Do you know how a photo is formed? A photo is “let the negatives shine and record the light at that moment”, so some people say that photography is “capturing light and shadow”. So what does a beginner photographer need to know? The answer is the light triangle: film sensitivity (ISO), Aperture and Shutter speed.


Negative Film Speed (ISO)

Before digital cameras were popular, photographers used film. The film usually has set numbers such as ISO50/ISO200/ISO1600. This number represents the sensitivity of the film. The higher the number is, the stronger the ability to absorb light, the more suitable it is to shoot in a dark environment. However, no matter how strong the sensitivity is, you can’t shoot in an environment where there is no light at all! So when should I use a photosensitive film (in a digital camera, it refers to the ISO number setting)? Under normal circumstances, the higher the ISO (the stronger the sensitivity), the stronger the noise. It will be more obvious in the darker areas of the photo. You can look at the following examples (Photos from ):

iso80 ISO100 iso200
iso400 iso1600 iso3200

Through in the above example, you can notice that ISO80 low-sensitivity photos are more detailed and smooth, while ISO3200 photos are full of noise, so the higher the ISO value the better.


Aperture is “a window hidden in the lens”. Imagine that the sun is very strong and the size of the window can control how much light enters the room. The same principle is used to control how much light enters the camera and shines on the film.

Shutter speed

The shutter speed is the time to open the “aperture” window. The longer the time, the more light can enter; otherwise, the shorter the time, the less light can enter. We can also imagine a water pipe. The longer the faucet is turned on, the more water flows out.

In Conclusion

How can the film sensitivity (ISO), aperture, and shutter speed affect each other? Let’s look at an example: Let’s say you want to record 10 units of light, then you can use the following methods:

  1. Use 1 unit of sensitivity per second and 1 unit of a large aperture to open the shutter for 10 seconds (1 x 1 x 10 = 10)
  2. With a sensitivity of 2 units per second and a large aperture of 2 units, let the shutter open for 2.5 seconds (2 x 2 x 2.5 = 10)
  3. With a sensitivity of 100 units per second and a large aperture of 1 unit, let the shutter open for 0.1 seconds (100 x 1 x 0.1 = 10)

As can be seen from the above, there are many combinations to absorb 10 units of light, and we have to make different judgments depending on the situation (how different combinations will make different results, I will explain it in another article). Remember the selection of the ISO number before? After reading this example, you should know that the noise of example 1 will be much less than that example 3. Example 3 is more suitable for use in dark environments (because it can absorb much more light in a short time).

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Author: Alex Tam