[Photography] F-number

In photography, the f-number is defined as the ratio of the lens’s focal length to the diameter of its aperture. F-number = \frac{f}{D}


In a DSLR camera, the F-number could easily been seen usually on the first line of the back screen (as showed in the picture above). As you can see, the measuring scale of F-number is 1 (\sqrt{2}^0) , 1.4 (\sqrt{2}^1), 2 (\sqrt{2}^2), 2.8 (\sqrt{2}^3), etc upto \sqrt{2}^n. From appearance, this scale should have be invented by an engineer.

For a fixed lens, the focal length could not be changed, so the F-number is also used to estimate the lens’ aperture. The higher the F-number, the smaller the aperture.

It turns out that the F-number, defined this way, is proportional to the depth of field. It is easier to remember that to increase the depth of field, you increase the F-number.

Here is a picture with F-number equal to 25, because some monuments of the city (Paris) in the background was supposed to visible.

Vieux ami de Quasimodo



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