Four Types of Dynamic Range!
Dynamic range is an important topic for photographers, so I'll do my best to make this article dynamic and interesting. Or at least I'll throw in a few key exclamation points for emphasis! (See.)
Defined as the ratio between the greatest value that something can assume to the smallest, dynamic range forms a useful metric in many areas dealing with signal processing. That it's a technical term doesn't make it sound very dynamic and exciting, but follow along with me here, if you will. In photography, we usually express this ratio in terms of doubling and halving, or "stops" of brightness. And there are four ways such dynamic range applies for photographers, and all four are helpful to understand.
Subject Dynamic Range versus Human Vision!
My glasses have a photo-grey coating that automatically darkens under bright light. I like that. It's like they become sunglasses on their own, so I don't have to squint so much in bright sunlight. From the deepest shadows to the most glaring highlights, brightness outdoors can vary widely. Our eyes automatically dilate to help compensate, but the glasses help, too. The more stops of light between the two extremes, the harder it is for our eyes to cope.
Of course, as photographers, what matters is the subject brightness range within a framed composition. Thankfully, we can create a more manageable problem simply by cropping out areas not of interest. The real problem begins when we can't frame an image with a workable subject brightness range.
Camera Dynamic Range!
Exposure outdoors can be challenging. Just like our eyes, our cameras have a hard time taking everything in. There are limits to what a sensor can record. If too much let gets in, your image will be overexposed. At least in part, the result will be burned out and white. The opposite can happen, too — underexposure results when too little light reaches the sensor during the time the shutter is open. We solve this problem by adjusting the aperture, shutter speed, or even the ISO sensitivity to compensate.
But cameras struggle when we ask them to do too much at the same time. We can't both open up the shutter and close it down in the same press of the shutter release buttom. When the dynamic brightness range of your subject exceeds what your camera can record, something has to give to avoid disappointment.
The traditional strategy to handle the problem was by using graduated neutral density (GND) filters. Such filters are clear at one side and progressively graduated towards darker grey (neutral density) at the other end. By aligning the filter so that the darker portion of the filter covered the brightest area of incoming light, it was possible to compress the dynamic range, hopefully enough to fit. It wasn't always possible to solve matters entirely or perfectly, but it helped.
Modern methods of tackling the problem involve shooting multiple separate frames at varying exposures and blending them digitally. With your camera locked in position, you can shoot a series of images at different shutter speeds and let your computer assemble the best from each later on. Digital blending allows you to merge exposures along edges beyond what you can with a traditional filter. I used to carry a huge stack of GND filters with soft edges, hard edges, reverse graduated, and so on, but never have I seen one with an irregular edge like a sloping hillside with a few sparse trees or a field strewn with boulders.
File Format Dynamic Range!
Taking things a bit further brings us to the realm of High Dynamic Range (HDR) imaging. An 8-Bit image like a jpeg can only record numbers between zero and 255 for each RGB color channel. 16-Bit applications like Photoshop and RAW image programs such as Lightroom have a leg up on simple jpegs but have their limits, too. Dispensing with the traditional color models and file formats used in the digital darkroom, HDR uses floating-point data to build a composite image and then "tone maps" the result into a final viewable image.
Monitor and Printer Dynamic Range!
But HDR images aren't perfect either. While they do have the dynamic range to handle anything you're likely to encounter in the real world, no monitor or printer made yet can do them justice. Tone mapping involves the use of clever software sleight-of-hand tricks to turn the nearly limitless dynamic range of an HDR image into something you can see and print. Good desktop LCD screens are better than most laptops, and OLED displays have the potential to come even closer, but we're a long way from being able to display a full HDR image in all its glory.
And let's face it, even if a monitor someday can do so, you may need sunglasses of photo-grey lenses even to squint at some HDR composites. For now, our hardware remains the weakest link in the chain, but it's improving quickly. In the end, it will be the dynamic range of human vision that we can't exceed.
At any rate, dynamic range is a difficult topic to make that dynamic, but I hope the article has been informative. Or at least occasionally entertaining.
Thanks for reading!