Last week I posted an introductory article on dynamic range. I want to start diving down into the subject a little more this week, starting with the best visual capture device I know: the human eye. The images here are both from my point and shoot here on the plains where I’m working, and also some from the archive.
Dynamic range, though technically expressed as a ratio, is more simply expressed in terms of stops, as in stops of light. If you hold shutter speed (and ISO) constant and open your aperture from f/11 to f/8 you have brightened the image by 1 stop. You’ve allowed twice the amount of light to enter the camera and be detected by your sensor or film. For every one stop you either double or halve the amount of light.
Our Amazing Eyes
So let’s start with your eyes. Although the internet has a wide variety of answers to the question, most agree your eyes have a dynamic range of about 20 stops from darkest to brightest. That means you can detect at least some detail in the darkest and brightest parts of that enormous range at the same time. We don’t usually need to deploy all this range at once of course. Instead we shift and adapt (very quickly!), using different parts of our dynamic range under varying conditions. If you allow for pupil dilation our range increases to 24 stops!
We do have our visual limitations. Our eyes start to fail us in very bright or very dark lighting conditions. We nevertheless can detect enough detail for recognition of threats when either looking into deep shadows at dusk or the rising sun at dawn. Remember each stop of light is accompanied by a doubling or halving of the amount of light. So despite the fact that some other animals put us to shame in other visual abilities, it’s quite the impressive dynamic range we’ve got.
Given the above, you probably are not surprised that cameras, even the best, cannot match the human eye’s abilities in the dynamic range department. At least when it comes to “normal” daylight exposures. At night you can leave the camera aperture open for extended periods and collect the scattered light photons that, while our eyes may detect them, simply don’t end up translating into good clear images in our brains. Our cones (color receptors) are especially lazy at night. It’s the reason we are so amazed when objects in the night sky are rendered in stupendous color.
So unless the future sees us evolve into nocturnal creatures, our eye-brain visual system will remain limited at night. Of course our brains have used our hands to invent devices to extend our vision into the nighttime realm. So enjoy those images of space you see, both the deep-sky telescope images and the star-scapes that have become such popular fare on the web.
Dynamic Range of Cameras
So let’s get to the question that sparked my interest in this subject in the first place: what can cameras see, and does it matter? If you look into this on the internet, you’ll come across all sorts of over-complicated camera test results, graphs and data until it’s coming out your ears! It may come as a surprise that film (at least negative film) can generally capture more dynamic range than digital sensors can. This is debated (of course) on the internet, but it’s pretty much true.
The reason I say “generally” and “pretty much” is that firstly, comparing film and digital in terms of dynamic range is a little like comparing apples and oranges. Film handles tonal variation differently (see Caveats below), and is nonlinear. Digital sensors are linear. In fact, in this way film is closer in behavior to your eyes than digital sensors are. Secondly, film is noticeably better at handling highlight dynamic range (bright end of the scale), whereas digital has advantages on the shadow (dark) side of things. Your eyes can also see more detail in shadow than in highlights. Digital cameras mimic your eyes in this way, but as any film enthusiast will tell you, they don’t do quite so well with highlights (they would describe it less kindly).
All that said, digital camera makers have been working hard on increasing dynamic range, while film is not receiving that much attention. So you can expect all this to be a moot point in the near future. Currently, the best digital cameras for dynamic range, the Nikon D800 and other high end DSLRs, and (especially) the new digital video cameras, can supposedly record dynamic ranges of up to 14 stops.
But that’s at low ISOs. Think about those starscapes I mentioned above, those images of an improbably colorful and bright Milky Way you see soaring over everything from mountains to farm tractors to the Eiffel Tower! That is not really a demonstration of the camera’s dynamic range but its low-light capabilities. When the photographer cranks up the ISO to create those images, she is cranking down on the camera’s dynamic range.
That’s enough for now. This little miniseries on Dynamic Range will continue next week. Thanks for reading!