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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.

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

The NEX-7 uses the same JPEG tone curve as the NEX-5N, and this means impressive highlight range compared to its mirrorless peers. So you get a full four stops in the highlights, with a gentle 'film-like' roll off. This is full extra stop over the Panasonic Lumix DMC-GH2 and Olympus PEN E-P3, and overall an impressive performance - as good as any SLR.

HDR and DRO

The NEX-7 features two means of expanding the dynamic range of its JPEG output - Dynamic Range Optimization (DRO) and Auto High Dynamic Range (Auto HDR). Here we illustrate the effects of each. Note however that the effects of Sony's DRO and HDR settings differ depending on the scene, so this test, performed using our 18 step wedge, isn't necessarily an accurate indication of 'typical' performance with a real-world subject.

DRO is designed to work by lifting the shadows, to reveal the most detail in these areas from a single exposure. It doesn't affect the camera's chosen exposure, which means that the highlight range remains unchanged. Increasing the DRO level simply increases how far the shadow regions are pulled up to reveal more detail.

Auto HDR mode combines three bracketed exposures to generate a single image, in effect combining the the mid tones from the 'correctly' exposed shot with the highlights from the 'underexposed' version and the shadow regions form the 'overexposed' one. The advantage of this method is that it increases both the highlight and shadow range, without increasing shadow noise. The disadvantage is that it's incompatible with RAW - indeed if you have RAW recording turned on, you can't even enable it.

These graphs show that the shadow regions are progressively lifted as the HDR increment is increased, but unlike DRO, the highlight range expands too. This also means that the overall tone curve diminishes in contrast, and at higher bracketing increments it gets very flat indeed. Images produced like this tend to be visually unappealing, and will often need further post-processing work to look right.

Colour Modes (Creative Styles)

The NEX-7 uses a series of similar tone curves for almost all of its Creative Styles, which differ in contrast while maintaining exactly the same highlight range. There are a couple of identifiable 'families' - Standard, Neutral and Night all use one curve, while Vivid, Landscape, Sunset, Autumn Leaves, and Black and White all use another, that's slightly higher in contrast. Light, as its name might suggest, produces slightly lighter images, while Deep is a bit darker.

The one exception to all of this is Clear, that uses a very high-contrast curve that clips abruptly to white just 2.6 stops above middle grey. Meanwhile Sepia uses much the same tone curve as Black and White, but with the white point depressed and the black point raised (such that 'whites' are rendered light yellow, and 'blacks' as dark brown).

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Sony Alpha NEX-7

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