#f-number

An aperture is an opening, hole or gap. 

In photography the aperture is the opening in the lens that controls the amount of light passed through the lens to the sensor. The aperture, or lens opening, is  analogous to the iris in an eye; which adapts to the amount of light present in our environment.

The size of the aperture is stated as an f-number which is the ratio of the lens’s focal length to the diameter of the opening. So, a 50mm lens with a 50mm opening would have an f-number of 1, that same lens with a 25mm opening would have an f-number of 2 and with a 6.25mm opening it would have an f-number of 8. 

The use of a ratio to express the aperture is good in that it allows comparisons between apertures on lenses of different focal lengths but it also can be a source of some confusion in that the smaller numbers indicate bigger openings while the larger numbers represent smaller ones; which is somewhat counter-intuitive. 

Standard f-numbers are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. These values represent full stop values for a lens and are written as f/1.4, f/8, etc.

I introduced the concept of a stop as a measurement of light intensity in my blog post on July 30, 2019. In that post I explained that a stop of light is a halving or doubling of light. So altering the lens by one full stop from f/2.8 to f/4 is going to make a smaller opening and let in half as much light; adjusting from f/11 to f/8 is going to open the aperture letting in twice as much light. 

Most modern cameras allow opening or closing down the lens in more granular increments than full stops, some allowing half, third and even quarter stop increments. 

(Lava and Ferns, Canon 5DMII, EF24-105 @ 82mm, 1/400s, f/5.6, ISO 1000)

As I mentioned in my post on November 19, ensuring proper focus across the full plane of the sensor is not a trivial task. This challenge is amplified by the need to accommodate varying apertures and is most difficult at the extremes of the aperture range of the lens. 

For example, the lens I have on my Sony a7RII is the FE 4/24-70. The 4 before the slash (/) represents the widest aperture (f/4) while the narrowest aperture is f/22. (The 24-70 is the focal length range: 24mm through 70mm). 

Lenses performs best and achieve maximum focal sharpness at the middle of their aperture range--in this example about f/8 or f/11--while it will be least sharp at the extremes. In a high-quality lens the fall-off will be minimal and perhaps even unnoticeable to the naked eye.