What the numbers mean
The resolution and frame rate shown are what your camera is sending to the browser right now, not what's printed on the box. They can be lower than advertised without anything being wrong.
The most common reason is light. In a dim room, a webcam keeps its shutter open longer to catch enough light, and that directly lowers the frame rate. A camera that does 30 frames per second in daylight can drop to 15 in the evening, and the video starts to look choppy. Plugging a webcam into a USB hub or a busy port can also limit the resolution it's able to send.
Black screen or no camera found
Check the physical stuff first, because it's the cause more often than you'd think. Many laptops have a small sliding privacy shutter over the lens. Others have a key with a camera icon that switches the camera off entirely.
After that, close every other app that might be using the camera. On Windows, usually only one app can use the webcam at a time. If Teams is running in the background, your browser gets a black screen.
Windows 10 and 11. Open Settings > Privacy & security > Camera and make sure camera access is on, including access for desktop apps.
macOS. Open System Settings > Privacy & Security > Camera and switch your browser on. Then quit the browser completely with Cmd+Q and open it again.
On a work laptop, your IT department may have blocked the camera for certain apps. In that case, this test won't be able to fix it for you.
Picture grainy, dark or blurry?
Light again, nine times out of ten. Webcam sensors are tiny. In a dim room the camera turns up its sensitivity to compensate, and that's where the grain comes from. Sit facing a window or a lamp. Light from behind you turns you into a silhouette. No good light nearby? A spare monitor or tablet works as a video call light.
Blurry or hazy? Wipe the lens. A laptop camera sits right where you grab the lid, so it collects fingerprints faster than you'd think.
Strange colours usually come from mixed lighting, like daylight on one side and a warm lamp on the other. The camera can't decide which one is "white". Use one type of light and the colours settle down.
Getting ready for a call? Check your microphone with the mic test too, since it is the other half of what people notice.
How this test works
The test asks your browser for the camera at up to 1920 pixels wide and then reports what the camera actually delivers. The resolution is read from the video itself. The reported frame rate is what the camera says it is doing; the measured frame rate counts the frames your browser really receives every second, in browsers that support it. A measured rate well below the reported one usually means the camera is slowing down to collect more light, which is another sign the room is too dark.
Nothing leaves your device. The picture is shown by your browser and goes nowhere else. Snapshots are only created on your own device when you click the button, and you decide whether to save them.