Camera Talk

So, I get that larger f numbers like f/22 actually mean that it's a smaller opening and that smaller f number like f/2 is a big opening.

But my question is this...WHY is the size of the aperture backwards from the numbers. Shouldn't f/22 be a big aperture since it's a big number and f/2 should be a small aperture since it's a small number???

I read the answer once, but it's been so long that I can't remember. It has to do with math. Mike Kovacs would know the answer.

It's a mathematical construct that displays the ratio of the focal length to the effective aperture diameter, Paul. A full explanation is here: https://en.wikipedia.org/wiki/Aperture

PF

F-number is a convention to simplify calculations of exposure and it is the ratio of the lens focal over the diameter of the iris; which allows you to calculate the f-number across focal lengths (i.e. f/8 in a a 600mm is equivalent to f/8 in a 50mm).
This the diameter of the diaphragm iris is a fraction of the focal length and as with all fractions 1/2 is larger than 1/16.

Each F-stop is 1/2 of the light of the previous; that is why modern f-Numbers are normally a progression of sqrt(2) ~1.4 because the light that passes is function of the area of the iris, which is the π*radius^2 = π*diam^2/4
The term f/ is used as the diameter of the iris is a fraction of the focal length (f).
So in a lens with 50mm focal you can measure the size of the diaphragm/iris and see that
f/ = radius = area
1.0 = 50mm = 625 π mm^2
1.4 = 35.7mm = 318 π mm^2
2.0 = 25mm = 156 π mm^2
....
16 = 3.125 mm = 2.44 π mm^2

The smaller the iris (i.e. less light that passes) this number is larger

As said above, the number you set for aperture is a denominator. This is why larger f number means less light.

Let us call f = focal length.

When you set aperture, technically you are setting the size of a hole that lets light through. The diameter of that hole is f/aperture number.

For a 50 mm lens, f/ 8 means a hole of 50/8 mm, i.e. 6.25 mm.

In old books, aperture was always referenced as f/something.

(Excuse the repetition!)

I have seen some very old some lenses (large format) when aperture was expressed in mm of aperture diameter, being the largest number the widest aperture and the smaller number then closest aperture.
I understand they took f/ as standard no matter what focal length the amount of light allowed is the same using f/

The light that goes throughout two lenses of different focal length is the same, if both lenses are dialed on the same f/ number.

Do not confuse "f stop" numbers with the old "Stop" numbers. Back when God was a boy, the amount of light transmitted by the lens of a plate camera was controlled by inserting a brass plate into the lens into which a hole of a certain size had been made. These were called 'Stops" because they "stopped" the light. The size of the hole had no relation to the aperture F numbers we use today. Alex would probably tell you more.

For a good tutorial on basic photography, I recommend the Photography Life website: https://photographylife.com/what-is-ape ... hotography

melek wrote: ↑

Fri Feb 26, 2021 8:52 pm

I read the answer once, but it's been so long that I can't remember. It has to do with math. Mike Kovacs would know the answer.

thanks

GrahamS wrote: ↑

Mon Mar 01, 2021 6:08 am

Do not confuse "f stop" numbers with the old "Stop" numbers. Back when God was a boy, the amount of light transmitted by the lens of a plate camera was controlled by inserting a brass plate into the lens into which a hole of a certain size had been made. These were called 'Stops" because they "stopped" the light. The size of the hole had no relation to the aperture F numbers we use today. Alex would probably tell you more.

For a good tutorial on basic photography, I recommend the Photography Life website: https://photographylife.com/what-is-ape ... hotography

ok thanks for your kind information