ISO Sensitivity

In this instalment of our photography fundamentals series we’ll briefly discuss ISO sensitivity.

What is it?

ISO sensitivity is a measurement that assesses the ability of the sensor in a camera to provide a defined response to a given level of light. Actual ISO sensitivity measurement is quite complicated and can involve any one of a number of highly technical calculations.

For those readers who would like to take an in-depth look at ISO sensitivity it is defined by ISO Standard 12232. This link provides details on how ISO sensitivity is calculated by Imatest
(be forewarned that this reference material is quite technical in nature).

In pragmatic terms…

Basically sensors with higher ISO sensitivity capability require less light to capture a good quality image in terms of it not having digital noise. When a sensor has higher ISO sensitivity it provides photographers with more flexibility when it comes to the range of exposure input settings (i.e. the combination of ISO, shutter speed and aperture settings) that can be used to capture and properly expose an image, while at the same time minimizing digital noise. This is especially important when a photographer shoots in low light situations.

What is ‘base’ ISO?

The ‘base’ ISO of a camera is typically the lowest ISO number of a camera’s sensor, and is the setting at which it will capture the highest quality images with the least amount of digital noise.

The ISO sensitivity scale

Photographers commonly use the following ISO scale:
100, 200, 400, 800, 1600, 3200, 6400, and so on.
You’ll notice that each number on the scale is double that of the previous number. Each step on the ISO sensitivity scale means that the sensitivity of the sensor has doubled. For example, ISO-400 is twice as sensitive as is ISO-200, and ISO-1600 is twice as sensitive as ISO-800. Each successive value on the ISO sensitivity scale represents one ‘stop’ of light.

How ISO sensitivity can affect shutter speed

As the ISO sensitivity is increased the camera’s sensor requires less time to gather the required amount of light to capture and produce a properly exposed image. Let’s say that based on lighting conditions a photographer needs 1/60th of a second to properly expose an image when their camera is set at ISO-100, at a specific lens aperture. By setting the camera’s ISO to 200 the photographer has doubled the sensitivity of the sensor and can now use a shutter speed that is twice as fast to get the correct exposure. That means that the same subject can now be shot at 1/125th of a second, without having to change the aperture setting.

The following ISO/shutter speed examples may help to illustrate this in more detail…
ISO-100 – 1/60 of a second
ISO-200 – 1/125 of a second
ISO-400 – 1/250 of a second
ISO-800 – 1/500 of a second
ISO-1600 – 1/1000 of a second
ISO-3200 – 1/2000 of a second
ISO-6400 – 1/4000 of a second

Higher ISO settings are often used when the subject is in motion and a faster shutter speed is required to ‘freeze’ the action.

ISO sensitivity trade-offs

As with most things photographic there are trade-offs to consider when using higher ISO settings, regardless of the camera with which a photographer may be shooting. The first consideration for most photographers is the amount of additional digital noise that can occur when higher ISO settings are used to capture an image.

Generally speaking larger sensors such as full frame will have better light gathering capability. They will be able to capture good quality images at higher ISO values than smaller sized sensors are capable of doing at that same ISO setting in terms of the amount of digital noise that is in the image.

There are two other very important trade-offs to consider when using higher ISO settings: the loss of dynamic range and colour depth.

In simple terms think of dynamic range as the ability of the sensor to simultaneously capture details in both the highlight and shadow areas of an image. The larger the dynamic range of a sensor the greater the amount of details from the brightest highlights to the darkest shadows that it will be able to capture. This is measured in ‘EV’.

Think of colour depth as the ability of the sensor to capture a broad range of colours and to differentiate between subtle differences in colour. This is measured in ‘bits’.

For a difference to be noticeable for the majority of people a difference of 0.5 EV is required for dynamic range and 1.0 bits for colour depth.

Every camera, regardless of the size of the sensor used in it, will lose dynamic range and colour depth as higher ISO values are used to capture an image…even if the sensor is able to keep digital noise to a minimum at the higher ISO values.

Let’s look at a few examples of various cameras with test data produced by DxOMARK. At each ISO setting the corresponding test data for dynamic range (in EV) and colour depth (in bits) will follow.

