Optical magnification

One of the performance parameters of an optical lens, optical magnification refers to the ratio of the image size formed by an object on the focal plane through the lens to the actual size of the object. In other words, it is the ratio of image to object.

Lenses generally specify a "maximum magnification," which refers to the magnification value of the lens under two conditions:

1. Maximum focal length (fixed for prime lenses);
2. The closest shooting distance at which a sharp image can be formed.

The magnification value under these two conditions is the maximum magnification of the lens.

For example: a 70–200mm zoom lens with a rated magnification of 1/6.5 means that when shooting at 200mm focal length at the shortest shooting distance that allows sharp imaging, the ratio of the image on the focal plane to the actual size of the photographed object is 1/6.5.

The magnification of most lenses is less than 1 — that is, most lenses actually produce reduced images, such as the cameras on common mobile phones, DSLR cameras, digital cameras, and webcams on computers.

(Figure: Most lenses produce reduced images.)

Of course, this is not absolute. Certain telecentric lenses and microscope lenses have magnifications greater than 1 — that is, the image is enlarged. These are mainly used for viewing small details. Such lenses are not commonly used in daily life; they are generally applied in more specialized fields.

(Figure: Microscope imaging produces magnified images.)

For prime lenses, a more intuitive way to calculate optical magnification is: sensor size / field of view, which gives the optical magnification of the lens. For example, with a 1/2" CCD with a length of 6.4mm, if the field of view is 64mm, the optical magnification of the lens is 6.4/64 = 0.1X. This value actually refers to the current magnification. For general FA (factory automation) lenses, because there is also a focus ring, different fields of view can be obtained at different distances while still forming sharp images, so the magnification at these distances differs. According to the statement above, the specified value is the magnification at the closest distance at which sharp imaging is possible — the closest distance also means the field of view is the smallest at that point.

Physical magnification

Physical magnification refers to the ratio between the image viewed through a visual device such as a monitor and the actual object — in other words, the ratio of the final image size to the original object size. If our monitor is 19 inches (4:3, approximately 386×290mm) and the field of view is 64×48mm, the magnification is 386/64 = 6.03x. This is calculated based on the monitor and the field of view, without considering the optical magnification of the lens.

In fact, the optical magnification also has an effect, because when the monitor is fixed and the camera's sensor size is fixed, the optical magnification of the lens determines the field of view. A larger magnification gives a smaller field of view; a smaller magnification gives a larger field of view. Physical magnification = monitor / field of view, while optical magnification = sensor size / field of view, i.e., field of view = sensor size / optical magnification. Therefore, physical magnification = monitor × optical magnification / sensor size.

For a single lens product, the term definitely refers to optical magnification. However, for certain imaging systems (a typical example being imaging systems that use VGA or HDMI interface cameras paired with microscope lenses to directly observe fine features of products on a monitor), it more often refers to physical magnification.

Additionally, the two can also be distinguished by the magnification value itself (this method is not absolute, but can serve as a reference): if the magnification is stated as 1x, 2x, 0.5x, or 0.3x, it usually refers to optical magnification. If the magnification is stated as tens or hundreds of times, it usually refers to physical magnification. This is because it is very difficult for ordinary lenses to achieve optical magnification of tens or hundreds of times — even reaching tens of times in optical magnification already carries high cost, and higher magnifications cost even more, so such high-magnification optical lenses are generally not used in the industry.