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Discussion in 'Digital Video' started by mtfield, Oct 30, 2008.
Could someone explain to me the idea of dynamic range in digital cinematography?
Dynamic range in audio refers to the range of levels between the quietest and loudest parts of the audio. Unless you're recording a test signal, a good dynamic range is important to maintaining realistic sounding audio.
Dynamic range in video refers to the range of gray scale between the black and white levels - sometimes referred to as contrast.
Here's a picture comparing low dynamic range with high dynamic range. You can see that the higher the range, the more areas are properly exposed.
thanks a lot for your answer guys... i see dynamic range mentioned often with various f stops... could you guys explain the connection?
The f/stop refers to the size of the iris opening. The larger the number, the smaller the opening. ie; on most camcorder lenses, f/16 is typically the smallest opening and f/2 is typically the largest opening.
Since dynamic range is controlled by lighting and exposure, the f/stop setting does play a part in the overall dynamic range of the image. Dynamic range will be reduced if the image is over-exposed or under-exposed. If the image is over-exposed, the highlights are blown out and you'll lose details in the brighter areas. Likewise, if the image is under-exposed, the blacks will be crushed and you'll lose details in the shadow areas. To get a good dynamic range, you'll need both proper lighting AND exposure.
Exposure is essentially controlled by two things, the size of the iris opening and the shutter speed. The iris controls the VOLUME of light reaching the image sensor while the shutter speed controls the DURATION of time that the light hits the image sensor. Think of it like a faucet; the more you open the faucet, the more water you'll get over any given duration ... and the longer you leave the faucet open, the more water you'll get at any given volume.
The iris setting can also play a part in determining depth-of-field. Depth-of-field can be altered by three basic things; focal length of the lens (longer lenses produce a shallower depth-of-field), distance from camera to subject (closer distances produce a shallower depth-of-field) and the iris opening (larger iris openings produce a shallower depth-of-field). In many scenes, most film makers try for a shallow depth-of-field to focus the viewer's attention on the subject. The same applies to still photography as well. By controlling any or all of the three things above as well as the lighting, you can achieve the "look" and "feel' that most people try to get when they're going for a "film look." Keep in mind that when you change the iris setting to get the depth-of-field you want, you'll have to change the shutter speed or add neutral density filters to the lens to compensate for the change in exposure.
One note: video cameras don't have an actual shutter. The term is a hold-over from the film camera days. When you set the "shutter speed," you're actually setting how many times per second that the electronic signal is downloaded from the image sensors to the image processing circuitry. In the U.S., we're on 60 cycle current so the default shutter speed is 1/60th of a second ... meaning that the signal is downloaded 60 times per second (2 fields per interlaced frame, 30 interlaced frames per second, otherwise known as 60i). In PAL countries, they're on 50 cycle current so the default shutter speed is 1/50th of a second (25 interlaced frames per second).
Thanks for the info DH, this is something I've been scratching my head over for awhile.