A wide-angle lens is something everyone getting into real estate must get before anything else because you can't get by shooting interiors with off-the-shelf kit lenses that come with cameras!
I've talked to several Real Estate Photographers recently in the process of purchasing new DSLRs and noticed that there is a misunderstanding about what lenses are appropriate for real estate photography on the new DSLRs.
First of all there are two types of DSLRs:
What does this have to do with wide-angle lenses? Everything! When you mount a lens, say for example a 18-55mm, on a full frame DSLR, it's a 18-55mm lens just like you expect. But when you mount the 18-55mm lens on a APS DSLR the lens doesn't act like a 18-55mm it acts like a 28.8-88mm lens! This is because the smaller sensor size of APS cameras have the effect of multiplying the focal length by a focal length multiplier (1.5 for Nikon and 1.6 for Canon). The term used to refer to the the focal length after the multiplier is 35mm effective focal length.
When talking about wide-angle lenses, it's important to understand that the sensor size of the camera the lens is mounted on has an effect on the angle of view of the lens:
A practical example of this is if you put a lens like a 12mm Rokinon on a Sony A6000, you must multiply the 12mm by the Sony cropped sensor multiplier (1.5) to get the effective focal length (18mm). So we say the effective focal length of the 12mm Rokinon on a Sony A6000 is 18mm.
This focal length multiplier is a big deal for real estate photographers because this means the standard kit lens that comes with most off-the-shelf cameras is not optimal for real estate work because after you multiply the effective focal length multiplier for the camera you are using, the lens may not have as wide a view as you think.
Real estate photographers find the effective focal lengths roughly between 16mm and 24mm to be the "sweet spot" for shooting interiors. It's best to have a zoom that covers this whole range between 16 and 24 but at a minimum, you need to work at 24mm or below. This is why the Sigma 10-20mm lens (available for both Nikon and Canon) is so popular with real estate photographers because with a 1.6 multiplier, it allows you to work between 16 and 32mm effective focal length and it's an inexpensive alternative. For full-frame cameras, the Canon 17-40mm, Canon 16-35mm, and Nikon 17-35mm lenses are popular choices for interiors.
What are the wide-angle alternatives for APS DSLRs? I'm only going to cover Canon and Nikon because I recommend that you stick with these two manufacturers. It will make your life easier and give you more flexibility and alternatives in the long run because 3rd party vendors provide accessories for these two brands.
Notice that I put the Canon 14mm fixed lens on the Canon list. This lens is legendary for it's lack of barrel distortion. It is pricey but as high a quality as you'll find. Zoom lenses all have varying degrees of barrel distortion that can be removed in Photoshop or other photo-editing software.
There are many wide-angle prime lenses that will work but for real estate, a zoom that covers the range of 16mm to 24mm, 35mm effective focal length will serve you best.
The bottom line here is that you need to pay careful attention to which lens you choose for real estate work. It may be the most important equipment decision you make for real estate photography. It's way more important than which camera body you choose.