Dynamic range is one of those onion phrases. The more you dig, the more complex it becomes. And that’s after you’ve narrowed your choice down to which discipline you’re talking about. Apart from photography, it is also used in audio engineering, electronics, music, and metrology.
The simple definition of dynamic range is this: the ratio between the largest and the smallest possible values of a changeable quantity, such as light.
That’s the easy bit. Thing is, in photography, what light are we talking about? The light in the scene? The light the human eye can see? The light your camera can record? The light it can convert into a file? The light your monitor can show you? The tonal range of the final print you produce?
For now, let’s stick to the human eye and your camera.
The human eye is a big fat liar
We all know that the human eye is able to cope with a much wider range of highlights and shadows than our digital cameras are. That’s a nailed on certainty, we all have that experience first hand.
On the face of it, our eyes can handle 20-24 stops of dynamic range. Your DSLR can’t hope to compete. You’d be lucky to get half that.
Except, it’s not that simple. Your eyes cheat. Big time.
For a start, your eyes aren’t static devices. Your pupils dilate or constrict to suit varying light levels. They do so rapidly, so that when you sweep your gaze across a scene, they are adapting to the changing light levels it contains instantaneously, giving the impression that you can see a wider range at any one time than you actually can.
In addition, your eyes adapt chemically to the amount of light available to them. Just think of how your eyes adjust in the dark, which in itself is dependent on a number of factors, including how efficiently your blood circulates. Alcohol and tobacco consumption are known to affect eyesight because they can impair circulation.
As if that weren’t enough variables, the ability of our pupils to dilate reduces as we get older, affecting our perceived dynamic range.
So, all in all, it’s tough to put a distinct value on the dynamic range of the human eye. Estimates range between 6.5 and 14 stops, depending on your source. Not exactly a narrow band.
Nevertheless, that brings our eyes closer to what DSLRs can manage.
Or does it?
Buckets and golf balls
Brace yourself. This is where your brain might start to hurt a bit.
Let’s start simple and go from there.
Digital imaging sensors are made up of individual cavities (photosites) that catch particles of light (photons) during an exposure. We commonly refer to the photosites on a sensor as pixels, which is short for picture element.
Now, remember that dynamic range is the ratio between the largest and the smallest values of a measurable quantity. Well, the smallest number of photons that a bucket can hold is one.
You might think that number should be zero, but bear in mind that dynamic range is a ratio. A ratio of x:0 isn’t possible. The only thing x:0 is good for is an interesting emoticon, but that’s about it. Ratios need to be x:y. Equally, we’re not going to entertain the notion of a fraction of a photon. As a result, the dynamic range of your digital camera sensor is going to be x:1.
The dynamic range of a sensor is determined by how many photons it takes to fill a photosite. A full photosite translates into white as far as the sensor is concerned.
Think of of a photosite as a bucket into which you pour golf balls – the golf balls being individual photons. Once the bucket is full (saturated), adding more balls isn’t going to make it any fuller. They will simply bounce off. The same goes for photons when they reach a saturated photosite.
The advantage of a bigger bucket (photosite) is that it can hold more golf balls (photons). In the case of a sensor photosite, that means the bigger it is, the wider its dynamic range range at the highlight end, and thus overall. In the case of an actual bucket, the more golf balls it can hold the longer you can go at the driving range before you have go for a refill, or steal some from your neighbour's bucket.
If the photosites on a sensor can hold a maximum of 1,000 photons and a minimum of 1 photon, its dynamic range ratio is going to be 1,000:1.
As photographers, we’re used to expressing that ratio differently, speaking instead of stops of exposure. If each stop is twice as bright as the preceding one, a dynamic range ratio of 1,000:1 is roughly the same as 10 stop increments (1 : 2 : 4 : 8 : 16 : 32 : 64 : 128 : 256 : 512 : 1024). That’s why we tend to talk of 10 stops of dynamic range*.
But wait, it’s not that simple
You know that often quoted statement about bigger photosites being better than smaller ones? Well, here’s the reason. Bigger photosites can hold more photons before they are full. That means they are capable of reproducing more shades of white than smaller sensors. What a small photosite sees as white, a big one sees as off-white. It’s capacity hasn’t been reached, so it won’t show up as quite white yet. In other words, a small photosite will report white at a lower level of light than a bigger one.
Because a big photosite can catch more photons, the dynamic range of a sensor with bigger photosites is going to be wider. If instead of being full at 1,000 photons (10 stops of dynamic range) a photosite that can take, say, 4,000 photons, it is going to have two stops more dynamic range (1,000 : 2,000 : 4,000).
So far, so good.
However, that’s only part of the story. Equally important is how well your camera converts the photons captured in its photosites into an image. A sloppy conversion will reduce the dynamic range. The reduction can be quite dramatic. Even a camera with a sensor capable of 10 to 14 stops of dynamic range could end up with an output equal to only five to nine stops. Bummer.
Does it matter?
In the end, the dynamic range of your camera depends on how big its photosites are and how well it can convert only a few photons in to a shade of grey and lots of photons into not-quite-white.
Even then, that could mean nothing when it comes to your final image.
You need to get your exposure bang on to make the most of the camera sensor’s dynamic range. Then you need process it sympathetically in your preferred post-production software, making sure you lose nothing along the way (sRGB anyone?). Finally, your display medium has to be able to cope with the dynamic range you want to show.
None of which is going to be at all relevant if the photograph is eyeball-combustingly bad to begin with. A great photograph with limited dynamic range will always trump a pitiful attempt with wide dynamic range.
*Not that photographers know what they are talking about. In optical science and in ISO standards, a stop is the actual physical aperture. What photographers call a stop of exposure is really a step, which could be a step too far for many of your reading this, so we’ll stop.
Sources:
http://www.hdrsoft.com/resources/dri.html
http://www.cambridgeincolour.com/tutorials/dynamic-range.htm
http://en.wikipedia.org/wiki/Dynamic_range
http://en.wikipedia.org/wiki/Human_eye
http://en.wikipedia.org/wiki/Exposure_value
http://www.telescope-optics.net/terms_and_conventions.htm
Shameless plug (we have to eat, you know)
Don’t forget, you can purchase the first episode of our landscape photography video series Dynamic Range for just €9.95, and as a bonus, the pilot episode is just €4.95! You’ll receive the download link via email as soon as you’ve completed the purchase.
Sorry, but there is a fundamental flaw in the logic of your golfball-bucket analogy (at the level of simplicity you’ve chosen). Because a narrow bucket fills up in the same rain exactly as fast as a wide one. Provided both are of the same depth, which is the case in both your drawing and real world image sensors.