Sunday, August 19, 2012

Dark side of ETTR

Three weeks back I was photographing a show of noctilucent clouds. Since there was enough light, I exposed the images to the right (or ETTR for short) to gain better signal-to-noise ratio. As exposure increases, the read noise of the camera can be regarded as a constant when exposures are short, hence better SNR. Images will have less noise, gradations will be smoother and colors better. ETTR works best when dealing with raw image files.

However, there were some unexpected consequences because of ETTR: as I brought the exposure down in Raw Therapee, I noted that there was visible posterization in the gradients on the sky. Paradoxically this artifact is caused by too good image quality, the steps between the tones in 8 bit image are so far apart that they become visible.

ETTR banding

As mentioned the problem only exists for 8 bit images displayed on 8 bit displays. Cheapest TN panels are usually 6 bit and they dither everything displayed. Dithering is the act of adding a small amount of noise to salient parts of the image to mask the posterization. Human vision likes details even if they're not real.

Above image has taken an adjustment of three stops in exposure, the posterization/banding is clearly visible in the sky part of the image. Below is an untouched exposure, here the noise from the camera acts as a dither, helping to hide the posterization. However, the image quality is clearly inferior due to the worse signal to noise ratio.

ETTR banding, alivalotettu kuva

From further testing I was able to rule out lossy NEF compression as the reason to this artifact, as both losslessly and lossy compressed raw files showed the same amount of banding (the lossy compression in the NEF files works by cutting some tones from the highlights as shown here).

Hopefully Raw Therapee implements some form of dithering for 8 bit images. I was unable to test other raw converters, but I assume the results will be similar - there would be too many people complaining about small amount of added noise because they wouldn't recognize it as a good thing.

This post was originally posted in Pallopanoraamablogi as "ETTR:n varjopuolet" at 2012-07-12.

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