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Tonal Range
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The tonal range of a digital image is the
number of tones it has to describe the dynamic range. These
conceptual examples show that an image with a large dynamic range
can have a narrow tonal range and an image with a low dynamic image
can have a wide tonal range.
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Wide Tonal Range
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Narrow Tonal Range
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High Dynamic Range
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Low Dynamic Range
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Dynamic Range and Tonal
Range of the Sensor
The dynamic range and tonal range of a sensor
are related. If a sensor has a dynamic range of say 1000:1 AND it
has an ADC of at least 10 bit, it automatically has a wide tonal
range. If a sensor with a 10 bit ADC is able to output about 1,000
different tones, the sensor must have a dynamic range of at least
1000:1. This is because the sensor is linear and ADC samples in
equal steps.
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Dynamic Range and Tonal Range
of the Image
Once you apply a tonal curve to
the linear sensor data, the dynamic range and tonal range of the image
can vary independently, depending on what tonal curve you apply. The
tonal curve can compress the dynamic range, the tonal range, or both
(*).
When shooting in JPEG, the rather
contrasty tonal curves applied by the camera may clip shadow and
highlight detail which was present in the RAW data. RAW images preserve
the dynamic range captured by the sensor and allow you to compress the
dynamic range and tonal range by applying a proper tonal curve so that
the whole dynamic range is represented on a monitor or print in a way
that is pleasing to the eye. This is similar to the more extreme example
below which shows how the larger dynamic range and tonal range of a 32
bit floating point image were compressed. |
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Dynamic Range and Tonal Range
of a Monitor or Printer - Compression
Monitors and printers have a
limited dynamic range. Therefore a tonal curve is applied to the linear
raw data to compress the dynamic range so that it fits in the dynamic
range of the monitor or printer. The tonal curve is chosen so that
detail is preserved where it is most noticeable. As a result, the image
looks pleasing to the human eye and the perceived dynamic range is
higher, even on a limited dynamic range print. |
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In this scene, the shadows were about 2,000
times darker than the highlights (11 stops). As explained in the
dynamic range topic, a digital camera could capture either image A
or image B. In image A, the highlights are clipped because the long
exposure needed for the shadow detail to be captured, caused the
highlight pixels to overflow. In image B, to prevent the highlights
from being clipped, exposure had to be so short that the sensor did
not receive enough photons to capture shadow detail, resulting in
clipped shadows.
In Adobe Photoshop CS2 you can combine several exposure bracked
images into a high dynamic range image. How do we represent such a
high dynamic range on a limited dynamic range monitor or printer?
The answer is "compression".
In the histogram of images A and B, the red and blue areas show the
unclipped shadow and highlight detail respectively. Compressing both
areas by describing them with fewer tones allows both areas to be
combined into a single image that looks pleasing on a monitor or
print. The catch is that in the actual scene the values of the
highlights were about 2000 times stronger than the shadows, while on
the monitor, and especially on the print, h:s will be much lower.
When viewing Image C on a monitor, it has a high perceived dynamic
range because it looks as if the whole dynamic range of the scene
was captured in single exposure by a camera with a very high dynamic
range. Tonal compression is better done in high bit environments as
it avoids posterization.
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(*) Here the
analogy with film applies. For instance, positive (slide) film has a
wider density range but less exposure latitude, while negative film,
has a narrow density range, but more exposure latitude.
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