In camera’s Long Exposure Noise Reduction – Good or Bad?

In this episode, I answer of your questions:

1. What is the Lon Exposure Noise Reduction feature modern DSL offer built-in?
2. Is it worth activating it?

In our previous episode, we reviewed the In camera’s High ISO Noise Reduction feature. The two features are completely different as there are 2 types of noise:

1. Standard/Random Noise which is the result of High ISO and can be treated by Noise Reduction tools such as found in Adobe Lightroom, Photoshop, DXO, DPP etc.
2. Thermal Noise which si the result of a long exposure and is not treated by the standard Noise Reduction tool feature in the software mentioned above.

The Noise we are covering today is the Thermal Noise.

What is the Thermal Noise?

To understand Thermal Noise, let me remind you how the DSLR image sensor works:

I had vulgarised this in the episode Raw vs Jpeg vs Tiff but as a quick recap, an image sensor consists of a matrix of light sensors which convert light into an electric charge. When light (photons) strike the image sensor, electrons are produced. These “photoelectrons” give rise to analogue signals which are then converted into digital pixels by an Analog to Digital (A/D) Converter.

When taking a long exposure (several second or minutes) the image sensor heats up and even more so if one uses a high ISO. The heat generated might free electrons from the image sensor itself, thus contaminating the “true” photoelectrons. These “thermal electrons” give rise to a form of noise called thermal noise or dark current.

The Thermal Noise appears in your photograph as 1 or multiple pixels (usually not touching) with very bright colour (red, Blue, Green or even purple). This is also known as Hot Pixel(s).

Full Frame & Thermal noise

I must admit I had not mentioned the sensor size in the previous episode In Camera’s High ISO Speed Noise Reduction – Good or Bad? and I should have. I suspect that by know if you read this article you are familiar with the concept of full frame and cropped sensor. If not I invite you to check this episode: How to choose your first DSLR

At number of pixel equivalent, a full frame will always be better than a cropped sensor camera because of the size or each Photosite (light sensor) and the space between them. That is the reason why Canon has always refused to follow Nikon in its race to high number of pixels sensor (except with the upcoming Canon 5DS and 5DSR). With regard to Thermal Noise, the space between each Photosite is an advantage against heat and dark current. Therefore Full Frame cameras are less subject to Thermal Noise.

Not confusing Hot Pixel, dead pixel and Stuck pixel

We often read or hear people using each expression interchangeably (maybe due to their appearance) when in fact there refer to 3 different state of a pixel.

Dead Pixel

A Dead Pixel is literally a Dead Pixel. It is physically a Pixel, aka a Photosite (one square on the image sensor’s matrix of light sensors) that is no longer receiving any current. It would appear black or often darker than the adjacent Pixel. A Dead Pixel as its name implies means it is damaged forever. One can find Dead Pixels in the LCD screen.

Stuck Pixel

Unlike Dead Pixels, stuck pixels always receive power, which results in a pixel that can be red, green, blue or any combination of these colours. Unlike Dead Pixels, Stuck Pixels do not change their colour from picture to picture. Stuck Pixels are very common, but not permanent like Dead Pixels – they might disappear over time. One can find Stuck Pixels in the LCD screen.

Hot Pixel

Unlike Stuck Pixels, Hot Pixels only show up when the camera sensor gets hot during long exposures or when the ISO is cranked up. Hot Pixels are very normal and they will show up even on brand new cameras, although manufacturers do their best to map Hot Pixels out during the quality testing process. Hot pixels will appear and disappear over time. Suffice to say that Hot pixels do not occur in LCD screens.

The 3 options for LENR

When selecting LENR in the menu one has 3 options to choose from:

1. Disabled: self-explicit.
2. Auto: Some kind of smart mode where the camera will figure out whether the LENR is worth running. Usually for exposure of 1 sec or longer.
3. Enable: Is activated each time the exposure is of 1sec or longer

The Dark Frame Subtraction

The first thing one notices when taking a >= 1sec exposure with LENR enabled is the camera becomes unavailable for twice the exposure time. If you shoot for 5 sec the camera will only let you shoot the next exposure after 10 sec from the moment you pressed the shutter. Why? This is caused by the LENR mechanism. As mentioned above, the Hot Pixels are a constant phenomenon when the exposure condition is unchanged. Meaning that if you take 3 identical long exposure exposures (only the setting matter and not the subject), captured in the same ambient temperature, the resulting photographs will have the same Hot Pixel aka same Thermal Noise.Therefore one can say they are somewhat identifiable. What the camera actually does when LENR is enabled, is taking the exposure you desire followed by a second exposure with the image sensor unexposed to any light (the mirror remains down). The first exposure (the one you wanted to take) is called the Light Frame and the second exposure that is captured by the camera is called the Dark Frame. Once the 2 have been captured, the camera performs what is called a Dark Frame Subtraction where the Hot Pixels of your photograph are removed by subtracting the Dark Frame from your photograph. See new episode about this Dark Frame Subtraction process.

The Test Case (find the test files converted to JPG )

In order to answer the initial question as to whether one should use the Long Exposure Noise Reduction feature found in the DSLR, one should run a test to see the effect of the NR. Here I have decided to use my Canon 60D to better illustrate the phenomenon. The test is simple and consists of taking two exposures (one with the LENR enabled and one without ) with the lens cap on with a 30sec exposure combined with a high ISO to increase the heat factor (as mentioned above). I decided to open the 2 exposures in Canon Digital Photo Professional (Canon’s photo editing software). To better see the Hot Pixels, I decided to crank up the exposure slider.

This is the photo with the LENR turned off

As one can see there are multiple single pixels that are brighter than the others (black since we shot the black back of the lens cap with no light entering the lens).

This is the photo with the LENR turned on

Those single Pixels have now almost disappeared and would lead to the conclusion that the LENR is a great feature. But is just another thing that only works because we are in Canon’s photo editing software DPP? Does LENR affect RAW?

RAW and LENR

To test, one only needs to open the same CR2 files from our test above in Adobe Lightroom. The first thing one can notice is the Hot Pixel are not as visible as in DPP. Therefore to better see them I am increasing the exposure by 5 and darken the shadow and black down to 0. Here again, we can see some of the Hot Pixels seen in the photo with LENR disabled have now disappeared. We can conclude that LENR does affect RAW; even when being interpreted by a 3rd party software such as Lightroom, Photoshop and DXO unlike the High ISO Noise Reduction we review in the previous episode (In Camera’s High ISO Speed Noise Reduction – Good or Bad? ).

Something worth noticing

It is interesting to see what happens when zooming at 200% and compare the effect of LENR more closely. Interestingly the photo with LENR enabled shows a huge amount of Standard/Random noise compared to the photo with LENR disabled.

Photo with LENR disabled and zoomed at 200%

Photo with LENR enabled and zoomed at 200%

Conclusion – Should we use LENR?

There is rarely a straight yes-no answer in Photography. “It depends” is the right answer!

Here it depends on what we are trying to capture. Given the LENR increases the time one has to wait before the next shot, it means this feature is not suitable when:

1. Shooting star trails or any other situation where we need the shortest interval possible between shots.
2. Shooting time lapses, unless our timer system happens to work with LENR enabled and the interval between shots is longer than the shutter speed.
3. We don’t have a lot of time and need to get as many shots as quickly as we can.
4. The only portion of the photo you’ll be using is a bright light source (as when shooting multiple exposures of car light streaks for layer stacking using the Lighten blend mode) since Thermal Noise/Hot Pixels are less visible in the bright zones of the frame.

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