Part-IV was a pre-cursor to understanding Aperture, Shutter Speed and ISO.
Let us now dive deep into understanding the most important aspect of Photographic Exposure, the Aperture.
Aperture is the opening in the lens that controls the amount of light that falls onto the sensor to make an exposure.
The opening in the lens can be controlled using the standard set of values for the Aperture. We looked at the hypothetical example in Part-IV and understood the following key concept:
Each Aperture value is half the previous value and double the next value.
Below figure illustrates the concept.
Aperture is represented in f-stops, where f stands for focal length. It is the ratio of the len’s focal length (f) to the diameter of the entrance pupil of the lens.
N (Aperture) = f (focal length of the lens) / D (Diameter of the entrance pupil)
Entrance pupil is a window opening at farther end of the lens that fits to the Camera body. The light that enters the lens travels through the various optical elements to finally pass through this window or entrance pupil before reaching the Camera Sensor.
This entrance pupil or the window opening can be controlled by using an adjustable diaphragm in the lens, thereby controlling the exposure.
For instance, a 300mm with an entrance pupil diameter of 75mm will have an aperture value of f/4. We can rephrase this sentence like this: A 300mm f/4 lens will have a maximum entrance pupil diameter of 75mm.
300mm (f) /75mm (D) = 4
300mm (f) / 4 = 75mm (D)
An aperture stop or an f-stop is usually denoted as f/aperture value [we will deal with denominator values in a little while]. If we look at the above equation, an increase in the denominator value leads to decrease in the entrance pupil diameter.
300mm (f) / 5.6 = 54mm (D)
300mm (f) / 8 = 38mm (D)
- Bigger denominator values decreases the diameter of the entrance pupil resulting in less light to fall onto the Camera sensor
- Smaller denominator values increases the diameter of the entrance pupil resulting in more light to fall onto the Camera sensor
Let us look at a pictorial representation of the same (Note the relative sizes of the circles).
An f-stop functions similar to that of a car door. You might have observed that the car door opens in steps. It opens upto some predetermined point and then stops. If you open it further, it opens free upto next predetermined point and then stops again, and so on.
F-stop works very similar to this concept by dictating how much a lens should be open or closed before it stops at a predetermined point. These predetermined points are standardized set of values that every manufacturer follows.
Why do you need f-stops?
Simple reason is to have control over the exposure. It enables you to precisely control how much light should enter the sensor to make a proper exposure. Since every scene requires a different exposure, it is necessary to have different f-stops in order to control the incoming light.
Here are the standard f-stops:
f/1, f/1.4, f/2, f/2.8, f/4, f5.6, f/8, f/11, f/16, f/22, f/32, etc
Each of these denominator numbers like 1, 1.4, 2, 2.8 and so on are the rounded values of the geometric sequence of √2.
For instance, (√2)0 = 1
(√2)1 = 1.4
(√2)2 = 2
(√2)3 = 2.8
These are called full stops, which lets-in:
- double the light compared to its next stop (bigger denominator number), and
- half the light compared to its previous stop (smaller denominator number)
- f/8 allows double the light than f/11 and half the light than f/5.6
- f/5.6 allows double the light than f/8 and half the light than f/4
Below picture, which is similar to the one used in Part IV, illustrates the concept. Note that the size of the circle (or diameter) decreases as the denominator value becomes larger.
Note that smaller the f-number bigger the size of the entrance pupil and vice versa.
Manufacturers also give fractional f-stops to have precise control over the exposure. The intermediate stops are usually in steps of ⅓ stops.
Here are the standard fractional f-stops in steps of ⅓ stops:
f/1, f/1.1, f/1.2, f/1.4, f/1.6, f/1.8, f/2, f/2.2, f/2.5, f/2.8, f/3.2, f/3.5, f/4, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16, f/18, f/20, f/22
For instance, consider the intermediate stops between f/5.6 and f/8 which are f/6.3 and f/7.1.
- f/6.3 is 1/3rd stop smaller than f/5.6 and 2/3rd stop bigger than f/8
- f/7.1 is 2/3rd stop smaller than f/5.6 and 1/3rd stop bigger than f/8
Reading an f-stop can be confusing for many. Let us understand the notation in a simpler manner.
- Bigger the f-number smaller will be the diameter of the entrance pupil resulting in lesser light to pass through (f/8, f/11, f/22 are usually considered as smaller apertures)
- Smaller the f-number larger will be the diameter of the entrance pupil resulting in more light to pass through (f/2.8, f/4, f/5.6 are usually considered as larger apertures)
In case of any confusion, just take the focal length of the lens you are using and divide it by the standard f-stop denominators like 2.8, 4, 5.6, etc. The resulting value will indicate the diameter of the entrance pupil.
If you remember the below diagram you will never be confused again!
I hope you will never get confused in the future.
Let me know if you had any difficulty in understanding the basic concept of aperture? Do you have similar examples which you would like to share with us?
In the next article we discuss about The Role of Aperture in achieving the required Depth of Field (DOF)
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