When I started learning about photography, I was a bit confused about F stops. F stop, like a nightmare boomed up everywhere, leaving me perplexed. What were these numbers f 1.4, f 2.8 etc. As a hobbyist I always thought, these complications were not for me. It was not until I realized, the importance of the same. There were a lot of questions that were to be answered. What stepping down actually mean? What is stepping? What role does it play in exposure? and lot more than this.
Starting with the most basic definition, F stops are nothing but a number related to the amount of light passing through the aperture. We are all aware of what apertures are. At least I'll assume this much. If we are not, then probably a revisit to the the this article will help.
Looking at the picture above, we know what aperture looks like. In better lenses the hole that we are talking about has more blades as compared to 5 here. This will give the opening a more circular look not a pentagon as shown.
Let us assume that our aperture is a circular hole through which we allow light to pass.
Now, aperture allows light into the camera. The wider the aperture, the more light enters the camera and vice-versa.
If we talk about a f number, it directly depends on the light we allow to pass into the camera with respect to the opening of aperture. We put these f numbers onto a scale, called a f scale. Every number on a f scale doubles the amount of light entering the camera.
Looking at the diagram above we get a clear picture of what f scale is all about.
Let us think that the Diameter for a particular aperture is d mm. The area of circle for radius d/2 mm is "a". Now, to double to amount of light passed (double the area) ie. for 2a we need to multiply d by √2. Not getting into this too much, but we now know that if diameter of aperture is multiplied by 2 then the area of the aperture is double and hence double the light.
Is that it? Is this how simple the f scale is? but then what does f 1.4 2.8 etc tells us. Where do these numbers come from? Definitely, there has to be something else that impacts the amount of light coming in.
Looking at the above diagram, we can deduce that focal length plays an equally important role.
Therefore, if F is the focal length and D is the diameter of the aperture we can say that any quantity x = F/D. Rearranging D = F/x. Here x being the f stop number, D is the diameter and F is the focal length.
Now lets say
D = F/x.
If Area is to be doubled then D is to be multiplied by √2. To keep the equation balanced, we can say that x is to be divided by √2.
Hence, if D is 20 mm, F is 40 mm then x is 2. To double the amount of light we know that D is to be multiplied by √2 and x is to be divided by √2. Thus, x now becomes 1.4 approx.
Clearly, to open the aperture such that we allow double the light to be passed, we divide the f stop by √2, or if the aperture is to be opened so that it allows half the light, we multiply the number by √2.
Therefore, lets say for a aperture f1 if we need to half the amount of light we multiply by √2. The next aperture hence, f1.4. Looking at the table below wold ease up our calculations a bit.
We know now how these numbers are arrived at. Also, we know the significance of these. Stepping up would mean increasing the f number to its next lower number. For example, if we are on f 4 and we realize that the exposure is a bit on the darker side, we can step up by a stop (f 2.8) and correct the exposure for us.
Hope the jargon is simplified know. We can say what f numbers are and how are they used. How is the scale calculated and read. Let me know if there is any issue understanding the same.
Starting with the most basic definition, F stops are nothing but a number related to the amount of light passing through the aperture. We are all aware of what apertures are. At least I'll assume this much. If we are not, then probably a revisit to the the this article will help.
 |
| Aperture |
Let us assume that our aperture is a circular hole through which we allow light to pass.
Now, aperture allows light into the camera. The wider the aperture, the more light enters the camera and vice-versa.
If we talk about a f number, it directly depends on the light we allow to pass into the camera with respect to the opening of aperture. We put these f numbers onto a scale, called a f scale. Every number on a f scale doubles the amount of light entering the camera.
 |
| F Scale |
Let us think that the Diameter for a particular aperture is d mm. The area of circle for radius d/2 mm is "a". Now, to double to amount of light passed (double the area) ie. for 2a we need to multiply d by √2. Not getting into this too much, but we now know that if diameter of aperture is multiplied by 2 then the area of the aperture is double and hence double the light.
Is that it? Is this how simple the f scale is? but then what does f 1.4 2.8 etc tells us. Where do these numbers come from? Definitely, there has to be something else that impacts the amount of light coming in.
Looking at the above diagram, we can deduce that focal length plays an equally important role.
Therefore, if F is the focal length and D is the diameter of the aperture we can say that any quantity x = F/D. Rearranging D = F/x. Here x being the f stop number, D is the diameter and F is the focal length.
Now lets say
D = F/x.
If Area is to be doubled then D is to be multiplied by √2. To keep the equation balanced, we can say that x is to be divided by √2.
Hence, if D is 20 mm, F is 40 mm then x is 2. To double the amount of light we know that D is to be multiplied by √2 and x is to be divided by √2. Thus, x now becomes 1.4 approx.
Clearly, to open the aperture such that we allow double the light to be passed, we divide the f stop by √2, or if the aperture is to be opened so that it allows half the light, we multiply the number by √2.
Therefore, lets say for a aperture f1 if we need to half the amount of light we multiply by √2. The next aperture hence, f1.4. Looking at the table below wold ease up our calculations a bit.
 |
| F Scale Numbers |
Hope the jargon is simplified know. We can say what f numbers are and how are they used. How is the scale calculated and read. Let me know if there is any issue understanding the same.
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