F STOPS

How do F STOPS WORK ?

If anyone can give me some feed back that would be great !

Timothy, you are expecting photo.net members to spend time explaining something covered by practically every photography textbook ever written. And there is loads of information on the internet, for example:

f-number - Wikipedia

Timothy, you are expecting photo.net members to spend time explaining something covered by practically every photography textbook ever written. And there is loads of information on the internet, for example:

 

f-number - Wikipedia

Thank you John

The key to the kingdom is getting the exposure correct and repeatable and transferable. If we can do this, another photographer with a completely different lash-up , can duplicate your exposure results. Further we can write exposure directions that will operate universally.

The problem is: The amount of light passes by an optical system is a variable based on focal length and the working diameter of the lens. Consider that there is a hodgepodge of different focal lengths and apertures, how could we tell some a photographer how to set their camera and yield a fateful image?

Ratio to rescue: We use what we call a “focal ratio”, that’s an f-number for short. A ratio is dimensionless. This means we can set any lens set to the same f-number as any other lens as this method will deliver the same exposing energy regardless of the size of the lenses involved. Suppose a giant telescopic camera has a focal length of 1000 inches and an aperture diameter of 125 inches. We obtain the f-number by division. Thus 1000 ÷125 = 8. We say the f-number is f/8. Another photographer using a 100mm lens sets his aperture diameter to 12.5mm. The focal ratio of this lash-up is 100 ÷ 12.5 = 8. In other words f/8. This works so well we can ignore the dimensions involved and just say f/8. No need to know the aperture diameter to work the problem. All is figured out for you when you set your camera to f/8. The good news is: The f-number system takes the muddle away.

Since the camera lens is a circular opening, the amount of light passed by the lens is created on the geometry of circles. Truism: If you multiply the diameter of any circle by 1.4 you calculate a revised circle that is twice as big in surface area. We use this scheme to calculate a series of circles, each with either twice or half the area.

Since we are dealing we lenses we use this scheme to calculating a set of f-numbers that deliver twice or half the amount of light passed to film or digital chip. The series is: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 -11 – 16 – 22 -32. Note: each number going right is its neighbor on the left multiplied by 1.4. Each number going left is its neighbor on the right divided by 1.4. Such a ratio set allows adjustment of a lens system that will yield a change in exposure based on 2x increments. Further the f-number system is universal, any lens set to the same f-number, passes about the same amount of light. This permits exposure settings that are unvarying between different all cameras.