The size of the lens can directly determine the subject of the actual photograph taken, and different focal segments of the lens can bring a specific Angle of view and depth of field effect. One of the biggest reasons many people don’t take as good pictures with the same machines is because they don’t choose the right lens. Every photography enthusiast, as long as the choice of the right lens, will be able to shoot with ease.

1. The lens type

The lens can be divided into zoom lens and prime lens according to whether the focal length is variable.

The focal length of the zoom lens is variable, and the aperture is medium. It is distorted and suitable for shooting scenery.

The focal length of the prime lens is very fixed, the aperture is relatively large, distortion, suitable for portrait still life shooting.

According to the focal segment, it can be divided into wide-angle lens, standard lens, telephoto lens, and fisheye lens with special effects, etc.

2. Principle of lens imaging and its application

The distance from the object plane to the lens is called object distance (S1), the distance from the lens to the sensor plane is called image distance (S2), and the focal length of the lens (F). Between them meet:

In actual application scenarios, the installation space of visual devices is often small, so it is difficult to select a lens that is fully suitable for the application environment. At this time, the cognition of the principle and law of lens imaging can help us solve this problem. According to the principle of imaging, when the space between objects is limited, the installation requirements can be met by appropriately increasing the image distance (that is, increasing the connection ring).

3. Adjustment of object thickness and depth of field

In object point imaging, due to aberration, the imaging beam cannot converge on a single point, forming a diffuse circular projection on the image plane, which is called a diffusion circle. If the diameter of the circle is small enough, it can still be seen by the naked eye as a focal image of a point. This maximum acceptable diameter is called the admissible diffusion circle diameter.

The distance between the two circles is called the depth of field. That is, before and after the focus, the image still has a clear range, which is the depth of field. Calculation formula of depth of field:

δ is the depth of field, S is the object distance, F is the focal length, F is the aperture value when the lens is shot, and D is the diameter of the diffusion circle. It can be seen from the formula that the factors affecting the depth of field are:

(1) Lens aperture: the larger the aperture (the smaller the aperture value), the smaller the depth of field; The smaller the aperture (the larger the aperture value), the greater the depth of field;

(2) Focal length of lens: the longer focal length of lens, the smaller depth of field; The shorter the focal length, the greater the depth of field;

(3) Shooting distance: the farther the distance, the greater the depth of field; The closer the distance, the less depth of field.

Since most scenes of visual application do not involve the height change of the measured object, the concept of depth of field is often ignored. However, in some special scenes, it is necessary to flexibly use depth of field to achieve high quality image effect. For example, when the measured object will gradually become thick or thin, and the position of the camera lens is fixed, at the beginning of selection and design, it is necessary to consider the maximum thickness change of the measured object, to ensure that the depth of field of the selected lens can be compatible with the changes of the measured object. The most common method we use is to adjust the aperture and choose the focal length of the lens to ensure that the object under test can be clearly imprinted within the depth of field.

Aperture and focal length are two important factors affecting lens imaging.

How skilful use of the focal length of the system, whether it is a wide Angle and telephoto, or standard focal length has its characteristics, the portrait of how we can think about using the focal length of the lens can achieve different effects, 35 mm lens to the focal length of lens operation range is wide, the composition have also changed, clap out effect is different also. Wide-angle prime lens is our common use, shooting sketches, environmental portrait is very good. Wide-angle lens: The wide-angle lens has a short focal length and a wide Angle of view. It is usually suitable for taking pictures of larger scenes, such as buildings and scenery. On the other hand, if you want to take pictures with a wide range of views (architecture, landscape, group photo), a non-full-frame camera will have to be paired with a wider focal length lens.

The larger the focal number, the longer the focal length, the narrower the field of view, the less scenery to accommodate, play the effect of closer shooting picture. The focal length of the standard lens is 45mm, 55mm, 80mm and other specifications, more suitable for portrait, landscape, and cultural photography. The focal length of the medium telephoto lens is generally 100mm, 135mm and other specifications, more suitable for portrait, landscape shooting, and telephoto for shooting birds, shooting month and so on.

The aperture value of the lens is also very important. The larger the aperture number, the smaller the aperture, the less light in, the darker the picture, the fainter and smaller the blur effect. The larger the aperture, the shallower the depth of field. Conversely, the smaller the aperture value, the deeper the depth of field. For example, the 85mm F1.4 or 50mm F1.4 and other large optical enclosure focal lens used for portrait shooting, the larger the aperture value, the better the background blur effect, which can well highlight the subject of the figure, so as to bring better portrait shooting effect.

In addition to looking at the type of lens, focal length, and aperture value, but also according to the weight of the lens choice, after all, the lighter the weight of the lens, the more convenient to carry out. Prime lenses have fewer and simpler sets of lenses than zoom lenses, giving them an advantage in terms of size and weight, and even larger apertures don’t weigh too much.

When choosing a camera lens, the above points can be used for reference. Of course, in addition to these, but also according to the type of camera, lens price and other aspects of the comprehensive consideration, only in this way to choose a suitable camera lens.