DSLRs and High Dynamic Range Imaging (HDRI)

This was filmed using two Canon 5D mark II DSLRs, each capturing the exact same subject via a beam splitter. The cameras are set so one records the subject overexposed, the other underexposed. After the footage has been recorded, a variety of HDR processing tools are used to to combine the video from the two cameras, yielding the clips you see above.

This can also be done with one camera such as Canon T2I with Magic Lantern.

High dynamic range imaging (HDRI) is a way to achieve a much higher contrast ratio (the range of values between light and dark areas) than normal photo techniques can typically achieve. The goal of HDRI is not to create an unnatural image, but one that accurately records the wide dynamic range of maximum black and the whitest white that occurs in one image.

For instance an image can have extreme contrast differences, but most photo and video technology cannot record these extreme differences. If you exposed for the darkest areas, you would over-expose the highlights and vice versa. Choosing what to expose properly and what to let go – ether under or overexpose is a judgement call of the photographer and is an important element of the art of photography. It applies equally to cinematography and videography.

If you took one photograph and exposed for the darkest areas and then from the same vantage point you took more photos and exposed for the lighter areas and selectively superimposed these photos into one image, you would get an image with remarkable dynamic range. So instead of allowing the windows to “blow out” or overexpose while keeping the room properly exposed, you can do both. This was the old way of doing this by combining the different shots in Photoshop or another program after the fact.

Taking different exposures with differing f-stops is called bracketing and is a time-tested technique in photography. Instead of taking three separate photos, each exposing for a different light value, today’s DSLRs can do this automatically. By taking just one shot, the camera will automatically take several different shots at different exposures.

Is it cheating? Well, perhaps in the same way that auto focus and auto iris are also cheating, not to mention all the other auto functions in today’s cameras. It’s really just another tool at your disposal.

HDR For Video

While most video camcorders cannot process multiple images for each frame, there are several that can process multiple images. The Magic Lantern for Canon records alternate frames at different ISOs. But this technique requires considerable post-production processing. Unfortunately any movement in the frame would lead to excessive motion blurring.

Red and Arri have gone a step further. Red Epic and Scarlet cameras have an HDR video solution called HDRx. It records two exposures within the interval that a standard camera would record only one frame. The primary exposure is normal, using the standard aperture and shutter settings. The secondary exposure uses a much shorter exposure for highlight protection. The two image streams are combined post to create a higher dynamic range.

The sensor in the Arri Alexa cinema camera uses a proprietary Dual Gain Architecture so it captures two separate signals at different levels of amplification, effectively increasing the dynamic range.

In the more affordable arena, DSLRs can capture multiple images per frame if the rate is much less than 30 frames per second. Time lapse video fits that bill because it has typical frame rates of 1 frame per second and less.

As TV sets are now featuring enhanced dynamic range, it won’t be long before TV and other video applications will find it commonplace.


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