2/3 of a stop brighter than the 24-105) - you can see it in the images and you can see it on the exposure meter. But this performance comes at a cost, literally! This lens is $2200, fairly heavy and does not provide IS.Canon 24-70 f/4 IS - Very sharp though maybe not quite as sharp at f/4 as the f/2.8 version, similar brightness as the f/2.8 with an excellent macro mode, I mean really seriously good and a nice size- feel really nicely balanced and weight is quite good and it has IS!!Now what isn't here is the Tamron 24-70 F/2.8 VC - I have a video of my reboxing and return of that lens, I was unhappy with the sharpness, I had gotten a bad copy and I could try again but I was also getting numerous lens communication errors. Those two issues combined soured me and I didn't want to try again. Now I know there are people that are quite happy and I have seen sample images that are much better than the one I got. Even in sample images the corners are much softer than the Canon 24-70s but you do get IS or VC as Tamron calls it. There is also a Sigma 24-70 but it is a good bit softer than the other lenses, it is also the cheapest.
Which is best for you?
It depends ;) If you are on a tight budget the 24-105 is a good lens, You get an L class lens for under $1000. Its a bit heavy but it does have IS and a nice range.If you have a bit more to spend the 24-70 F/4 is VERY versatile, sharp, excellent IS and that macro mode. Because the glass is so durn good it lets in enough light that you are shooting faster than f/4, about 2/3 of a stop faster. For $200 less you can pick up the Tamron, not quite as sharp, no macro mode but it does offer f/2.8 and IS.If you need the ultimate performance in low light with unrivaled sharpness in a zoom lens get the 24-70 f/2.8 Mark II.