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 cameras) black to clipped white (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' 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 the camera can capture before it clips to white. Shadow range is more complicated; in our test we stop measuring values below middle gray 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.
Picture Style options
The EOS 550D, like all current Canon EOS cameras, features a range of Picture Style presets - image parameters which determine the color and contrast balance of the camera's JPEG output. As well as a set number of preprogrammed (but editable) Picture Style presets, the EOS 550D also has space on board for a further three user customizable profiles. These can be created using the supplied Picture Style Editor software and uploaded to the camera.
In terms of dynamic range, the difference between the inbuilt Picture Style presets is subtle, as you can see from the graph shown below. The gentler tones of the neutral and faithful presets are paired with a slightly lower contrast output which gives fractionally more dynamic range, and a slower 'rolloff' in the highlights, but the difference is very small in everyday use. Effective dynamic range remains almost the same across the span of settings, at an impressive 8.8 stops.
Highlight Tone Priority
Highlight Tone Priority was introduced into Canon's DSLR line in 2007, and has since become a standard feature. HTP is designed to recover roughly one stop of extra tonal information in highlight areas, which is especially useful in scenes containing a range of tones - the classic example being a bridal couple, in a dark suit, and bright white wedding dress respectively. This sort of scene is one where normally, you might have to trade highlight for shadow detail, or vice-versa.
Highlight Tone Priority works by reducing the amplification of the sensor by one stop, so that it takes one stop more light before it clips to white (and, as a result, you cannot select ISO 100 because this amplification mode is needed to provide HTP ISO 200). A different tone curve is then applied so that images are rendered at the correct brightness.
This one-stop reduction in amplification results in a raw file that is effectively push-processed by the new tone curve and would be underexposed if the normal tone curve were applied. There's a downside, of course - that additional highlight dynamic range comes at the expense of slightly greater shadow noise.
ISO Sensitivity and Dynamic Range
The Canon EOS 550D offers a very wide ISO span by traditional standards, and one of the widest in it's class, from ISO 100-12800 (equivalent). Impressively, dynamic range remains very stable throughout the span of available ISO settings, only decreasing (due to noise swamping signal in the shadows) at the highest ISO setting of 12800.
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 100||-5.5 EV||3.3 EV||8.8 EV|
|ISO 1600||-5.5 EV||3.3 EV||8.8 EV|
|ISO 6400||-3.7 EV||3.4 EV||7.1 EV|
|ISO 12800||-3.0 EV||3.3 EV||6.4 EV|
Dynamic Range compared
The Canon EOS 550D compares very well to its nearest competitors and is a near match for its close relation the EOS 500D, as we can see from the graph shown here. Total dynamic range from all four cameras plotted on this graph is very similar, although the Nikon D5000 does display slightly better highlight range. The Pentax K-x has the steepest curve here, indicating a tendency to clip highlights more abruptly, although the clipping point is almost identical to the Canon DSLRs shown here.
|Camera (base ISO)||
|Canon EOS 550D||-5.5 EV||3.3 EV||8.8 EV|
|Nikon D5000||-4.8 EV||4.0 EV||8.8 EV|
|Canon EOS 500D||-5.1 EV||3.4 EV||8.6 EV|
|Pentax K-x||-5.7 EV||3.2 EV||8.9 EV|
The wedges below 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).