Long Exposure Noise Reduction
There are two types of noise reduction. One is designed for photographing at very high ISO’s, the other when making very long exposures.
Long Exposure Noise
The shutter speed at which noise reduction is applied likely varies from camera to camera. But, for the sake of this discussion, let’s say it kicks in at an exposure time of ten seconds.
Hot Pixels and Noise
During very long exposure times some pixels in your camera’s sensor can become quite hot.
My understanding is that it’s the same pixels under the same conditions (i.e., ambient temp, exposure time, etc.) that will be effected in this way.
These hot pixels show up as a kind of artifact, often like a red dot, similar to noise in an image.
To counter this phenomena cameras, when long exposure noise reduction is switched on, make two separate exposures: one of the scene in front of you; the other, at the same exposure time, of a dark frame.
In other words the shutter stays closed during the second exposure so no light actually enters the camera.
The dark exposure that results produces no image to speak of, but it does have the same hot pixels and resulting noise as the original exposure.
The camera then looks at what’s similar about the two images (i.e., noise) and does its best to remove that noise from the original image.
If all goes well the end of the process will reveal a relative clean (i.e., noise free) single image.
This is incredible technology, but it comes at a cost.
A one second exposure now, effectively, takes two seconds
A one minute exposure takes two minutes
A one hour exposure will mean that it’s two hours before you’re able to use your camera again. Assuming, that is, your camera’s battery hasn’t gone flat in the process.
What’s The Best Way Forward?
What’s the best way forward?
It depends upon the individual photographer, their workflow and the camera in question.
Be prepared to establish a routine and, after working with it for some time, to change it based upon experience gained under a variety of circumstances.
Be prepared to fail as failure is the best teacher.
For example, with exposures of 10 seconds or less I think it’s fine to leave long exposure noise reduction set to AUTO.
This is particularly the case when photographing in JPEG mode, as there’s an implied assumption that you’re unlikely to want to process the image on the desktop.
The above image was made at dawn at Milford Sound on the south island of New Zealand. The exposure required was 105 seconds at an aperture of f/11 at ISO 100 on my Sony A7Rii camera.
I chose a long exposure as a way of smoothing out the textures on the surface of the water.
I was fortunate that it was a very still morning, otherwise the grasses in the foreground would have moved during the long exposure.
I feel the final result explores the dualities of texture and smoothness, drama and tranquility quite well.
Noise Reduction and Sharpness
I’d be wary of switching your camera’s long exposure noise reduction to ON as that may apply noise reduction to all your images, even relatively fast exposure times with no noise to speak of.
AUTO is a better option because you’re allowing the camera to determine if and when long exposure noise reduction should be applied. I think that’s a safer option.
Given that noise reduction can result in a loss of sharpness, it should only be applied, whether in camera or on the desktop, when it’s needed.
With exposures of, say, 4 minutes or longer some folks will decide that it’s better to make more photos during a photo session and will switch long exposure noise reduction OFF.
This option is better suited to folks photographing in RAW mode as, by doing so, they’re already committed to post-processing their images on the desktop.
Practice Brings Certainty To Your Photography
I’d recommend tests where you make a series of exposures with and without long exposure noise reduction turned to Auto in camera.
You would need to photograph the type of subject matter (e.g., moving water, night sky, etc.), under the kind of conditions (e.g., time of day, weather), where you’re mostly likely to be employing significantly long exposures.
Folks working in RAW mode will need to bring their images into their software application of choice (e.g., Lightroom) to determine if they’re able to approximate or improve on the noise reduction applied in camera to other images from the same session.
If you regularly make prints you should also print both versions of the image to help you determine which workflow works best for you.
Tests can be a bit of a hassle, but they pay off in the long run. And don’t worry if, at first, you get it wrong.
It’s a digital camera which means that, unlike film, there’s no financial cost associated with trying.
Pain Is Often The Best Teacher
It’s your choice whether you decide to experience pain associated with the production of poorly conducted test images or, for example, after messing up photographs made of Mount Everest under a clear, night sky.
I made it to base camp at Mount Everest, on the Tibetan side, in 1988 and I was extremely fortunate to photographed the mountain, at night, under a clear sky.
Believe me, at that altitude, you’ll be so focused upon loss of energy, incoherent thoughts and an inability to get sufficient air into your lungs that the last thing you’ll want to have to think about is how best to use your camera.
Thus the need for tests beforehand.
If you'd like help with your photography, through a specially designed one-to-one private photography session, feel free to contact me directly.