Nikon D7000 w/ nikkor 50mm 1.8g?

If it's a Nikkor lens, then it will fit your D5200. The Nikon lens mount has been the same for 50+ years (my 1980s F-mount lenses still fit my Nikon DSLRs). If you got the inexpensive ($120) 50mm f/1.8D lens, it is only AF which means you need to manually focus the lens on your D5200. The AF-S (f/1.8G) version of the 50mm lens is a $220 lens which will autofocus with your camera. In the future, get AF-S lenses which support auto focus on your camera.

If the shutter speed is below 1/50th of a second, use a tripod, obviously the subject will have to be standing still. Or get more light if you want faster shutter speeds. 1/125th of a second should be enough to stop camera shake for hand held shots with a 50mm lens.

I use the Canon EF 50mm f/1.8 which is similar - I almost never have to crank up the ISO. If you have to go above ISO 400/800 then you need to get more light.

No, the lens has nothing to do with noise. Only high ISO causes noise. Lenses with wider maximum apertures allow you to set lower ISO settings, which produce less noise.

Focus problems with wide maximum apertures are common if you haven't used one before. Because you have a shallow depth of field at wide apertures, getting focus on a whole subject, or multiple subjects might be difficult. You may need to choose a narrower aperture to get sufficient depth of field for the subject. Just because you have an f/1.8 doesn't mean you have to shoot at f/1.8

Instead of using shutter priority, you might want to try using aperture priority if control over depth of field is crucial. For example if you are taking a shot of two people sitting together, f/1.8 is going to give you too shallow a depth of field to get both people in focus. Try f/2.8 or f/4 instead. f/1.8 might even be too shallow for a regular one person portrait, sometimes at f/1.8 it's not possible to get both eyes in focus if the person is not face on to the camera.

If you want to test your lens, you must do it on a tripod. Otherwise, you have hand-blur as a factor. I'd imagine what you are seeing is VR compensating for your motion and making the image look sharper.

When you go as open as f/1.8, often, it is beyond the ability of many camera to accurately acquire focus in phase focus mode. You may want to switch to LV (Live View) mode. In this mode, it WILL accurately focus. But, you'll have to use tripod to get it right for testing. (I won't go into lengthy explanations here)

An odd thing I experienced with an older version of your lens, 50mm 1.8D. I ordered like 5 lenses and all of them had soft image wide open. It was SO soft that printed 5x7, I could tell it was soft. I kept the best one and immediately sent it for repair/adjustment. To my surprise, this warranty repair included replacement of the front element (lens). It came back very very sharp. So I wouldn't discount the possibility you may experiencing something like this either.

As I said, I recommend using a tripod and LV mode. See what it can do. Then go from there.

Nikon D7000 50mm 1.8g

"Can a quality of lense impact the noise in pictures?"

Not directly. The sensor and the chosen ISO sensitivity are the main factors in noise. Higher sensitivity results in more noise.

However, using a wider maximum aperture (which this lens allows) or longer shutter speed allows for sensitivity to be reduced for the same exposure. The tradeoff is more blur in unfocused areas (shallower depth of field due to wider aperture), and/or potential for motion blur (due to longer shutter speed).

Along with a wider aperture, this lens also offers sharper rendition, less distortion, and less vignetting (darkening at the edges) over any of Nikon's kit lenses.

A way that a lens can affect the noise is by its light transmission/transmittance. While both lenses may be at the same aperture, other elements of the lens will affect how much light is hitting the sensor. There can be as much as 40% difference.

http://www.canon.com/technology/s_labo/light/003/0...

So if your 50mm has better lens transmittance, more light is hitting the sensor which will decrease the amount of noise that the sensor's electronics will amplify.

Lens quality has absolutely nothing to do with image noise.

All sensors generate image noise, but at low ISO settings the image is strong enough to swamp the noise so that it's not noticeable. As light levels fall, you have to dial up the ISO which effectively just amplifies the signal from the sensor, but that also amplifies the noise as well. So at higher ISO the noise signal forms a far larger proportion of the overall image signal and becomes increasingly noticeable. The effect is described as signal to noise ratio, or S/N ratio, and the higher the ratio, the better the performance.

Your 50mm f/1.8 has a faster maximum aperture than you've been used to. So at low light levels it can allow in just over two to three stops more light than your kit lenses ever can if they have maximum apertures of f/4 or f/5.6. That means that instead of having to use what is often a very noisy ISO3200, you can select a far less noisy ISO of about 800 (two stops better) or 400 (three stops improvement).

If you wish to use your faster aperture to best effect then use the camera in aperture priority mode. That way you choose the aperture to take advantage of the shallower depth of field possible with a faster lens.

AF speed and AF calibration are different things. If your camera has slow AF lock in lower light levels then you can speed it up by activating the AF assist lamp in your camera menu, but that can attract unwanted attention as it's fairly bright and alerts everyone to the fact that you're about to take a photo. AF calibration relies on the fact that AF systems don't usually focus precisely, but focus well enough to appear sharply focussed. They do that by taking advantage of the depth of field available with different aperture settings, and that is usually set by the manufacturer with a certain bias. So if the AF is set to usually ensure that the subject is about one third of the distance into the total depth of field, then calibration might allow you to alter that to halfway or two thirds as common alternatives.

Depth of field requires more detail than I've skimmed here, but you can find out a lot more by searching "depth of field" and "circles of confusion" which are exactly why depth of field exists. Note that depth of field is NOT the same thing as depth of focus; that's a different but related effect which occurs behind the lens where the sensor is located.

Finally, the fact that you noticed less noise at ISO3000 with your new lens compared to your kit lens is purely an effect of Buyer's Bias, also known as "post-cognitive rationalisation". It's a common cognitive effect where someone genuinely believes they can see improved performance because they're expecting to, and the effect has been shown to increase in proportion to the price paid. The performance gain they claim is always non-existent or insignificant when measured objectively. A common example of that are people spending a fortune on "high performance" digital connection cables, or ultra expensive loudspeaker cables. The effect you saw cannot exist, because as stated earlier, lenses make zero difference to image noise.