My Nikon D7100 is getting a lot of noise when I bump up the ISO to even 1000. How do I reduce that? It says it can go up to 6400 which is one of the reasons I got the camera.
It also depends on the subject. If your subject has a lot of flat black areas the noise will show up much more than on a patterned colourful/white area. As already stated the camera's max ISO is not the max it will be able to make a usable picture. Noise will be noticeable but controlled for different ranges depending on the quality of the camera and unusable or very poor above that.
You can improve the appearance with software afterwards - lightroom has very good and easy to use noise reduction tools - but it will lose detail.
The fact a camera can use a high ISO (e.g. 20 000) doesn't automatically mean it will be noise-free when using high ISO. ALL cameras have noise, only its amount is different.
D7100 is almost a 5 years old camera, obviously there are better options today, but it should be acceptable at about 800-1000 ISO. Maybe you're looking too close at your images or have unrealistic expectations?
If you want to reduce the noise, you can apply noise reduction either within the camera or in the software you use to process the images. It's usually a one-click process and should give you sufficient results. If not, then we have a more advanced approaches that you can surely find online :) The biggest problem with noise reduction is usually some loss of sharpness, so be careful when you do it.
Amount of noise is subjective, you can't say "iso 1000 is very noisy". You should compare it to some other iso value photograph of the same scene. You can't even compare iso values if images are different: details such as subject, lighting, motion blur might hide noise differently.
Here is my image, shot with Nikon D7000 (even older than yours d7100) at ISO 4500. Kit lens 18-55, exposure 1/60s, f/5.6, 55mm focal length
Here is image at Google photos
Does it have a lot of noise? I guess so. Does it still preserve details? Yes. It is usable image? Very much so (well, depending on situation :) Skin tones are preserved, even some eyelashes details.
Is it gonna win awards? Probably not, but not because of the noise.
That is normal. Actually, there is noise at every ISO, only it is less strong at low ones. For each stop above, noise increases, usually by small steps until you hit the Expanded ISO range.
The camera can go to ISO 6400, sure, but that is always going to be noisier than ISO 200 lets say. It there, in case you need it. Should you want better high-ISO performance, you will need to buy a newer camera or one with a larger sensor. Even so, there is always a point where the camera can shoot. In fact, the Nikon D7500 can reach a stellar ISO 1,683,400 (yes, ISO 1.6 million) and I find it completely unusable but it is there.
Take a lot at the sample crops in my review of the D7500 which compares it to the D7200. Looking down the column shows ISO increasing and you can see that noise always increases, fairly steadily until ISO 51200 and there there is visible jump in noise and once more at ISO 819,200.
Noise can get reduced by applying noise-reduction but that removes details, so there is no way for high ISO not to have an impact, you can only fine-tune the compromise between detail and noise.
Noise is a granularity akin to grain seen in chemical based photo film. We are likely to see noise in our images when we set the ISO high. We would do this if the lighting is feeble or if we a super-fast shutter is needed or if a super-tiny aperture is desired, or some combination.
What makes noise? The imaging sensor chip is covered with photosites. Each receives light energy during the exposure. This energy generates an electrical charge. The charge is always frail so each site photosite has an amplifier used to boost the signal strength. When we turn up the ISO we are upping the amplification. This comes at a price. The price is: The signal to start with is not pure, it consist of good signal and static (noise). Amplification boots both. The problem is;, each photosite is working at a slightly different efficiency than its neighbors. This delivers a serviceable signal that is overlaid with a bad signal. We see this bad stuff as a granularity which we see but it is particularly noticeable in mundane areas like dark sky or uniformly toned expanses.
The bottom line is; digital imaging is not perfect. What we want is a faithful image. We have yet to achieve. The super high ISO setting generate noise however these setting allow us to image when conditions are meager as to illumination.
Sometimes, when I need to push limits a bit (fast action under low light), be it ISO, or shutter speed, I use raw and deliberately underexpose one stop. For some subjects, I find the exposure correction artifacts (RAW -> JPEG!) are less intrusive than higher ISO noise, especially above 640. Please take care: this is not a panacea and must be dosed carefully.
You can underexpose a photo at ISO 100 severely enough and it will be noticeably noisy. If you properly expose a photo at a much higher ISO setting it can look less noisy than the underexposed shot taken at ISO 100. This is because what we usually describe as noise is really what is more properly referred to as the signal-to-noise ratio. The more signal (light) we have to work with, the more noise we can tolerate before it becomes noticeable. Conversely, the less signal with which we have to work, the less noise we can tolerate before the signal-to-noise ratio becomes low enough that it looks noisy to our eyes.
Here are a couple of examples from this answer I submitted to What is the highest acceptable ISO to use for weddings with Canon 7D?
This image illustrates that even at ISO 3200, if the subject is well lit it can be shot at high ISO without a lot of noticeable noise. The image was shot using a Canon 7D, which is notoriously noisy at high ISO. Since the background was almost totally dark, it was very simple to reduce the exposure of the shadows to deal with the noise in the dark area surrounding the subject. The singer was illuminated by at least one high intensity spotlight as well as colored stage lighting mounted above and below the front of the stage as well as above the back of the stage. ISO 3,200, 1/1,600 second @ f/2.8.
If you decide to pixel peep, however, even the best of images will break down. 100% crop of a section of the same photo:
On the other hand, further back on the same stage, but without the benefit of the spotlights directly on the subject, this photo shot at the same ISO for twice as long shows more noise. ISO 3,200, 1/800 second @ f/2.8