What Is Dynamic and Tonal Range?

If you have ever wondered how dynamic range and tonal range affect your digital photography results, you are not alone. These two photographic terms can be a little confusing at first, but you can improve your DSLR photography by learning how they work.

What Is Dynamic Range?

All DSLR cameras contain a sensor that captures the image. The dynamic range of a sensor is defined by the largest possible signal that it can generate divided by the smallest possible signal.

A signal is generated when the camera image sensor's pixels capture photons, which they then turn into an electrical charge.

This means that cameras with a larger dynamic range are able to capture both highlight and shadow details simultaneously and in greater detail. By shooting in RAW, the dynamic range of the sensor is preserved, whereas JPEGs may clip the details because of the file compression used.

As already stated, pixels on the sensor collect photons during the exposure of an image. The brighter the exposure, the more photons are collected. For this reason, the pixels collecting the brighter parts of the image collect all of their photons more rapidly than those pixels collecting darker parts. This can cause an overflow of photons, which can lead to blooming.

Issues with dynamic range can most often be seen in high contrast images. If the light is too harsh, the camera may 'blow out' the highlights and leave no detail in the white areas of an image. While the human eye can adjust for these contrast and noise details, the camera cannot. When this happens, we can adjust the exposure by stopping down or add more fill light to reduce contrast falling on the subject.

DSLRs have a larger dynamic range than point and shoot cameras because their sensors have larger pixels. This means that the pixels have enough time to collect photons for both bright and dark parts of the image without any overflow.

What Is Tonal Range?

The tonal range of a digital image relates to the number of tones it has to describe the dynamic range.

The two ranges are related. A large dynamic range combined with an Analog to Digital Converter (ADC) of at least 10 bits automatically equates to a wide tonal range. (The ADC is part of the process of converting pixels on a digital sensor into a readable image.) Similarly, if a sensor with an ADC of 10 bits is able to output a large number of tones, it will have a large dynamic range.

Because human vision is non-linear, either or both dynamic and tonal range need to be compressed by a tonal curve to be more pleasing to the eye. In reality, RAW conversion programs or in-camera compression tend to apply a vaguely S-shaped curve to the data in order to compress the larger dynamic range in a way that is visually pleasing in a print or on a monitor.