Canon PowerShot G1 X Review
Canon G1 X Dynamic Range (JPEG)
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.With no DR enhancement features switched on the G1 X's default tone curve is a little steeper in the highlights, with about half a stop less highlight range than some cameras in the same class. This steeper tone curve means there can be a rather abrupt transition from near-white pixels to clipped data, for example in clouds or an overcast sky.
The G1 X offers two Dynamic Range enhancement options - DR Correction and Shadow Correction. 200% DR Correction is the same as Highlight Tone Priority (HTP) on Canon DSLRs and is designed to retain highlight detail in high contrast scenes. However, in addition to the 200% DR option (equivalent to HTP) it also offers a 400% DR mode for even more highlight range. It does this by applying less amplification to the signal coming from the sensor, then compensating for it by using a different tone curve to ensure the correct brightness in the final image.
At the 200% setting ISO 200 becomes the minimum sensitivity and ISO 400 at the 400% setting respectively. In our test the 200% option gives you one additional stop of highlight range, the 400% option two stops, making it a good option for high-contrast scenes. When using the system the effect is the same as underexposing an ISO 100 shot by one stop for the 200% setting (two stops for 400%), then pulling up the mid tones and shadows to compensate. On the downside you loose some low contrast detail in the shadow areas due to increased noise reduction.
Shadow Correction is the same as the Auto Lighting Optimizer (ALO) on Canon DSLRs and is designed to slightly lift the shadows and increase shadow detail. You can choose between 'Off' and 'Auto' settings for the feature. Unfortunately our dynamic range test scene doesn't trigger the system but you can see real-life samples which were taken with both DR Correction and Shadow Correction on the 'Photographic Tests' page of this review.
The G1 X used Canon PoweShot series tone curves which are a little different to what we've seen from Canon DSLRs. The Neutral setting is unusually steep in the highlights and while the tone curves of most other settings variy a little bit in the mid-tone range they all clip around the same point as the neutral setting, thus not offering any additional highlight range. Sepia is the only setting that offers a tone curve with a noticeably smoother roll-off and about half a stop additional highlight range.
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