# [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.