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Crop factor

Translated from Wikipedia, a free encyclopedia

The clipping factor is the ratio of the camera frame to the reference frame. Generally, this term applies to digital SLR cameras, and accordingly, 35mm Quan Huafu film is taken as the benchmark. The ratio of the diagonal length of 35 mm film to the diagonal length of digital SLR camera sensor is the most commonly used cutting factor.

This ratio is often referred to as the focal length multiplier ("FLM") because multiplying the focal length of a lens by a cutting factor or focal length multiplier will produce the same field of view (FOV) as using a lens with the same focal length (multiplied by this factor) in a reference system. The term format factor will also be used, which is more neutral and more in line with the corresponding concept in German vocabulary-format Faktor.

brief introduction

The term crop factor or focal length multiple was coined in recent years to help photographers who use SLR cameras understand the performance of their existing lenses on emerging digital SLR cameras with sensor sizes less than 35mm. Taking the focal length multiple of 1.5 as an example, a photographer may say that a 50mm lens behaves like a lens with a focal length multiplied by 1.5 on his digital SLR. What he means by this is that the camera gets the same angle of view as the 75mm lens on the film full-frame camera that photographers are more familiar with. Of course, the actual focal length of a photographic lens is determined by its optical structure and will not change with the sensor format placed behind the lens.

Most digital slrs (especially entry-level ones) on the market use the nominal APS-C sensor, which is smaller than the standard 24x36 mm (35mm) film frame. For example, the sensor size used by many consumer digital SLR is 15x23 mm. Therefore, the image data collected by the sensor is smaller than the 35mm frame area, which is equivalent to cutting off the corners of the 24x36 mm film Quan Huafu. Due to the above cutting, the effective viewing angle decreases proportionally from the smaller sensor size to the Quan Huafu (reference system).

For an entry-level digital SLR, this coefficient is1.5-2.0x. For example, the wide-angle viewing angle obtained by a 28mm lens on a full-frame camera is moderate, but on a camera with a cropping factor of 1.6X, the viewing angle of an image produced by the same lens is the same as that produced by a 45mm lens on a full-frame camera (28x16 = When you want to get a wide angle, narrowing the viewing angle is a disadvantage-super wide angle becomes wide angle, and wide angle becomes "normal". However, when you want to get a narrow perspective, the cutting factor once again becomes an advantage. When a photographer shoots a distant object with a telephoto lens, cropping makes it easier for the target to fill the picture. The viewing angle of a 300mm lens on a camera with a cropping factor of 1.6X can only be obtained by installing a 480mm lens on a full-frame camera.

Digital lens

In order to meet the needs of wide-angle lens users, most digital SLR and lens manufacturers have designed lenses with shorter focal length and optimized them to adapt to the format of digital SLR. In most cases, these lenses are designed to project a smaller imaging circle, which cannot cover a 24x36 mm frame, but is enough to cover a smaller 16x24 mm (or smaller) sensor used in digital SLR. Because the projection imaging circle is smaller, the lens can be optimized to use less glass, and can have smaller physical size and weight than the lens designed for a full-frame camera.

The lenses designed for smaller digital photo frames include Canon EF-S lens, Nikon DX lens, Olympus 3/4 system lens, Sigma DC lens, Tenglong Di-II lens, Pentax DA lens and Sony Alpha (SAL) lens. These lenses are usually smaller than the imaging circle projected by the lenses designed by Quan Huafu 35mm camera. However, the cropping factor or focal length multiple has the same influence on the viewing angle and focal length of other lenses, although the projected image is not really strictly "cropped". In this sense, the term "clipping factor" has some vague meanings and should be more accurately described as "focal length multiple".

Cutting coefficient and fool camera

Smaller non-SLR consumer cameras, typical fool cameras, are also suitable to be represented by the cutting factor or focal length multiple relative to the 35mm frame, although the lens cannot be changed or the frame size is different. For example, a sensor with a diagonal length of 9mm is called a "11.8 inch" frame. Compared with a 35mm film frame with a diagonal length of 43.3mm, its cutting factor is 5, so the lens focal length of these cameras is 1/5 of that of a typical 35mm film camera. Many times, manufacturers will mark the actual focal length value on their own lenses, but sometimes they will choose to multiply the focal length of the lens by the cutting factor or focal length multiple and mark the focal length value of "35mm equivalent". Evaluators sometimes use 35mm equivalent focal length as a general term to describe the range of visual angles.

For example, Canon Powershot SD600 indicates that the actual focal length range of its lens is 5.8-17.4 mm.. However, it is sometimes described as having a lens of 35–105 mm because its cropping factor is 6 ("1/2.5 inch" frame).

amplification coefficient

The cutting factor is sometimes called the "amplification factor". This statement comes from the telephoto effect. For a lens with a given focal length, a camera with cropping coefficient will get a larger magnification than a full-frame camera. It is worth noting that no matter what camera it is installed on, the lens always projects the same image, which is why it has the same magnification on all cameras. Just because many digital SLR sensors are small and the viewing angle is narrow. The end result is that a lens always produces the same magnification, and the image produced by a smaller sensor is enlarged to the same size as the image taken by a full-frame camera with a longer focal length when it is output (print or screen). That is to say, the commonly defined magnification (of the lens) has not changed on the focal plane, but for the system, the magnification has increased from the subject (subject) to printing.

Other influences

When the lens designed for a 35mm full-frame camera is used in a digital SLR with a smaller frame, it may produce other effects, such as depth of field, perspective, camera motion-induced blur and other shooting parameters.

The depth of field may change, depending on the contrast. Shooting in the same position, using the same aperture value, and shooting an image larger than the reference system (note: it should mean that the magnification is large and the viewing angle is small) will lead to the depth of the scene. On the other hand, compared with the image of the same content shot by a 35mm camera with the same focal length, the image shot by a camera with a smaller sensor has a greater depth of field.

Perspective is an attribute that only depends on the observation point (camera position). However, if the lens is placed on a small-format camera, the photographer will be far away from the subject, and the perspective relationship will be affected.

Due to the extra magnification brought by the smaller frame, the blur caused by inaccurate focus and camera motion (jitter) is strengthened. Therefore, for a given shutter speed, the focal length value of a clear image can be reduced by hand-held shooting. The old rule is that the shutter speed required for hand-held shooting is not lower than the focal length value. This rule is still available if the focal length value of the lens is multiplied by the focal length multiple first.