So in preparation for Retirement from Law enforcement I started selling Real Estate. I'm trying to figure out what lens would work best for taking indoor picture of smaller rooms. Like a wide angle type lens. I don't want a fish eye type.
I'd go with a wide angle zoom instead of a prime (fixed focal length lens). Room sizes change, and using a prime lens won't be an effective one size fits all approach.
Don't worry about wide aperture (the f number like f/2.8). Larger aperture number (like 5.6 vs 2.8) will get more of the entire pic in focus rather than a small part with the background being blurred (bokeh). While this looks great for portraits, it's not what you need for what you're doing. You could have two pretty much identical lenses, but one will go to 2.8 and the other will only go to 4.0, and the 2.8 one will cost double for something you don't personally need.
You need sharp corner to corner, not middle sharpness. And low distortion. This makes good lenses far more expensive than cheap and/or kit lenses.
People worry about vingetting, but I don't think it's pretty much a non-issue as it's very easily corrected in editing. It's a simple button click.
Distortion is critical because distortion correction can blur images in editing.
Get a tripod. They range from $20 or so to several hundred or even thousands if you look around. A cheap Sunpack brand one will fit the bill here.
I say tripod because you can use longer shutter times and not worry about camera shake (blurring your pics from your hands slightly shaking).
I've used (but never owned) Canon's 17-40 mm f/4 L lens. I think that would probably fit the bill if I had to make a general recommendation. I haven't used it for real estate photography specifically, but I don't foresee it not working well. I think it would be more of using the camera properly and lighting properly than the lens itself and which camera at that point.
Vignetting is pretty much a non-issue IMO. I used a double negative earlier.
Also, good photo editing software makes a difference. I'm not talking about doctoring up photos to dupe customers, but editing to correct exposure, resizing, stuff like that. If you have a Canon, you can download their free software which does probably anything you need.
And shoot RAW not JPEG. It's far easier to correct RAW images and you have more options.
I never did real estate photography. But when I used to photograph crime scenes, I used as wide a lens as was needed. That could be an 18mm, 24mm, 28mm, or 50mm. I don't think there were 18mm to above then, at least not usable for what I was doing. What I found more important was viewing distance, so a prosecutor could instruct a jury at what distance to hold a print for no distortion.
I think that isn't perhaps so important in real estate photography. Just don't be too obvious that you are showing apparent roominess where it isn't, or at least warn clients of the effect of any wide angle lens you use. They will after all wish to see for themselves. People appreciate agents that are up front with them.
Back then, I used film only (still do mostly). Now, as mentioned, more people want to use digital and it has many advantages. Lenses are better now, and many zooms are quite usable. In fact, I have a film kit with an 18mm to 28mm, 28mm to 70mm, and 75mm to 150mm. Those lenses are surprisingly good and get most of the shots I want.
Do get a tripod. Real estate photography isn't racing photography. You can take the time to get the focal length, depth of field, and any distortion out of the way as much as possible.
How wide you need depends on the format of your camera. 20-24mm is very wide on a full frame DSLR, but probably not going to wide enough for your purposes on a crop sensor camera. Most good, non-pro dSLRs have a sensor smaller than a full-frame SLR, which is equivalent to 35mm film. That means the photo will come out "cropped". Wide enough on a crop sensor camera is going to be in the 15-18mm range.
Mirrorless Interchangable Lens or Micro 4/3 cameras have a smaller sensor still, so you'll want an even wider lens. I'm not as familiar with that format, you'd need to research or talk with a camera shop or other realtors.
For the web, those smaller sensors will probably produce excellent photos and would be less expensive and more portable. I agree regarding a good quality wide - normal zoom. Which one depends on your camera system.