How will the lens angle and shooting range change when the focal length of the lens is short and the film size is large?

The size of the negative is constant, and the shorter the focal length, the larger the viewing angle and photographic range;

When the focal length is constant, the larger the film size, the larger the viewing angle and photographic range.

In other words, shortening the focal length has the same effect as enlarging the negative.

The focal length of digital camera lens is always based on the lens of 135 camera. Mainly 135 camera is the most popular, the article introduces the most, and people know the most. The focal length of camera lens is an important parameter to measure the shooting range of lens, and lenses with different focal lengths adapt to different shooting needs. Fixed focal length lenses are generally divided into wide-angle lenses, standard lenses and telephoto lenses.

A standard lens refers to a lens whose focal length is approximately equal to the diagonal length of the photosensitive surface (negative film, CCD or CMOS). The perspective of "seeing" (shooting) an object is very similar to that of the human eye (of course, the afterlight of the human eye cannot be taken into account). For 135 camera, the size of the photosensitive surface is 36×24mm and the diagonal length is 43mm, so the lens with a focal length of about 43 mm is the standard lens, and it is customary to take the lens with a focal length of about 50mm as the standard lens. For 120 medium-sized camera, there are three sizes of photosensitive surface: 45×60mm, 60×60mm and 90×60mm, so the focal length range of its standard lens is 75 ~ 1 10 mm, and a wide-angle lens with a focal length smaller than that of the standard lens is called a telephoto lens with a focal length larger than that of the standard lens. The viewing angle of a 50mm standard lens (the angle formed by the connecting line between the two diagonal points of the photosensitive surface and the center of the lens) is 47 degrees. Obviously, the wide-angle lens has a larger viewing angle than the standard lens and can shoot a wider range. The telephoto lens has a smaller viewing angle and a smaller shooting range than the standard lens. A simple rule is that the greater the focal length, the smaller the viewing angle, the smaller the focal length and the larger the viewing angle.

The size of the photosensitive surface (CCD or CMOS) of a digital camera varies from camera to camera (the photosensitive surface of 135 camera is fixed). It is difficult to compare the shooting ranges of different cameras only by the real focal length of the camera lens, so they are all converted into the lens focal length of the equivalent 135 camera and then compared. Up to now, except for Canon EOS- 1Ds, Kodak DCS 14n and Contax N DIGITAL, the light-sensitive areas are the same as those of 135 camera, others are smaller than those of 135 camera, so the focal lengths of these three cameras are no different from those of 135 camera.

The converted focal length can be divided into two categories. One is a digital camera with the ratio of the size of the photosensitive surface to the length and width of 3: 2, which is the same as the photosensitive surface of 135 camera. These cameras are all SLR cameras with interchangeable lenses (except Canon EOS 300D, which are basically professional cameras). The other type is a digital camera with a 4: 3 aspect ratio of the photosensitive surface, which is the same as the aspect ratio of the computer monitor screen. Most of these cameras are civilian cameras (a few are professional SLR cameras, such as Olympus E- 1).

For the first kind of digital cameras, although they all use the lens of 135 camera, the angles of view are different. How to convert their lens focal length into equivalent focal length? The basis is that the diagonal perspective of the two must be equal. Let f be the equivalent lens focal length (unit: mm), f 1 be the real focal length of digital camera lens (unit: mm), and d 1 be the diagonal length of digital camera photosensitive surface (unit: mm), which can be easily obtained from the similar properties of triangles.

that is

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It's called the lens factor.

For example, if the CCD size of Nikon D 1x is 23.7× 15.6mm, then d 1=28.37mm, and α= 1.5, then if the zoom lens used in the camera is 17 ~ 35mm, The former is a zoom lens for 135 camera from ultra-wide angle to general wide angle, and the latter is a zoom lens from general wide angle to standard lens, with a much smaller shooting range. So it's a little wasteful to use this 17 ~ 35mm zoom lens here. Nikon Company specially designed a zoom lens with a focal length of 12 ~ 24mm for its professional digital camera. This lens is used here, and the equivalent focal length is18 ~ 36 mm.

For the second kind of digital cameras, the aspect ratio of the photosensitive surface is different from that of 135 cameras, so camera manufacturers have different methods to convert it into the equivalent lens focal length of 135 cameras. Generally, there are two conversion algorithms:

1, diagonal viewing angle equality conversion algorithm: From the previous description,

2. The algorithm that the horizontal viewing angle of the photosensitive surface (the angle formed by the connecting line between the two endpoints of the horizontal line passing through the midpoint of the photosensitive surface and the central point of the lens) is equal: the similarity of triangles can also be obtained:

Obviously, the equivalent focal length f obtained by the first conversion is smaller, that is, the viewing angle is larger, but the difference is not too much. For example, the CCD photosensitive surface of Nikon coolpix 5700 is 8.8×6.6mm, and the focal length of the lens is 8.9 ~ 7 1.2 mm, so d1=1mm, according to the first algorithm, the equivalent focal length is 35 ~ 280 mm, which is equivalent to that given by the manufacturer. According to the second algorithm, the equivalent focal length is 36 ~ 290 mm, and there is not much difference between them. Some people will say that the 10mm difference between 280mm and 290mm lenses should be a lot, but anyone who has used this focal length knows that the difference between the two focal lengths is very small.

It is said that some digital camera manufacturers also convert the equivalent focal length in this way: the focal length of the lens of a digital camera is equal to the diagonal length of the photosensitive surface. As a 50mm standard lens, it is equivalent to a 135 camera, and other focal lengths are converted in proportion, that is,

namely

This conversion is much worse than the previous two conversions. I guess the first two conversions are still dominant. These formulas are very easy to calculate, but there is a problem d 1 that is generally unknown. Although there are CCD sizes in the technical reference materials of digital cameras, such as11.8, 1/2.5, 2/3 inches, etc. This is not the true diagonal size on the photosensitive surface, and the diagonal size on the real photosensitive surface is less than this value. However, because the equivalent focal length will be given in the manual of the digital camera, we can use these formulas to calculate the real size of the photosensitive surface. This real photosensitive surface size can provide necessary data for us to estimate the depth of field of digital cameras.

About the depth of field of digital cameras, I will come again in the future.