Adjusting exposure in Raw Therapee

Exposure is the most intuitive way to adjust the brightness of a digital photo. Exposure correction simulates the effect of changing exposure in the camera (changing shutter speed, aperture or ISO sensitivity). This post compares two methods for adjusting exposure in Raw Therapee (RT, version 4.08). I found that scaling of the raw photo RGB channels is more accurate than exposure compensation in the RGB working space.

How exposure correction works

Digital cameras are linear devices and exposure correction is simply a factor applied to linear RGB data that simulates the result of more light (factor > 1) or less light (factor < 1) being recorded by the camera sensor.

Light meters measure exposure in stops and photographers think in stops. The relationship between the linear exposure correction factor and exposure compensation (EC) stops is exposure factor = 2^EC. Here is a table that might be helpful.

Fractional stops Decimal stops Linear factor
-1 -1.00 0.50
-5/6 -0.83 0.56
-2/3 -0.67 0.63
-1/2 -0.50 0.71
-1/3 -0.33 0.79
-1/6 -0.17 0.89
0 0.00 1.00
1/6 0.17 1.12
1/3 0.33 1.26
1/2 0.50 1.41
2/3 0.67 1.59
5/6 0.83 1.78
1 1.00 2.00
Equivalent exposure corrections, stops and linear.

Increasing exposure

Here is an example photo which, to my eye, has a satisfactory exposure. Scene brightness covers a wide range and there is potential for clipped highlights.

Satisfactory exposure, taken with Canon 400D (Digital Rebel XTi), ISO200, f/8, 1/100 seconds. Processed with Raw Therapee. All photos in this post use the same, manual white balance.

Here is the same scene underexposed by -0.68 stops. This is a real capture. The scene composition has not shifted because I was using a tripod.

Underexposed, taken with Canon 400D (Digital Rebel XTi), ISO200, f/8, 1/160 seconds. Processed with Raw Therapee.

Next, I have applied +0.68 stops exposure compensation to the underexposed photo in RT (Exposure tab). This resulted in clipped red and blue channels and a pink sky. The pink cast is more obvious in the RT preview than in the JPEG output.

Underexposed photo with +0.68 stops exposure compensation, detail at 50% size. Screen grab from Raw Therapee. Note the pink highlights.

Underexposed photo with +0.68 stops exposure compensation, detail at 50% size. Raw Therapee JPEG output. The pink highlights are subtle.

To increase exposure in RT, raw white point linear correction factor > 1 (Raw tab) gives better results. Increasing raw exposure will increase highlight clipping, of course, but the results accurately portray how the image would have resulted if the exposure was increased at the moment of capture.

Underexposed photo with raw white point linear correction factor x1.60 (+0.68 stops), detail at 50% size. Processed with Raw Therapee.

To increase exposure and retain highlights, try using RGB curves (Exposure tab in RT). This post is focussed on basic exposure adjustments and not highlight recovery.

Decreasing exposure

Here is the same scene overexposed by +0.74 stops. The raw photo histogram in RT showed plenty of highlight clipping.

Overexposed, taken with Canon 400D (Digital Rebel XTi), ISO200, f/8, 1/60 seconds. Processed with Raw Therapee.

To decrease exposure in RT, raw white point linear correction factor < 1 (Raw tab) often results in false highlight colours, i.e. the relative proportions of RGB are lost when the raw data are clipped. When this happens, highlight reconstruction (Exposure tab) can save the image. Note well: highlight construction works on the raw photo RGB channels, before colour management and conversion to the RGB working space. Highlight construction only makes a difference when the raw photo RGB channels are clipping.

Overexposed photo with raw white point linear correction factor x0.6 (-0.74 stops), detail at 50% size. Processed with Raw Therapee. Note the pink highlights.

Overexposed photo with raw white point linear correction factor x0.6 (-0.74 stops) and highlight reconstruction by colour propagation, detail at 50% size. Processed with Raw Therapee. Highlight reconstruction was successful in this example.

Exposure compensation (Exposure tab) in RT takes place in the RGB working space and does not recover highlight details. The highlight recovery tool (amount and threshold) is similarly ineffective, it can turn white to grey but apparently does not reconstruct and recover details.

Overexposed photo with -0.74 stops exposure compensation, detail at 50% size. Processed with Raw Therapee. Some highlight details are lost and, yes, white has turned to light-grey.

Compared to Canon Digital Photo Professional

I have also processed the same images, with the same exposure corrections and white balance, in Canon Digital Photo Professional (DPP) version 3.10.1.0. It appears that Canon DPP applies negative exposure in the working RGB space. This strategy avoids colour artefacts, but some highlight details are lost.

Satisfactory exposure. Processed with Canon DPP, detail at 50% size. White balance the same as RT examples.

Underexposed photo with +0.68 stops brightness adjustment, detail at 50% size. Processed with Canon DPP.

Overexposed photo with -0.74 stops brightness adjustment, detail at 50% size. Processed with Canon DPP. Some highlight details are lost and white has turned to light-grey.

Recommendations

Exposure corrections applied to the raw photo RGB data are nearly equivalent to changing exposure in the camera. Just beware of false highlight colours when decreasing raw exposure in RT and attempt highlight reconstruction if required.

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