JPEG Tone Curves / dynamic range
Our Dynamic Range measurement system involves shooting a calibrated Stouffer Step Wedge (13 stops total range) which is backlit using a daylight balanced lamp (98 CRI). A single shot of this produces a gray scale wedge from the camera's clipped white point down to black (example below). Each step of the scale is equivalent to 1/3 EV (a third of a stop), we select one step as 'middle gray' (defined as 50% luminance) and measure outwards to define the dynamic range. Hence there are 'two sides' to our results, the amount of shadow range (below middle gray) and the amount of highlight range (above middle gray).
To most people highlight range is the first thing they think about when talking about dynamic range, that is the amount of highlight detail above middle gray the camera can capture before it clips to white. Shadow range is more complicated; in our test the line on the graph stops as soon as the luminance value drops below our defined 'black point' (about 2% luminance) or the signal-to-noise ratio drops below a predefined value (where shadow detail would be swamped by noise), whichever comes first.
This page features our new interactive dynamic range comparison widget. The wedges below the graph are created by our measurement system from the values read from the step wedge, the red lines indicate approximate shadow and highlight range (the dotted line indicating middle gray).
The E-P5 has a fairly simple DR expansion mode - Auto Gradation. This combines an adaptive shadow brightening algorithm that Olympus calls Shadow Adjustment Technology, with a change in the camera's metering that will slightly reduce exposure to capture a fraction more highlight detail. Below you can see the effect on our DR test and in a real-world example.
Exposure: 1/250th sec, F5.6, ISO 200
Exposure: 1/320th sec, F5.6, ISO 200
|100% crop from shadow region||100% crop from shadow region|
Here you can see the real-world impact of this change, when shooting a high contrast scene. There's not a huge difference in the highlight regions (which is to be expected with just a 1/3EV difference in exposure), but the shadows have been lifted significantly. Lifting the shadows helps balance the image tonally, while retaining local contrast.
As you can see, the Olympus's tone curve is fairly similar to that used by the Sony NEX 6 - offering a decent amount of highlight dynamic range and a gentle roll-off to clipped regions. The shadow half of the tone curve is closer to that of the Nikon D5200, showing a slightly higher-contrast shadow response than the Fujifilm or Sony.
Changing the Gradation setting changes the exposure, so the top of the dynamic range graph shifts. The additional highlight tones captured are sensibly incorporated in the the image (as shown in the real world example above). The result is comparable to Fujifilm's DR 200% setting (or Canon's Highlight Tone Priority mode that does essentially the same thing as the Fuji), but with an adaptive tonal response that responds to the specific image, rather than applying a single tone curve.
Low ISO setting
As was common on many Olympus models since the E-30, but missing on more recent cameras like the E-M5, the E-P5 offers an ISO 100 equivalent setting with reduced highlight range. This is because the ISO Low and ISO 200 settings are derived from the same sensor amplification setting - ISO 200 images are exposed to less light, protecting highlights, compared to ISO 100. The two settings have different tone curves applied so that both give the same image brightness, despite the difference in exposure.
The upshot of this is that the ISO Low shots include less highlight detail but with 'cleaner' shadows, while the ISO 200 shots strike the opposite balance.