The core parameters of an image lens determine its imaging capability and applicable scenarios. Below are the most critical categories of key parameters, explained clearly and focused on the essentials:
1. Focal Length
Core definition: The distance from the optical center of the image lens to the imaging plane (e.g., camera sensor), measured in millimeters (mm).
Practical impact: A shorter focal length means a wider angle of view, capable of capturing more scene content (e.g., wide-angle lenses); a longer focal length results in a narrower angle of view, which can magnify distant objects (e.g., telephoto lenses).
Supplementary note: Marking methods are divided into "fixed focal length" (e.g., 50mm) and "zoom" (e.g., 24-70mm); zoom lenses are marked with a focal length range.
2. Aperture
Core definition: The light-transmitting aperture that controls the amount of light entering, commonly represented by "f/value" (e.g., f/1.4, f/2.8, f/4). The smaller the f-number, the larger the aperture opening and the more light that enters.
Practical impact:
Light intake: A large aperture (small f-number) is advantageous for shooting in low-light environments (e.g., indoor, night scenes) as it allows more light to reach the sensor.
Depth of field: A large aperture creates a shallow depth of field, blurring the background to highlight the subject (ideal for portrait photography); a small aperture (large f-number) achieves a deep depth of field, keeping both foreground and background sharp (suitable for landscape, architectural photography).
Supplementary note: Zoom lenses often have a variable maximum aperture (e.g., f/3.5-5.6), while prime lenses typically feature a fixed large maximum aperture (e.g., f/1.8).
3. Maximum Relative Aperture
Core definition: The ratio of the entrance pupil diameter to the focal length when the aperture is fully open (equivalent to the maximum aperture of the lens).
Practical impact: A larger ratio (smaller f-number) indicates stronger low-light shooting performance and better background blurring effect.
4. Minimum Focusing Distance (MFD)
Core definition: The shortest distance between the image lens and the subject at which the lens can form a clear image.
Practical impact: A shorter MFD enables close-up shooting of small objects (e.g., macro lenses with MFD as short as a few centimeters), while a longer MFD limits the ability to shoot nearby subjects.
5. Angle of View
Core definition: The angle of the scene that the image lens can capture, divided into diagonal, horizontal, and vertical angles of view (usually the diagonal angle of view is referenced).
Practical impact: Closely related to focal length—on the same sensor size, shorter focal length = wider angle of view; longer focal length = narrower angle of view. For example, a 16mm wide-angle lens has a diagonal angle of view of about 100°, while a 200mm telephoto lens has only about 12°.
6. Sensor Compatibility (Format)
Core definition: The maximum size of the image sensor that the lens can fully cover (e.g., full-frame, APS-C, Micro Four Thirds).
Practical impact: Using a lens designed for a smaller sensor on a full-frame camera will cause vignetting (dark corners); using a full-frame lens on a smaller sensor camera will result in a "crop factor" (e.g., 1.5x for APS-C), effectively increasing the focal length (e.g., a 50mm full-frame lens becomes equivalent to 75mm on APS-C).
7. Resolution
Core definition: The ability of the lens to clearly distinguish fine details of the subject, measured in line pairs per millimeter (lp/mm).
Practical impact: Higher resolution means sharper image details; it is particularly important for high-megapixel sensors (e.g., 50MP+ cameras require high-resolution lenses to fully utilize sensor performance).
8. Modulation Transfer Function (MTF)
Core definition: An important index to evaluate image lens quality, which reflects the lens's ability to transfer the contrast and detail of the subject to the image plane. The MTF curve plots MTF value (vertical axis) against spatial frequency (horizontal axis).
Practical impact: A higher MTF value at 0 spatial frequency (closer to 1) and a slower decline with increasing spatial frequency indicate better lens performance (sharper details, less contrast loss).