Enhanced Dynamic Range EDR

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Contents

Background

Enhanced Dynamic Range (EDR) is a technique where a non-linear transfer function is used to allow a greater dynamic range of luminosity to fit in the same number of luminance data bits. There are many choices for the shape of the EDR non-linear transfer function. The edgertronic camera uses a transfer function with two knees - approximately located at 30% and 60%. The portion of the tranfer function from 0% to 30% matches the same linear transfer function as when EDR is off. Stated another way, when EDR is enabled, only pixels above 30% saturation are effected by the non-linear transfer function. The edgertronic camera image sensor supports a dynamic range of 57 dB when EDR is off and 90 dB when EDR is enabled.

Remember, Enhanced Dynamic Range only effects pixels that are more illuminated than the first knee on the non-linear transfer function.

Enhance Dynamic Range goes by other names as well, including High Dynamic Range (HDR), extended dynamic range, wide dynamic range (WDR), and other trademarked terms.

EDR settings

The edgertronic camera supports 5 levels of EDR - off, A, B, C, and D. There is no direct relationship with ISO, so the levels were given generic names.

EDR off
EDR A
EDR B
EDR C
EDR D

In the images above, all were taken with a color camera with ISO 1,600, 1/500s, f1.8 and force monochrome enabled. Notice that for all EDR settings, you can still make out about the same detail in the barn on the left. This is due to those pixels all having a luminance that is in the linear region of the transfer curve. As the EDR settings increase from off, A, B, and C, you can see more and more light bulb filament detail. At EDR D setting, the glass bulb and the glass structure holding up the filament have less detail.

EDR and color

For color cameras, enabling EDR will cause a hue shift in the non-linear regions of the transfer function. When EDR is enabled, high luminous pixels will have a purplish tint. A force monochrome feature was added for color cameras so when you have EDR enabled, you may want to enable force monochrome so you do not get false coloring in your video.

EDR off
EDR A
EDR B
EDR C
EDR D

In the images above, all were taken with ISO 1,600, 1/500s, f1.8. You can see the purplish tint where the non-linear EDR transfer function effected the highly luminous areas where the darker pixels have the proper hue.

EDR and ISO

Each doubling of the ISO (e.g. going from ISO 6,400 to ISO 12,800) halves the number of photons required to fully saturate a pixel. Since the EDR non-linear transfer function is applied as the photons accumulate, the effect of doubling ISO is moving the two knees up the transfer function so only more luminous pixels are in the non-linear range. Example: say a pixel accumulates photons until it is 66% saturated. With EDR enabled, that pixel is above the 2nd knee, so photon accumulation will have a higher attenuation. Now if you double the ISO, the pixel will go from 66% saturated to 33% saturated, moving it just above the first knee so it will be much closer to the linear region of the EDR non-linear transfer function. Stated another way, with EDR enabled, turning up the iso effectively gives you a wider luminous midrange. You can change the lens F-stop by one setting and double the ISO to get the same image brightness with a wider midrange at the cost of more analog and digitally induced noise.

ISO 1,600 f1.8
ISO 3,200 f2.8
ISO 6,400 f4
ISO 12,800 f5.6
ISO 25,600 f8

In the images above, all were taken with EDR set to level D. As you move from left to right the ISO is doubled and the F-Stop is increased by one setting. The effect of changing both ISO and F-Stop keeps the overall image brightness the same. This is most easily seen by comparing the barn, which is in the linear region of the EDR transfer curve, as you scan left to right. As the ISO is increased, the two knees in the EDR transfer function move up so only brighter pixels are effected. Somewhere around ISO 350, the EDR transfer curve becomes essentially linear (and thus is the same as EDR off) since the two knees are pushed way to the top of the transfer function. This can be seen by comparing the image labeled ISO 400 f4 to the image labeled EDR off. The noise increases as you move from left to right, but because of the subject, it is hard to see.

Notes

The above JPEG images were taken with an edgertronic camera using the Nikon 50 mm f 1.8D lens that typically comes with the camera. The background is an acrylic farm scene painted by my grandmother. I wanted a color background to show the hue shift that occurs with the higher saturated pixels.

The purple filament you can see prominently in the image labeled ISO 25,600 f8, and in other images as well, is caused by lens flare.

For all the images above, the other camera settings not mentioned are shown below.

Shutter:         1/500   Seconds
Frame Rate:      498.256 Frames/Second
Horizontal:      1264    Pixels
Vertical:        1008    Pixels
Sub-sampling:    Off     sub-sample
Genlock:         Off     genlock
Smart calibrate: On      using smart calibrate
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