JPEG tone curves / dynamic range

By Rishi Sanyal

Our Dynamic Range measurement system involves shooting a calibrated Stouffer Step Wedge (13 stops total range) which is backlit using a daylight balanced lamp (95 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.

Note: 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).

Cameras Compared

Our comparison chart shows the GH4's default tone curve to have a less of the familar S-curve we're used to seeing with default renderings of most cameras. While the tone curve has S-shape character for the darker tones, the lighter tones above middle grey take a slow approach to clipping. This results in lower contrast around middle grey, since lighter tones aren't brightened as much as they are with, say, the Sony A7S. The more linear approach to clipped whites has another consequence: highlight tones do not roll off to clipped white as its rivales. Thus, one might expect more harsh transitions to clipped white with the GH4.

The GH4 offers slightly brighter rendering of shadows compared to the Sony a7S, which pushes its clipping point in our tests further down the wedge. The combination of a near-linear approach to clipped white and brighter shadows means overall image contrast will be slightly less than what you'd expect from a default Sony JPEG rendering, and it's really a matter of taste whether or not this is desirable.

DR modes

The GH4 uses the latest version of Panasonic's 'iDynamic' dynamic range compensation feature for coping with high contrast scenes. It's an 'active' system that can reduce exposure to capture more highlight tones, while boosting the shadows and mid-tones to give the correct image brightness. There are three levels of iDynamic, plus an Auto setting that attempts to use the most appropriate setting for the scene being captured.

The graphs below show the progressively increasing shadow and midtone boosts higher levels of iDynamic apply. Exposure is constant, as the camera did not choose to apply any exposure shift for our wedge scene. iDynamic High applies quite an aggressive shadow/midtone boost that is just shy of what the Sony a7S applies at its highest, Lv5, DRO setting.

It's worth mentioning that Panasonic's particular implementation here is almost midway between Sony's DRO (Dynamic Range Optimizer) and Nikon's ADL (Active D-Lighting) features. While Sony's DRO modes are inactive (no exposure adjustment), Nikon's ADL mode typically applies a pre-determined amount of negative exposure shift. Panasonic has chosen a middle way: the camera appears to make a scene-dependent decision on how much, if any, exposure shift to apply.

It's rather conservative in its approach - for high contrast scenes we typically noted 0 and -1/3 EV exposure shift for 'Low' and 'Standard' iDynamic settings, respectively, and -1/3 to -2/3 EV for the 'High' setting. Lower contrast scenes often, appropriately, showed no exposure shift at all. Given the GH4's reticence to apply large exposure shifts, you won't typically see drastic changes in highlight retention with progressively higher iDynamic modes. However, you can expect to see more balanced handling of high contrast scenes, as our real-world example shows below.

Off
Low
Standard
-1/3 EV
High
-2/3 EV
Auto
-1/3 EV

You'll note above that at higher settings, the camera reduces the exposure slightly to retain more highlights, while applying enough of a shadow/midtone boost to render these tones brighter than they'd have otherwise been. We've indicated the amount of exposure shift the camera chose to apply, if any. 'iDynamic' Auto chose a setting of Standard in the scene above, which is fairly reasonable.

DR modes with exposure compensation

Although the GH4's iDynamic modes are rather conservative in terms of how much exposure shift they apply to capture additional highlight detail in high contrast scenes, you can yourself manually combine more aggressive exposure reduction with iDynamic to achieve a more significant effect. Below we show you how much exposure shift you can apply, for any iDynamic setting, while still achieving the proper midtone brightness. You'll note that the highlight range gained can be so extensive (up to 1.3 EV) that the highlight cutoff falls off the right side of the chart. Despite our chart not having enough highlight range to show you where the highlight cutoff occurs, you can expect to gain back in highlights whatever negative exposure shift you apply - while still maintaining proper midtone brightness.

Below we demonstrate the real-world effect of manually combining exposure compensation with iDynamic. The results roughly follow what the graphs above lead us to expect - the tone curves used in iDynamic Low, Standard, and High provide enough shadow/midtone lifts to compensate for around -0.3 to -0.7*, -1, and -1.3 EV reductions in exposure, respectively. This means you can gain between 0.3 and 1.3 EV of highlight range using these modes, when you manually apply some exposure compensation or expose for the highlights in Manual mode.

iDynamic Off
iDynamic Low
-0.3EV
iDynamic Standard
-0.7EV
iDynamic High
-1.3EV

*Note that even though our tests indicated that iDynamic Low should be combined with -0.7EV exposure shift, we chose to apply -0.3EV exposure shift for our real-world iDynamic Low example as we felt the city tones at the bottom half of the image were more well maintained.