How to calculate the focal length of a single camera lens?

Generally, digital cameras have a conversion ratio, which is called lens conversion ratio. Take Canon EOS 300D as an example.

The focal length of the lens (35mm, 50mm, etc.) is determined by the relationship between the format of the final photo and the focal length, so "standard" is the most common format, that is, 24x36 mm, so when the matrix on the sensor used is less than 24x36 (the sensor of EOS 300d is 15. 1x22.7 mm), the focal length of the lens will be the same. When the surface of the illuminated object decreases, the angle of the scene also decreases. Actually, the 35mm lens is used to take photos in 24x36 format. If the surface of the object is actually 24x36mm, the photo taken by a sensor of15.1x 22.7 mm (EOS 300d) will be smaller, because the scene angle becomes smaller. So according to the size of the sensor, the conversion rate is used to calculate the size that needs to be increased. So this means that the sensor used in EOS 300D must be added at the ratio of 36/22.7 = 24/15.1= 1.6x. Specifically, the 35 mm lens is "multiplied by"1.6, which is equivalent to the traditional "24x36".

So the focal length of the lens specially designed for digital use comes from the surface of the sensor, but this does not mean that most people can understand it. An experienced amateur photographer will know the meaning of a "35- 105 mm zoom lens", but he may not know much about the mysterious 17.4-34.33mm lens.

Therefore, the 18-55mm lens provided on EOS 300D will be 28.8-88mm corresponding to the traditional 24x36 size (35mm). In short, the zoom photos of the traditional 24x36 camera will be "exactly the same" as those of the 18-55mm camera installed on EOS 300D. Do you understand this?