Long Exposure Noise Reduction

Many cameras have a “Long Exposure Noise Reduction” feature hidden in their settings. As the name implies, it plays a role during long exposure times. We take a look at this feature and how it works.

What is it?

If your camera has the “Long Exposure Noise Reduction” feature , you have may have wondered how it works. Some cameras, like Canon’s 1-series, the 5D and the 30D, have an Automatic setting as a default, so your picture may be benefitting from this without you even knowing it.

Many photographers however, knowing about this feature, may prefer to control its use and will toggle the “On” or “Off” setting accordingly. In the Canon models, this feature is found under the Custom Functions, Number 4 in this case (C.Fn-4). In the 30D’s case, the function only applies to exposures of 1 second or longer, hence its name, LONG Exposure Noise Reduction.

How does it work? When you switch it on, you will most likely notice one particular thing, the exposures will take twice as long! Perhaps, marginal at 1 or 2 seconds, but try an exposure of 25 or 30 seconds and you’ll be waiting for quite a time before the camera is ready to shoot again.
What is going on?!

Well, the Noise Reduction funtion is using a technique called “Dark Frame Subtraction” which works pretty much as the name may suggest.

For short exposures the noise of a digital camera’s sensor is acceptable, but for longer exposures, the noise gets amplified along with the subject of the image. The longer the exposure, the more noise.

However, it’s generally true that sensors are noisy in predictable ways in the sense that some areas of the sensor are usually noisier than others. This means that if you were to know WHERE that noise is, you can compensate for it. And the way to do that is: Dark Frame Subtraction. What happens when this custom function is set, is as follows:
You take your normal picture, and the camera then takes another, a “dark frame” of equal length and exposure settings (which is why the time doubles when you use this feature). The idea of the dark frame is to gather the noise without any subject matter. The camera then does a “subtraction” between the two, and voila, the noise is reduced, if not eliminated. This happens in camera, BEFORE the image is written to the card, hence it also applies to RAW images, not just to JPEG as some people may think. The results can be stunning.
Give it a try. Happy shooting.

By the way, if your camera does not support this function, it is possible to do it yourself, using an application such as Photoshop. All you need to do, after you’ve taken your long exposure, it to put the lens cap on and take another. Do not change any of the camera settings. This will give you that second, dark frame. Now you can do the extraction in Photoshop.

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