What does the lens mm of SLR camera mean? For example, 30mm, I hope it is more detailed.

The mm number of SLR camera lens indicates the focal length of the lens, which refers to the distance from the main point to the focus after the lens is optically. This value determines the size of the object imaged on the film or CCD, which is equivalent to the ratio of the object to the image. When shooting the same subject at the same distance, the image with long lens focal length is large, and the image with short lens focal length is small.

A 30mm lens means that the distance from the main point to the focus of the lens is 30 mm after optics.

Extended data

First of all, two laws of focal length

1, the smaller the focal length, the wider the field of view and the wider the framing range; The more pictures you can take, the smaller the proportion of scenery in the picture;

2. The greater the focal length, the narrower the field of view and the narrower the framing range; The fewer pictures you can take, but the greater the proportion of scenery in the picture.

Second, the relationship between focal length and theme

1, less than 24 mm

Generally, we call a lens with a focal length below 24mm "super wide-angle lens", which has a large angle of view and is very suitable for shooting large scenes and scenery.

2, 24 mm and 28 mm

These two focal lengths are very standard wide-angle focal lengths, which are often used to shoot landscape themes.

3, 35 mm

The focal length of 35mm is called "humanistic eye" by many people because it has a good humanistic effect. The most important thing in shooting humanistic subjects is to show the relationship between people and background. 35mm can not only shoot the background, but also get a good depth of field.

4, 50 mm

50mm is called the "head" by everyone, and it is very good to take portraits and humanities.

5, 85 mm

The focal length of 85mm is very suitable for shooting portraits, with good illusory background and good image cropping ability.

6. 100 mm

The focal length of 100mm is basically a macro lens, so macro photography is best used naturally.

7.135mm

This focal length is also very suitable for taking portraits. It is recommended to stay away from the model.

8, 200mm and 300mm

This focal length is telephoto, and you can shoot portraits, sports and the like; You can also take portraits because the illusion effect is good.

9. Super telephoto

Generally, a lens with a focal length of more than 300mm is called a super telephoto lens, which is very suitable for shooting wild animals, the moon and the sunrise.