I've seen a lot of people talking recently about if they should get a manufacturer lens (NIkon, Canon, etc) or if they should get the third part equivalent (Sigma, Tamron, Tokina, etc). There's several popular arguments for the third party lenses. They are less expensive, they are smaller, they are "just as good as [insert camera company]." I've also hear people saying stuff like I have this camera and this third party lens but the quality of my photos isn't great so I want to upgrade my body. The biggest factor when it comes to poor photo quality is the quality of the lenses you are putting on your camera. Yes technique and settings play a part as well but the optics is the biggest part.
If you take the most expensive camera in the world and put a low quality lens on it, it will take low quality photos. Just like if you take an inexpensive camera and put a high quality lens on it the quality of the photos will increase. Now yes, this is where technique and settings come in as well. If you're trying to handhold at night shooting a 1 second exposure the photo is going to turn out bad no matter what camera/lens combo you're using.
The comparison I did I set up my D500 on a tripod and fired it using a remote trigger. I shot with the Sigma 28-70 f2.8 at f2.8, f4, f5.6, and f8. Then all I did was replace the Sigma lens with the Nikon 28-70 f2.8 and repeated the test. I shot both at 70mm and didn't change the focus point.
What I found was with the Sigma at f2.8 the photos were very soft and mostly un-usable. The Nikon at f2.8 was tack sharp. The first time they were equal in sharpness was at f5.6. The other thing I found was the color on the Sigma seemed to be washed out and dull compared to the Nikon lens.
One of the big things that the Nikon has over the Sigma is the Nikon has 15 elements in 11 groups (2 ED glass elements and 1 molded-glass aspherical lens). This is a lot of high quality glass packed into the lens.
The bottom line is you get what you pay for. Yes the third party lens may be less expensive than the manufacturer lens however the quality might not be as good. You can download the .dng files below to check out some of the photos below that I took. Refer to the file names for lens and aperture.