Csdrhrt

Honestly the higher quality Third Party Lenses have really upped their game in terms of quality, focus speed and sharpness. I recently got a Tamron 15-30mm F/2.8 which I have now used to replace my Nikkor 24-70mm F/2.8 and it just gives me what I look for. I find generally, when looking for a wide angle, I rarely wish it could go longer, but I always wish it can go wider. The Tamron 15-30mm fit the bill perfectly and really gets some great dramatic shots when used appropriately.

Some samples for you.

I've probably shot more images on full frame cameras with an EF 24-105mm f/4 L IS than any other lens. But for what you want to do in low light, it's not the best choice. At f/4, it's just a bit too slow.

I'd suggest a 24mm, 28mm, or 35mm prime lens. While 24mm f/1.4 primes can get expensive because they are such a challenge to design and produce well, there are more than a few affordable 24mm f/2.8, 28mm f/1.8, or 35mm f/2 lenses on the market. Compared to your old crop body, 24mm would give the same field of view (FoV) with the 5D Mark III as 15mm does on your Rebel T2i/550D. 28mm on FF translates to 18mm on the 550D, and 35mm on FF is the same FoV as 22mm on a 1.6X crop body.

Another great option would be the EF 16-35mm f/2.8 L III, but it is even more expensive than the EF 24-70mm f/2.8 L II. The earlier versions of the 16-35/2.8 L are not near as good optically as the "III", or as the more economical EF 16-35mm f/4 L IS (if one can do without the f/2.8 maximum aperture). For wider landscapes using a FF camera, including cityscapes at night, the 16-35/4 L is a great lens. But I think you'll find it not fast enough for group portraits in low light.

If your budget doesn't allow for the EF 24-70mm f/2.8 L II and you are dead set on a zoom lens, my advice would be to go with the less expensive Tamron SP 24-70mm f/2.8 Di VC. It's sharper than the original EF 24-70mm f/2.8 L (which I still use because it gives me what I need for a 24-70/2.8), but not quite as sharp as the newer "II" introduced in 2012. The Tamron also offers something the Canon does not: Vibration Compensation, which is Tamron's term for 'Image Stabilization'. I've got several friends who have the older version of the Tamron, and they are extremely happy with it. Tamron just released a 'G2' ('Generation Two') update a few months back that adds the ability to update firmware and calibrate AF using the Tamron 'TAP-In' USB dock. it is currently selling for a little more than the older one was before it was replaced, but it is still several hundred dollars cheaper than the Canon EF 24-70mm f/2.8 L II.

The existing answers cover lens choice very well so I'm going to answer with a non answer.

P.S. I'm interested in portraits, group photos, landscape and night photography (city skyline etc)

For portraits, lighting is key. Start looking into off camera flash. If using existing light in dim environments (e.g. A bar or club) go with a very fast prime.

For group photos, see above. Double emphasis on the lighting.

For landscape, lens speed isn't important. Invest in a tripod, remote shutter, polarizer, and neutral density filters.

For night city skyline, see above. Tripod, remote shutter, and potentially some filters like a graduated neutral density.

My point in adding this answer is to call attention to the fact that the lens is only part of the equation in taking a photo. As you've upgraded to full frame, your efs lens needs replacing, for sure. But don't assume that just because you replace it with a similar angle of view or wider angle or faster aperture that you'll be set.

You should examine the focal lengths of your existing images. If you frequently hit 18mm, you should look into wider lenses. Similarly, if you frequently hit 55mm, you should look into longer lenses. Otherwise, 24-70/2.8 or 28-70/2.8 should be fine.

• On full frame, a 24-70/2.8 lens will give the same FOV a 15-44/2 lens on crop-sensor, so you'll have a bit more room to work with than with the 18-55/3.5-5.6 kit lens. Whether that is enough for you depends on the size of the group and room.




• The 24-70/2.8 lens will give you an extra 2/3- to 2-stops than the 18-55/3.5-5.6 kit lens. This is the difference between ISO 3200 and ISO 800. A 24-105/4 lens will give only one extra stop at the long end in exchange for additional zoom. Whether the trade off is worthwhile depends on whether you need the additional zoom.



• 
  

  Since you've switched to a larger sensor, you should be able to use a 1.6x longer shutter speed than with a crop sensor when hand-holding the camera. So if you were able to hand-hold your old camera at 1/120s, you should be able to get away with about 1/75s (or even slower with image stabilization).

  
  
  

  This point doesn't apply if you are using a tripod or if you are trying to prevent blur caused by motion within the scene. For people pictures, the speed you should select depends on the activity. For instance, you should expect to require a faster shutter speed for people playing sports than for someone standing at a podium.

  
  

For lenses faster than F2.8, aside from the Sigma 24-35mm f/2 DG HSM Art, you'll mainly be considering primes.

• You can find a number of lens reviews, along with sample images, at The Digital Picture.

On a full-frame the 24mm is roughly equivalent to a 15mm on an APS-C, still a bit long for interiors. You should aim for 16-17mm at the short end:

• CANON 16-35mm EF f/4 L IS USM


• CANON 17-40mm EF f/4 L USM

For a lot of money, there is also a Canon 11-24mm EF f/4 L USM or, less expensive in the same range, the Sigma 12-24mm f/4 DG HSM Art.

The Canon 16-35mm seems to fit your purpose but I'm not sure about price.
Another really good lens that is not expensive is Tamron 17-50mm 2.8 for Canon

You mention groups, so I wonder if you have thought about whether you want the distortion that wide angle offers, or whether you would prefer a wide shot but without the distortion that can curve lines (and make people look a bit odd, depending where they are in the scene).

I have had an aspherical wide angle that corrects the distortion for fifteen or twenty years and I absolutely love it because it captures a wide shot but does not do funny things to the lines. So, for example, a wide shot of a forest, but the trees don't curve outward in the middle! It is also wonderful for scenes including people. This is the one that I have https://www.imaging-resource.com/lenses/sigma/15-30mm-f3.5-4.5-ex-dg-aspherical/review/