@
normy123 I won't really give more information, but instead try to say in more plain English.
The 50mm f/1.8D lens is made to also work on decades-old camera bodies. I'm sure you see the aperture ring (where you set f-stop) and want to know why it does not seem to work when you change it. Your D7100 and really all Nikon digital SLR's set the aperture (f-stop) by the wheel on the front of the camera under where the shutter-release button is. On the back side where your thumb normally would sit is the other wheel and that controls the shutter speed. But to make this work right, the aperture ring on the lens must be set to the highest number (f/22 I believe on that lens, might be f/16) and left there. I'm also sure there is a lock that can be set to keep the ring on that setting and not accidentally move it. Once done, in "A" mode you will see the f-stop value change as you rotate the wheel on front.
The Scenes will also work. The way to view the current scene setting and change is to rotate the back-side wheel that normally changes the shutter speed. The screen on back will display the setting. The camera is going to work in full-auto mode and will try to select appropriate settings for the conditions that match the preset. Any lens that will work in Auto mode should work the same using a Scenes preset.
And please download that camera manual PDF. It is really easy to look things up on PDF since you can click on an index line in the back and it takes you right to the page. I personally have a PDF viewer on my phone and I keep the camera manual on the phone and I actually look things up on it. Easier than looking for where I left the printed book.