Part-IV was a pre-cursor to understanding Aperture, Shutter Speed and ISO. In Part V and VI, we understood the basics of Aperture, Depth of Field (DOF) and its importance in photography. In Part VII we understood the basics of Shutter Speed and its effect on resulting photograph.
Let us now dive deep into understanding another most important aspect of Photographic Exposure, ISO.
What is ISO?
ISO stands for International Organization of Standardization. In photographic terms, ISO represents the sensitivity of Camera Sensor to the light.
- Less sensitive the Sensor is to the light, more time it takes to expose for a scene, resulting in fine quality photographs
- More sensitive the Sensor is to the light, lesser time it takes to expose for a scene, resulting in lower quality (noisier) photographs
Standard ISO Values
There are standard set of ISO values, similar to that of Aperture and Shutter Speed, to measure the sensitivity of a Sensor. Usual set of ISO values that are found in most Cameras are:
100, 200, 400, 800, 1600, 3200, 6400
However, there are some Camera variants that support a lower ISO 50 and higher ISO values from 12800 up to 409600.
Note that the numbers are geometric progression of power-of-2.
ISO values work very similar to that of standard Aperture and Shutter Speed values. Each ISO value:
- Doubles the sensitivity of the Sensor compared to its previous value, and
- Halves the sensitivity of the Sensor compared to its next value
For example, ISO 200
- Doubles the sensitivity of the Sensor compared to ISO 100, and
- Halves the sensitivity of the Sensor compared to ISO 400
By increasing or decreasing the ISO value, we can control the amount of light that is needed to make a proper exposure.
This doubling and halving mechanism of Aperture, Shutter speed, and ISO values help you in achieving the desired exposure based on artistic needs.
Intermediate ISO Values
Every manufacturer gives intermediate ISO values/steps that will help to fine tune the sensitivity of the Sensor.
Complete set of ISO values including the intermediate values is:
100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200, 4000, 5000, 6400
Values in red color represent the standard ISO values.
In between two standard ISO values there will be two intermediate ISO values; one for 1/3rd stop and other for 2/3rd stop.
For instance, consider the intermediate stops between ISO 100 and ISO 200 which are ISO 125 and ISO 160.
- Sensor at ISO 125 is 1/3rd times more sensitive than ISO 100 and 2/3rd times less sensitive than ISO 200
- Sensor at ISO 160 is 2/3rd times more sensitive than ISO 100 and 1/3rd times less sensitive than ISO 200
The intermediate steps are very important as we can fine tune the ISO sensitivity in order to reduce the noise with higher ISO values.
Effects of ISO values on Image Quality
Every Sensor has the base ISO at which it will produce the highest quality image (higher Signal-to-Noise Ratio). Base ISO generally is ISO 100 but some models will have ISO 200 as their base ISO.
As we increase the ISO from its base value, the quality starts to degrade since we are increasing the sensitivity of the Sensor to the light.
Let us consider a simple example to see how the quality degrades with increased ISO values.
Consider that you are photographing an object on a table in your living room at night. You have the constant source of light which is either Tube light or CFL. There is no ambient light present, which means the light does not change.
Now, when you meter the scene using a reflective light meter or your Camera’s metering sensor, you will get the metering for the proper exposure. Say for instance, you are using a 50mm f/1.8 lens and the metering says you need to use an Aperture of f/1.8 and Shutter Speed of 1/15th of a second.
You know that Shutter speed of 1/15th of a second will make the image blurred since you know that you need a Shutter speed of atleast 1/60th of a second (assuming full frame camera) to get a sharp image.
There is a gap of 2-stops between 1/15th and 1/60th of a second. One way is to decrease the Aperture by 2-stops to allow more light. But we are already at the maximum Aperture. Or you can use Tripod. Let us assume you do not have one.
Now, the only option is to change the ISO from ISO 100 to ISO 400 so that you can use a Shutter Speed of 1/60th of a second.
What happens when you increase the ISO?
In our example, the light is same, scene has not changed, Aperture opening is same and Shutter Speed is decreased to allow less light! Then where is extra light coming from?
In reality there is no extra light here. Only the Sensor is tuned to become more sensitive to the light by virtually shifting the incoming signal level (or boosting the signal) by a certain factor.
When the signal levels are shifted (could be multiplied by a factor), the noise level also gets shifted resulting in lower Signal-to-Noise Ratio (SNR), which in turn result in poor image quality.
Loosely speaking, this could be related to Optical Zoom Vs Digital Zoom. Optical zoom uses the actual mechanical components like lens elements to do the zoom. When sensor captures the zoomed image it is always at its highest resolution.
Whereas, in case of Digital Zoom, the sensor captures at its highest resolution with no zoom and later the image is cropped to get the required zoom and re-scaled back to the original resolution. So, there is always a loss of quality in case of Digital Zoom.
Making changes to Aperture and Shutter Speed to achieve the required Exposure can be equated to Optical Zoom, whereas a change to ISO is similar to that of the Digital Zoom.
How far can you push the ISO?
It depends on you and the Camera that you are using.
Today’s DSLRs have amazing ISO capabilities. Most of the manufacturers have found ways to reduce the higher ISO noise within the Camera using proven techniques. In some Cameras you can go as high as ISO 1600 without too much of noise in the image.
The best possible way to know is to shoot at different ISO levels. Basic rule of thumb is to use the lowest ISO possible at all conditions.
Only when the situation demands, you should push the ISO levels up.
Most important thing is:
- Know your Camera
- Know its ISO capabilities
- Experiment with one subject and different ISO numbers
- Experiment in different lighting conditions
Note that noise usually resides in the lower lit areas or shadows. If you have good enough light, you may not have a bad quality image with higher ISO. But you may get really poor quality image if there are several objects in your scene that are in shadow.
In summary, experiment with different ISO values with different lighting conditions and find out the tolerance limit, both for your Camera and You!
A Word on Post Processing to reduce ISO Noise
Though it is possible to reduce the ISO noise in post, you should remember that reducing the noise makes the image look smooth. Because, a noise reduction filter is a low-pass filter that smoothen out the edges.
This could be a real problem when you want to reduce noise levels on the subject with lot of interesting textures and fine details like in bird’s feathers.
Therefore, it is always better to use the lowest possible ISO to avoid any kind of disappointments.
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