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One of the major
difference between a consumer digital camera and a digital Single Lens Reflex
(DSLR) is that the former produces images with a lot of noise when using high
ISOs and long exposure times, and the latter is practically noise-free (though high ISO performance varies depending on camera manufacturer and model). Noise
is apparent by the presence of color speckles where there should be none. For
example, instead of a blue sky, you notice faint pink, purple and other color
speckles amongst the otherwise blue sky.
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Just what is noise and how can we eliminate or minimize it? This article tries
to explain all of this in as a non-technical way as possible. [This also means
that the explanations may not be 100% technically correct. If you spot any mistake,
please email me. Thanks.]
Digital Image
When we hear 'digital' we automatically tend to think of high quality. Digital
sound does not degrade no matter how many times you play it. Digital images
can be saved forever and will still print in its pristine form.
But the image in a digital camera is sourced from a non-digital component:
the CCD or CMOS image sensor. Understanding how light striking the image sensor
is converted into digital form makes clear what noise is and why it is present.
From Analog To Digital
An image sensor is typically comprised of a matrix of light sensors. A light
sensor can be thought of as simply a device that converts light into an electric
charge.
Each square of the image sensor matrix is a photosite, usually with one light
sensor 'painted' on it. A photosite generally corresponds to one pixel in your
digital image.
When light (photons) strike the image sensor, electrons are produced. These
"photoelectrons" give rise to analog signals which are then converted
into digital pixels by an Analog to Digital (A/D) Converter.
Causes of Noise
There are a number of sources of noise contamination.
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.
Another type of noise is more akin to the 'grain' obtained by using a high
ISO film. When we use a higher ISO, we are amplifying the signal we receive
from the light photons. Unfortunately, as we amplify the signal, we also amplify
the background electrical noise that is present in any electrical system.
In low light, there is not enough light for a proper exposure and the longer
we allow the image sensor to collect the weak signal, the more background electrical
noise it also collects. In this case the background electrical noise may be
higher than the signal.
So why is using a larger image sensor better?
Each photosite itself generates electrical noise that can contaminate its neighbor.
In a larger image sensor, the photosites can be physically further apart and
thus be less affected by that contamination.
A larger image sensor also means that the photosite can be larger, thus have
a larger light gathering capacity. It is therefore able to generate a larger
signal to noise ratio.
That is why a digital camera with 6 million pixels crammed into a 1/1.8 in.
image sensor has more noise (especially at high ISOs) than a 6MP digital camera
using the much larger half-frame (APS-sized) image sensor.
In-Camera Reduction Of Noise
Camera manufacturers have therefore incorporated in their firmware noise reduction
algorithms that kick in when a slow shutter speed and/or high ISO is used to
try to reduce the noise. Depending on the quality of the algorithms, these work
only to a certain extent: they do not completely remove all noise and the smoothening
effect of noise reduction is frequently accomplished at the expense of fine
image detail.
Noise Reduction Software
There is a number of image editing software that can be used to reduce noise
in a digital image after you have taken them. Your image editing software may
already have such an action, or you may download one free from the Internet.
The better noise reduction software applications (such as NeatImage,
Noise Ninja [review]
and NoiseWare Pro) can
take a long time to process one image and so may not really be a viable solution
if you have lots of pictures with noise. They have their place though in a photographer's
toolbox and for that one photograph that you have to take with noise or else
miss an incredible shot, these software applications are your perfect noise
reduction tools. In fact, no photographer should be without one.
Why Are DSLRs Practically Noise-Free?
If it has not yet occurred to you to ask it, you should. Really, why are DSLR
images almost noise-free? The answer is simple: a larger image sensor!
See, with a larger image sensor, each pixel can be larger and each photosite
can be a bit further away from its neighbor (of course, there is an optimum
distance beyond which we'll have 'gaps'). This extra distance is often enough
to prevent signal leakage from one photosite onto another -- hence much less
to almost no noise!
You will find that the high ISO performance of a DSLR varies. Entry-level DSLRs are practically noise-free up to ISO 400. Better models can capture noise-free images up to ISO 1600. ISO 3200 is a stretch, even for the top line models. Turning noise reduction (NR) ON will help eliminate noise but this can happen at the expense of losing fine image detail. So far, the traditional camera manufacturers, such as Nikon and Canon, build DSLRs with better high ISO performance. Olympus, Panasonic, Pentax and Sony DSLRs seem to struggle with ISOs higher than 400. But that can change anytime.
Though the general statement that "DSLRs are practically noise-free" is true, readers should bear in mind that we have had new non-traditional camera manufacturers that have joined the DSLR bandwagon recently, and some of these relatively newer camera manufacturers do not seem to understand the importance of low noise at high ISOs and their sensors start producing noise even at low ISOs. Therefore, it pays to read the reviews carefully.
Hurrah for Bigger Image Sensors!
Why therefore do camera manufacturers not use the bigger image sensors in consumer
digital cameras? A bigger image sensor means the need for a bigger lens. Unlike
film that can capture light incident on it at an angle, an image sensor requires
that light falls on it straight on. Bigger lens add costs, need a bigger body,
etc. etc. You get the idea. You quickly end up with a camera body the size of
a... DSLR.
The biggest
image sensor on a prosumer digital camera is 2/3 in. sized at 8.8 x 6.6 mm (though
most of them now use an improved 1/1.8 in. type). We wait for the day when an
APS-sized image sensor is used in a prosumer model!
