30/05/2025
Common Resolution Units Explained
Resolution is an important indicator for assessing the performance of digital imaging devices. It reflects the device's ability to capture image details. Different resolution units are suitable for various applications. Below are the common resolution units and their characteristics:
Megapixels. Marketing often uses the total pixel count to describe the resolution of digital cameras, such as "this camera has a resolution of 10 megapixels." This metric actually refers to the total number of pixels on the sensor, rather than the actual image resolution. Pixel count is only one factor influencing image quality. A camera with more pixels but less sensor efficiency may produce images with a lower resolution than one with fewer pixels but higher efficiency. Thus, merely focusing on pixel count is insufficient for evaluating resolution.
LP / mm. This unit is mainly used for assessing lenses in the era of film cameras, representing the number of black and white line pairs that can be resolved within one millimeter on the film or sensor plane. A line pair consists of one black line and one white line, which have the same width and orientation. If a lens has a resolution of 100 LP / mm, it can resolve 100 black lines and 100 white lines within one millimeter. In digital imaging, since resolution lacks a physical range, using only this unit does not fully describe device performance. Additionally, using LP / mm to evaluate lens resolution requires knowledge of the sensor’s specific dimensions, which is difficult to obtain in many devices.
LP / pix. Digital devices commonly use LP / pix to measure resolution, as it describes the number of line pairs a single pixel can resolve. Since a line pair consists of black and white lines, the theoretical maximum resolution is 0.5 LP / pix. If a camera has a resolving capability of 0.4 LP / pix, it reaches 80% of the maximum resolution. This unit not only facilitates comparisons of different camera performances but also reflects the efficiency of pixel utilization.
LP / PH. To comprehensively evaluate the combination efficiency of resolution and pixel count, LP / PH is often used, representing the number of line pairs within the image height. For instance, if camera A has an efficiency of 0.5 LP / pix but only 320 pixels, while camera B has an efficiency of 0.4 LP / pix but has 2000 pixels, camera B will capture more details in the same scene. The LP / PH unit offers a more thorough representation of resolution performance, especially for comparisons between devices.
PPI. PPI is mainly used for scanners, indicating the number of pixels sampled within one inch. During scanning, PPI reflects only the sampling density and does not directly indicate the retention of image details. In practice, most scanners do not achieve the expected sampling rate defined by PPI. For example, many scanners approach the theoretical value of 0.5 LP / pix at 200 PPI but perform less efficiently at higher PPI values like 300 or 400 PPI. It is worth noting that PPI is different from DPI (dots per inch), which is typically used to describe printing precision for devices like inkjet printers.
Megahertz. In the era of analog video, since image signals were transmitted via time encoding rather than defined pixels, megahertz was used as a resolution unit. The image width can be represented using time parameters, such as microseconds. This differs fundamentally from spatial resolution units in digital imaging.