Nikon D810 (full frame sensor)
ISO-100, 14.36 EV, 25.3 bits
ISO-200, 13.48 EV, 24.5 bits
ISO-400, 12.63 EV, 23.2 bits
ISO-800, 11.67 EV, 21.8 bits
ISO-1600, 10.72 EV, 20.2 bits
ISO-3200, 10.03 EV, 18.7 bits
ISO-6400, 9.08 EV, 17.2 bits
Low light: 2853 ISO
Notice how quickly the dynamic range and colour depth of the Nikon D810 drops off as higher ISO values are used.

Canon 7D Mark II (APS-C sensor)
ISO-100, 11.78 EV, 22.4 bits
ISO-200, 11.76 EV, 21.6 bits
ISO-400, 11.39 EV, 20.4 bits
ISO-800, 10.73 EV, 19.0 bits
ISO-1600, 10.2 EV, 17.7 bits
ISO-3200, 9.52 EV, 16.2 bits
ISO-6400, 8.48 EV, 14.7 bits
Low light: 1082 ISO
The dynamic range of the Canon 7D Mark II starts off much lower than the D810, and initially does not drop off as quickly as higher ISO values are used. You’ll notice that by ISO-800 there is less than 1.0 EV difference between the two cameras.

Sony A6000 (APS-C sensor)
ISO-100, 13.14 EV, 24.1 bits
ISO-200, 12.44 EV, 23.1 bits
ISO-400, 11.95 EV, 22.0 bits
ISO-800, 11.06 EV, 20.1 bits
ISO-1600, 10.17 EV, 18.2 bits
ISO-3200, 9.25 EV, 16.5 bits
ISO-6400, 8.32 EV, 14.9 bits
Low light: 1347 ISO
Notice the difference in performance between the APS-C sensors in the Sony A6000 and Canon 7D Mark II.

Panasonic GH4 (M4/3 sensor)
ISO-100, 12.76 EV, 23.2 bits
ISO-200, 12.54 EV, 22.8 bits
ISO-400, 11.75 EV, 21.3 bits
ISO-800, 10.96 EV, 19.7 bits
ISO-1600, 10.14 EV, 17.8 bits
ISO-3200, 9.18 EV, 16 bits
ISO-6400, 8.16 EV, 14.3 bits
Low light: 791 ISO
Notice how the performance of the M4/3 sensor in the GH4 compares with the Canon 7d Mark II up to ISO-1600.

Nikon 1 J5 (1″ sensor)
ISO-160, 12.04 EV, 22.1 bits
ISO-200, 11.84 EV, 21.7 bits
ISO-400, 11.16 EV, 20.3 bits
ISO-800, 10.3 EV, 18.5 bits
ISO-1600, 9.63 EV, 17.1 bits
ISO-3200, 8.86 EV, 15.5 bits
ISO-6400, 7.85 EV, 13.7 bits
Low light: ISO 479
The same drop off in sensor performance with both dynamic range and colour depth occurs with the small 1″ sensor in the Nikon 1 J5.

As you can see with all of the examples above, there is a penalty to be paid when shooting at higher ISO’s in terms of reduced dynamic range and colour depth, regardless of the camera being used and regardless of sensor size.

Photographers wanting to maximize overall image quality should shoot at base ISO whenever possible, regardless of the camera they are using.

Manufacturer-stated ISO versus Measured ISO

Many folks make the assumption that when they set the ISO on their camera that it is actually shooting at this level of ISO sensitivity. This is not necessarily the case. There can be a difference between the Manufacturer-stated ISO (i.e. the ISO setting on your camera) and how the sensor in your camera is actually performing.

Just because your camera says it is shooting at ISO-800 for example, it doesn’t mean that’s actually how the sensor in your camera is performing.

Let’s have another look at DxOMARK test data in terms of the 5 cameras noted above and see how their Manufacturer-stated ISOs compare with each camera’s actual measured ISO. NOTE: click on image to enlarge.

manufacturer vs measured ISO

From a practical perspective the vast majority of photographers would not even notice these differences between manufacturer-stated ISO and measured ISO. Where it could become an issue is when shooting video with more than one camera, and the photographer is using all manual settings including ISO. Under identical manual settings the exposures could look sufficiently different between cameras to require some work in post production to correct the exposures so that they match one another.

Understanding how ISO sensitivity settings can affect exposure settings and image quality is important for all photographers. Since there is a trade-off in terms of reduced dynamic range and colour depth, as well as the risk of more noise in images when higher ISO settings are used, photographers should use the lowest possible ISO settings whenever possible.

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