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 Mode options
The E-620 has a selection of Picture Modes that can be altered depending on the subject you're shooting and the look you wish to achieve. As you can see,everything except the portrait and mono modes are using the same tone curve and are clipping at the same point in both the highlights and the shadows (implying there's no contrast distinction between them).
Shadow Adjustment Technology
Like most contemporary DSLRs, the E-620 dynamic range compression mode that attempts to pull a wide range of tones into the final image in order to give a greater dynamic range in a way that recreates the scene as you would have perceived it. This Shadow Adjustment Technology is activated by changing the Gradation setting to 'Auto,' the result is a greatly improved dynamic range but, as the gentle slope at the bottom of the tone curve implies, an awful lot of near-black detail that risks being clogged up with noise. Our noise tests with Gradation set to Auto showed significantly higher noise in dark regions of the image.
The thing that jumps out about the E-620 is what happens to the dynamic range and response curves at ISO 100 and 200. ISO 100 is showing considerably less highlight dynamic range and a much less subtle roll-off to white (resulting in a harsh transition to blown-out regions beyond the camera's dynamic range). Whereas ISO 200 and above use a more gentle and highlight-preserving tone curve.
It's worth noting that the ISO 100 tone curve is virtually identical to that used by the E-520 - the E-620's ISO 200 mode is effectively that ISO 100 underexposed by a stop, with a different tone-curve applied to pull the data back up to the correct brightness. This is essentially what Canon does with its Highlight Tone Priority mode, or Pentax with its DRange expansion mode, where the sensor is intentionally under-exposed and the data pulled-up in order to retain highlight detail.
However, this is not to say that anything underhand is occurring. There is no fundamental distinction between this and the behavior of the D5000 that over-exposes its ISO 200 in order to mimic an ISO 100 setting - because the ISO standard for digital imaging provides several different methods of determining ISO and many manufacturers use a definition based on JPEG image brightness, rather than any inherent sensor property. This is something we plan to cover in greater depth soon.
|Sensitivity||Shadow range||Highlight range||Usable range|
|ISO 100||-5.7 EV||2.7 EV||8.4 EV|
|ISO 200||-5.4 EV||3.9 EV||9.2 EV|
|ISO 400||-5.1 EV||3.8 EV||8.9 EV|
|ISO 800||-4.3 EV||3.8 EV||8.2 EV|
|ISO 1600||-4.0 EV||3.8 EV||7.8 EV|
|ISO 3200||-3.0 EV||3.8 EV||6.8 EV|
Dynamic Range compared
Don't get distracted by the usable range figure, it's the Highlight range value that's most impressive about the E-620. As you can see, even with gradation set to normal (Shadow Adjustment Technology turned off), the E-620 has a very flat tail to its response curve - meaning it's risking showing noise in the shadows by attempting to preserve detail, rather than clipping to black as early as its rivals. The result can be increased noise in the shadows (having studied our sample shots, the effect is rarely disastrous). But, while this is caused by the revised tone curve, it's not a direct effect of the increased highlight range - following the more abrupt dip to black taken by most other cameras would hide the problem.
|Camera (base ISO)||
|Olympus E-620||-5.4 EV||3.9 EV||9.2 EV|
|Nikon D5000||-4.8 EV||4.0 EV||8.8 EV|
|Canon 500D||-5.1 EV||3.4 EV||8.5 EV|
|Panasonic G1||-5.0 EV||3.0 EV||8.0 EV|
|Sony Alpha A350||-4.9 EV||3.7 EV||8.5 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).
Experience has told us that there is typically around 1 EV (one stop) of extra information available at the highlight end in RAW files and that a negative digital exposure compensation when converting such files can recover detail lost to over-exposure. As with previous reviews we settled on Adobe Camera RAW for conversion to retrieve the maximum dynamic range from our test shots.
As you can see the default Adobe Camera RAW conversion delivers less dynamic range than JPEG from the camera (a more contrasty tone curve and less noise reduction in shadows). It's possible to get considerably more than this out of the file, (for example, our ACR 'Best' parameters) but doing so results in a very 'flat,' unrealistic image. The point is that this additional information is there if you wish to selectively recover and blend-in this detail from a series of differently processed versions of the raw file.
- ACR Default: Exp. 0.0 EV, Blacks 5, Contrast +25, Curve Medium
- ACR Auto: Exposure -0.85 EV, Recovery 7, Blacks 0, Brightness 0, Contrast 0, Curve Medium
- ACR Best: Exposure -1.25 EV, Recovery 0, Blacks 0, Brightness +79, Contrast -50, Curve Linear
|ACR Default||7.2 EV|
|ACR Auto||9.2 EV|
|ACR 'Best'||11.3 EV|
To test how much exposure latitude the raw file actually gives, we intentionally over-exposed an image (by three stops) to ensure that highlights were clipped to pure white JPEG file. We then used digital exposure compensation on the raw file to see to what extent those highlights could be recovered. We found that we got no additional information beyond around -1.3 EV, and that even at this point the color accuracy of the recovered areas is imperfect.
|+3EV image with -1.3EV digital comp in ACR (ISO 200)||100% Crops|
Let's see what we could have captured in the higlights with a second shot of the same scene taken at 2.0 stops over the metered exposure (in other words, 1 stop less than the first example):
|+2EV image with -0.3EV digital comp in ACR (ISO 200)||100% Crops|
In this less exposed RAW file, all of the color could be brought back with -0.3EV digital compensation (the net result being the same image brightness as the previous example), so it's clear that there is something less than 1.3EV of reliable information beyond that which appears in the default output.
And, given that we already know that ACR's default tone curve gives around 0.4EV less highlight dynamic range than the JPEG, it suggests there's less than 1 stop of latitude for highlight recovery in the RAW files. Although this figure may be a fraction lower than we're used to seeing in APS-C cameras, the difference is slight and in part is a reflection on the JPEG engine's ability to make the most of the raw data.
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