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The dynamic range

In digital photography, the dynamic range or contrast ratio is a major parameter influencing the image quality of a sensor. The higher the dynamic range that a camera can record, the more natural and detailed are the images captured. Colours will appear vibrant, with a high level of contrast. Of note, the price of a camera is no indicator for the image quality it delivers. In terms of dynamic range, all-round DSLRs can easily match expensive special-purpose microscope cameras and in most cases even outperform them.

dynamic range

In photography, dynamic range means the ratio of luminance between the darkest and the lightest points in an image. If, for example, a white point in the image is 3,000 times lighter than a black point, the contrast is 3,000:1. Typical camera systems have a ratio of 3,000:1. The higher the dynamic range, the more capable a camera is of differentiating various levels of luminance.

Dynamic range human eye vs. dynamic range camera sensor:

Dynamic range human eye

As a rule, the specimen (object) has a much wider dynamic range than the camera system, which means that when a picture is taken, a lot of image detail is inevitably lost. It goes without saying that the human eye – with an average dynamic range of 17,000,000:1 (!) – is vastly superior to any camera because the mechanism of commodation enables the eye to optimally adapt to the respective lighting conditions (e.g. dark room or bright surroundings and daylight). This is also one reason why images seen through the eyepiece of a microscope appear more beautiful than when viewed later in a photograph.

Bit Precision of analog/digital Converter dynamic ratio
8 256:1
10 1.024:1
12 4.096:1
14 16.384:1
16 65.536:1

The dynamic range is commonly represented as a ratio or in aperture stops (f-stops).

Comparison of some current cameras:

DSLR/Microscope camera aperture stop dynamic ratio
DSLR Nikon D800 14,4 21.619:1
DSLR Canon EOS 700D 11,2 2.353:1
DSLR Canon EOS 7D 11,7 3.327:1
DSLR Canon EOS 650D 11,2 2.353:1
DSLR Nikon D7000 13,9 15.287:1
DSLR Nikon D5300 13,9 15.287:1
DSLR Canon EOS 70D 11,6 3.104:1
Mikroskop Spezialkamera Zeiss AxioCam MRc5 10,4 1.300:1
Mikroskop Spezialkamera Leica DMC2900 9,2 600:1
Mikroskop Spezialkamera Leica DFC295 9,2 600:1
Mikroskop Spezialkamera Leica DFC290 HD 9,2 600:1
Mikroskop Spezialkamera Leica DFC345 FX 10,0 1000:1
Mikroskop Spezialkamera Leica DFC450 9,9 900:1
Mikroskop Spezialkamera Leica DFC450 C 9,9 900:1
Mikroskop Spezialkamera Leica DFC495 9,6 800:1
Mikroskop Spezialkamera Nikon DS-Fi1c 10,0 1.000:1
Mikroskop Spezialkamera Nikon DS-Vi1 9,2 600:1
Mikroskop Spezialkamera Leica DFC3000 G 9,9 900:1
Mikroskop Spezialkamera Leica DFC365 FX 11,5 2.818:1

 

In the case of stationary specimens, the HDR (high dynamic range) feature makes it possible to increase the dynamic range by taking multiple shots at different exposures which are then merged into one image. The HDR feature is either included in the camera’s software, or the images can be post-processed on a computer using special software. HDR imaging produces significantly brighter, more detailed photographs with a much higher dynamic range.

HDR doesn’t work very well for moving objects. Since there is always some time lapse between the different exposures, the individual images cannot be perfectly aligned afterwards.

24.07.2015

 

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