I downloaded one of your shots, the one with the butterfly. I checked the exif data and saw you used the viewfinder instead of live view. I have that same 70-300mm Nikkor lens and a Nikon D7000. I found that the D7000 (and most likely your D7100) uses phase dection AF with the viewfinder, and contrast detection AF when in liveview. The consensus is that contrast detection AF in liveview (using the lcd) is a sharper focus than phase detection using the viewfinder on the D7000, even though phase detection is faster. I checked your settings, (f8, 1/200 sec, iso-100) and they are fine. The main problem with your shots is they are out of focus, nothing else. To find out if the lens is at fault, set the camera on a tripod, use the 10 second delay and don't touch the camera or tripod after achieving focus and pressing the shutter, and take some pics in live view. It could be your technique is rusty, maybe hitting the shutter instead of squeezing it, or pulling the camera when you shoot. Also shoot a lot of shots. I find the quality and sharpness of my shots varies a lot from pic to pic, even with the same settings. I may get one good sharp one in a dozen shots. It's what is called spray and pray. and there's nothing wrong with it.

Cowboy ...... I'm going to step back now, 'cos I certainly understand that "learning digital" can set off a "system overload" in many people, especially those whose basic grounding was in the 35mm film era. I, for one, have sent you loads to digest, and yes, it does take time.

I'll stick with what I've suggested already ........ learn how to use your camera and those two Nikkors that were designed for it. Leave other makers' lenses and adapters alone.

And yes, if you were at ISO 6400 you were probably way too high for "everyday" lighting conditions. I think that, in those conditions, put your ISO to the "base" number (100) and don't move it upwards unless you cannot get a good exposure from aperture and shutter speed. You'll get lots of good help from members here if you ask about when and how to use ISO. You might get some flak (eg "use the search facility" or "it's been asked thousands of times") if you ask about "The Exposure Triangle", so be careful how you word your questions ! And be ready for "info overload" from some members too - lots of 'em never bother to do a little background legwork on the poster !

Google will provide you with many answers too !

I'm just gonna follow your progress from afar. Good luck in your journey......... and, checking here on the D7100 base ISO, this article will help you too ......

http://photographylife.com/recommended-nikon-d7100-settings/