image resolution

Resolution answers how finely an image is divided, and therefore how much detail it can possibly hold. The everyday sense is a pixel count: a 1920 x 1080 image has 1920 columns by 1080 rows, about 2.07 million pixels. More pixels means the sampling grid is finer, so smaller features in the scene can land on distinct pixels rather than being averaged together.

But pixel count alone is not the same as visible detail. The detail you actually capture depends on the whole imaging chain: lens sharpness, focus, motion blur, sensor noise, and crucially the mapping from scene to sensor. A face that fills the frame is rendered by far more pixels than the same face seen across a stadium, even though the camera resolution is identical. Hence practitioners distinguish spatial resolution (the pixel grid density) from effective or optical resolution (the finest scene detail the system can truly distinguish, often measured in line pairs per millimeter or via the modulation transfer function).

Resolution is also expressed relative to a physical reference. Pixels per inch (PPI) describes how densely an image is printed or displayed; ground sampling distance (GSD) in remote sensing states how many real-world centimeters one pixel covers on the earth. The key insight is that upsampling — enlarging a 640 x 480 image to 4K — increases pixel count but cannot create detail that was never sampled; it only interpolates. Genuine resolution is fixed at capture by the sampling, the optics and the geometry together.