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Noise reduction: Overexpose, underdevelop

Have you heard this formula? If you first camera was Kodak Instamatic you probably have. If you first camera was digital you probably haven't.

Overexpose, underdevelop is an older (though still widely used in practice) way of shooting. In short it means that one should give more exposure to film then it should be for given ISO and luminance of the scene. What the overexposure gives us is detail preservation in shadows. Isn't it great?! Well, not exactly. Because overexposure blows out highlights. To preserve highlights one needs to underdevelop. Thus this approach allows better detail preservation/grain reduction in shadows. You may dig Internet for better details why it happens. In shot, the approach uses non-linearity of sensitivity curve of film and developing process.

Can it be used with digital cameras?

No. It should be used with digital cameras, particularly if you shoot RAW, although JPEG shooter will also benefit. In the digital world, it sometimes called exposing to the right (moving histogram to the right).

In the digital world, one needs such an exposure that a histogram of the captured image just started clipping on the right side. Highlight warning mode is at a great help here to ensure that nothing significant is clipped. This is an overexposure step.

As well as in the film world, the overexposed image need to be underdeveloped in a camera raw processing software to bring washed out image to an appearance of normally exposed image. Generally, it means you need to move an exposure slider in an RAW processing application toward the left.

And the result? Yes, it will be exactly the same - detail preservation in shadows and noise reduction. Yes, noise reduction. And yes, not only in shadows but also on saturated colors, like blue skies and red sunsets. Yes, you're absolutely right it will be full frame noise reduction, just because during the capture and development noisy bits of the image are suppressed.

And yes, it's absolutely free of charge and nobody yet patented it.

Unfortunately my images which I could use as samples were rejected due to excessive noise. But fortunately it's not very difficult to see the advantages of the technique. Just go out and shoot blue skies or if you're missing ones then look for deeper shadows and smooth darker tones.

This two images were overexposed during capture and underdeveloped during RAW processing stage. It helps reduce noise on darker part of the tea cup (right image). I did not have much of latitude due to specifics of light so I overexpose about 2/3 stop. The left image was shot with about 2 stop overexposure since lighting was even and contrast was low. Though blades of windturbines lost their details but it was not that important for this particular shot. No post-processing except exposure adjustment, sharpening was applied for both images.

How much to overexpose? As you may see above - it depends on particular lighting conditions and also on your camera. So turn on you highlight warning and histogram and experiment. On Canon 350D I have about 1/2 so called headroom. That's mean that if an image histogram has reached the right side, I still can add about 1/2 stop without loosing details in highlights. The headroom vary from camera to camera and also depends on light source. Typically cameras optimized of day light and departure from this type of light reduces camera's headroom.

JPEG shooters can use the same technique but they should be more conservative on the overexposure side, since they don't have much of headroom.

Photo credits: Actionwatcher.

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September 20, 2007


Interesting read. Thank you.

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