What is film sizing for?

Is the sensitivity, the original film has a "degree" or "fixed number", which is the sensitivity measurement unit under the two systems, one-to-one correspondence, such as

100 degrees 2 1200 degrees 24.

The so-called "degree" is the current ISO, and 100 degree is ISO 100.

The sensitivity of ISO200 is twice as high as that of ISO 100, so ISO200 with the same exposure is twice as bright as ISO 100, which means "one step brighter" in photography.

Similarly, the sensitivity of ISO400 is twice that of ISO200, so the brightness and brightness of ISO400 are twice that of ISO200 for the same exposure.

The same scene, the same aperture and the same shutter are two grades brighter than the photos taken by ISO 100.

So sensitivity, like aperture and shutter, is a parameter to control exposure.

When you encounter some restrictions: for example, if you want to take a photo with a large aperture during the day, but the photo taken at the highest shutter speed of the camera is still too bright, then you have to switch to a film with lower sensitivity, and the digital opportunity will set the ISO lower.

However, when shooting at night, the maximum shutter speed of the aperture is still very low, so it is difficult to shoot by hand, and it is necessary to change the film with higher sensitivity.