Flags in the mist

When do we need Long Exposure Noise Reduction?

Roger Clark had some comments about Canon cameras and Long Exposure Noise Reduction. He basically wonders if we really need it most of the time. And he suggests that you can just take a series of separate dark frame exposures at different temperatures and just keep those to manually subtract from the image in Photoshop. The noise map apparently doesn’t change very quickly.

Why do we even think about using Long Exposure Noise Reduction (LENR) or as it is more generally known dark frame subtraction? Sensors heat up as they are used, if we are talking a typical short exposure, maybe 1/100th of a second, the sensor doesn’t heat up very much. But once the exposure lengths get into the multi-minute range that sensor will get hotter. And a hot sensor is a noisy one.

Astrophotographers actually cool their sensors down, sometimes to very low levels–hundreds of degrees below zero–to hold down noise. That option isn’t open to us. But taking a dark frame image essentially maps the noise generated by the sensor and you can have the camera automatically subtract that from the image. Hopefully this reduces the overall noise in the image. The dark frame file, captured separately by shooting a shot with the lens cap on, can be subtracted later in post production.

The problem with letting the camera do the subtraction, is that it doubles the duration of exposures and seems to me it may heat up the sensor even more since you are keeping it on twice as long. That also uses up the battery faster.

Doing some exploration on the web, and this pertains to Canon DSLRs mainly, there is some info on the matter. Canon suggests on their European site:

The long exposure noise reduction performs a dark field subtraction – in other words, it calculates the fixed pattern noise and then removes it from the image you have just shot. In practice, it’s not worth turning on unless you are planning on taking images with an exposure time of more than 2 to 3 minutes.

Andy Frazier a well known night photographer and Nocturnes member suggests an even longer duration, 5 or 6 minutes. And DPReview has reported no benefits on the Canon 40D with or without LENR for 30 second exposures. Joe Reifer has some tests on the Canon 5D where he suggests you have a problems around 8 minutes or so. And that noise becomes a problem on older Nikons around 3 minutes. But the 5D is older technology now, superseded by lower noise sensors and Nikons too have better sensors. Joe Reifer has some comments about the Nikon D200 here.

So from all this I’d say that we probably don’t need to worry too much about sensor noise on shots less than 4 or 5 minutes on newer Canons and probably something like that on newer Nikons. And possibly for even longer exposures. This would depend on the age of your camera, older models could be noisier. If you need to know where the break is you should do some tests on your own system. Any of you using other brands may want to think about using Google to see what noise characteristics your model is.

Not using LENR will certainly speed up operations at night and let me capture twice as many shots.