When talking about the factors affecting the exposure, Aperture and Shutter Speed are the factors that actually control the amount of light hitting the sensor. However, there is another factor that controls exposure.
How could the exposure be adjusted without changing the amount of light hitting the sensor? It is done by adjusting the ‘Sensor Speed’ (or in early days ‘Film Speed’). In simple words, it is done by adjusting the sensor’s sensitivity to light.
The Word in Not ISO, It is the Sensor Speed
First things first. When the question is asked, ”what are the factors affecting the exposure?”, the most common answer would be ‘Aperture, Shutter & ‘ISO’. That is incorrect and even highly experienced professional photographers are making that mistake. The correct word would be ‘Sensor Speed’ (or Film Speed) not ISO. ISO is the unit of measurement for sensor speed.
It is like when someone asked; ”According to Newton’s Second Law, what are the factors deciding the amount of force?” (F=ma) and instead of answering as ‘the mass and acceleration’, answering as ‘kilogram’ and acceleration. Hope you got the point.
How to Measure?
As I mentioned earlier also, the measurement of sensor speed is ISO. It simply stands for the ‘International Organization of Standardization’. Sounds weird, right? Let me explain.
In the film era, there was a number of methods to measure the film speed. They were different from region to region and manufacturer to manufacturer. Some of them are Hurter & Driffield, Scheiner, DIN, Weston, etc. I am not going to discuss them here, but if you are interested, read this extensive Wikipedia article on Film Speed.
With the passage of time, people might have felt the necessity of a common method of measurement. In 1974, a common international method (standard) was established with the collaboration of International Organization of Standardization, called ISO. Ever since everyone around the world adopted that method.
A typical ISO value series would be like,
100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200, 4000, 5000, 6400, 8000, 10000, etc
My camera Nikon D7200 has ISO values up to 25600. High-end cameras have way higher values than that. As an example, Sony Alpha A7S Mark II has them up to ISO 409600. Many new high-end cameras have sensor speeds less than 100 also.
A Factor Affecting the Exposure
As mentioned above, the sensor speed can control exposure without changing the actual amount of light hitting the sensor. So can we expect the same quality when the exposure is increased, without increasing the amount of light actually hits the sensor? Absolutely NO. It is pretty much obvious, right? Then what do we lose? Let’s get back to it later.
Just by the word Sensor Speed, you may understand that the exposure increases when it is increased. What really happens there is adjusting the sensor’s sensitivity to the light. The amount of light gets recorded on the sensor depends on that sensitivity. So increasing sensor speed means capturing more light. And that leads to higher exposures. The same way around, decreasing it leads to lesser exposures.
Let us do a little demonstration on the Sensor Speed’s effect on exposure. The same object has photographed in the same position with the same aperture and the shutter speed (f/3.2, 1/320s) except for Sensor Speed.
It is clearly visible how the exposure rises along with the sensor speed.
When to Use?
When the exposure cannot be adjusted by aperture and shutter anymore, and you don’t have any additional sources of light, sensor speed is your saviour. You can increase the exposure without changing the actual lighting condition of the scene. Therefore, it is really helpful in low light conditions.
Some times we even have to increase the sensor speed in daylight also. In some scenarios, there are some limitations to adjust the exposure by aperture and shutter, because there will be more important aspects to be considered, such as depth-of-field and motion blur.
Imagine you have to shoot a bunch of flying birds, and they are really fast. To avoid the motion blur, you are forced to increase the shutter speed really high. And to capture everyone in the group equally sharp, you are also required to increase the depth of field by increasing the aperture value (f-number).
Now both the aperture and shutter speed settings lead to lower exposure. In such kind of a situation, sensor speed is your only solution to get the exposure correct, even in the broad daylight. But make sure you consider to increase it as the last option since it will reduce the image quality.
Effect of Sensor Speed
As I mentioned earlier, increasing exposure by increasing sensor speed comes with a price. That is the ‘Image Noise’ or ‘Digital Noise’ (or Film Grain in early days). What is image noise? Have you ever noticed in some photographs, there are light coloured dots all over them that does not belong to the original scene (grainy look)? That is called ‘image noise’ or ‘digital noise’ in photography.
The original details are missing on those pixels, instead, those pixels are filled with random colours. That is a highly undesirable scenario in photography.
Image noise is an undesirable by-product of image capture that obscures the desired information.Wikipedia
The word noise originally means the ‘unwanted signal’. The word is used for this scenario also because, by analogy, it is an unwanted signal.
The main reason for this noise is higher sensor speeds. There are some other minor factors also, we will discuss them in another article in details.
A Little Demo
Let us do another little demonstration to see how the noise behaves with the sensor speed. The same object is captured in the same lighting condition and same aperture. Sensor speed is increased gradually, and to compensate that shutter speed is reduced accordingly to keep the correct exposure.
If you observe carefully, you will notice the image noise is being introduced and developed as the sensor speed increases. If you can’t clearly notice it, let’s zoom in each photograph.
Here you can clearly see the development of noise. You should note that the noise becomes more visible when the photograph is enlarged.
Modern high-end cameras have mitigated this noise issue up to great lengths, even in significantly high sensor speeds. The noise cannot be eradicated in high ISO values, but it is getting lesser and lesser with the technology.
Let us discuss the other factors that cause noise and how to get rid of it, in another article. Some say the image noise can be used to create artistic photographs. Personally, I would never agree to that. Noise is something I would want to get rid of 100%. But it is up to you to decide. Anyhow, photographs can be made to look old intentionally using the noise.
Those are the basics of sensor speed. Feel free to leave a comment if you have any doubt.