The next size
down is 1/1.8 in. (sized at 7.2 x 5.3 mm) and is prevalent in most of the 5MP,
6MP and 7MP consumer digital cameras today.
Notice how camera manufacturers have 'squeezed' more megapixels into the same
1/1.8 in. image sensor. That is one reason some people say that a digital camera
at a lower megapixels resolution gives images that are more noise-free than
one at a higher megapixels resolution -- on the same size image sensor. More
megapixels on the same sized image sensor means the pixels are closer together
-- thus more noise. Of course, better in-camera noise reduction algorithms in
the newer digital cameras can counter this tendency toward more noise to a certain
extent. Photographers must balance the advantage of higher megapixels versus
more noise (albeit reduced with the in-camera noise reduction algorithms), although
camera manufacturers leave us with few choices as they all move to the higher
megapixels image sensor to compete with one another.
The dSLRs
have image sensors that are much larger than 2/3 in. Some dSLRs have an APS-sized
(or, 'half-frame', approx. 23.7 x 15.6 mm) image sensor.
When
we talk about a 'full-frame' image sensor, it is in relation to a 35mm film
and is therefore sized at 36 x 24 mm. Compare these with the 2/3 in. image sensors
in prosumer digital cameras sized at 8.8 x 6.6 mm, and you'd agree that the
size difference is indeed substantial. No wonder dSLRs produce practically noise-free
images.
Are we ever going to see bigger image sensors in prosumer digital cameras?
Bigger sensors mean bigger lenses mean more expensive cameras. So that is why
most of the work being done now is focused more on improving the small image
sensors and writing better noise reduction algorithms. However, we believe it
is inevitable for the APS-sized image sensor to eventually move down to consumer
digital cameras, starting with the prosumer models.
What Can You Do?
There are a number of things to remember about noise:
A slow or long exposure introduces noise.
A higher ISO introduces noise.
Noise Reduction Software will clean up the noise in some images, and sometimes
it's done well enough that you can't really tell the original image had unacceptable
noise level in it.
If you must have the option of using high ISOs (e.g. to capture fast
action in low-light situations), then get a DSLR. Since many amateur photographers
purchase 35mm SLR cameras today (especially since they are relatively cheap),
I don't see why they should not be able to use a DSLR as easily. The only
roadblock to owning a DSLR for many today is its high price, though even that
is steadily coming down as competition heats up among camera manufacturers.
We can now obtain a DSLR for under $1000, including the lens.
Noise is a fact of life in consumer and prosumer digital cameras, and is going
to stay with us for some time longer until camera manufacturers engineer better
and small noise-free image sensors. Until then, what can you do to reduce the
amount of noise in your digital images?
Avoid slow or long exposures.
Avoid using high ISOs.
When comparing digital cameras, all things being equal, favor the one with
the larger image sensor. For
example, Camera A may be 3MP using a 1/2.7 in. image sensor (sized at 5.3
x 4.0 mm), while Camera B may also be 3MP but uses a larger 1/1.8 in. image
sensor. Camera B will probably produce images that have less noise than Camera
A.
Purchase a noise reduction software. This will allow you to take pictures
and not miss some great shots. You'll find that a good noise reduction software
usually does a pretty good job of 'cleaning' up the noise to an acceptable
level in most of your noisy images.
Here is a comparative illustration of the approximate sizes of
the currently most popular image sensors:
Image Sensor
Size (approx.)
full frame
36 x 24 mm
half frame (APS)
24 x 15 mm
2/3 in.
8.8 x 6.6 mm
1/1.8 in.
7.2 x 5.3 mm
1/2.7 in.
5.3 x 4.0 mm
Addendum:
Lately there have been many "experts" that have come onto the forum boards to prove that more MP does not equate to more noise in tiny sensors and, as you would expect, they have the equations and formulas to prove it. Some contend that it is the limitations of our monitors or other technology that make it seem that way. Scientific facts are what they are. However, they miss the point. When we say that more MP in a tiny sensor results in noisier images, we are not making a scientific claim here; we are simply reporting our visual experience using the limited monitors and technology at our disposal -- and that is what matters.
Our Readers
Write Back
We don't pretend to be experts in all aspects of digital photography and
therefore are very happy to learn together with our readers. Their feedback
to this article are published below.
From: Leo P Purcell
In Aug/Sep [2005] I drove 4800 miles in western US and
Canada with my new Panasonic FZ20. It took me quite a few days to realise that
carrying the camera in the trunk in the [hot] Nevada temperatures was making
a major contribution to the noise [level of the images]. Carrying the camera
in the air-conditioned car interior made a significant improvement.
From: Darren Krakowski
In regards to the "What Is...Noise?" article,
in which you requested additional info, I found it only really lacking in one
respect, and that is with regard to noise as a function of ISO. The article
eludes to the fact that noise increases as ISO increases, but does not do much
to explain why. As an electrical engineer, I’d like to pitch in my two
cents.
Varying the ISO in a digital camera is like turning up
the volume on a stereo. You increase the “gain” to get more of something.
In a stereo, increasing the gain amplifies the audio signal, obviously resulting
in greater volume. On a digital camera, increasing the gain amplifies the available
light, meaning the light needs to be collected for a shorter period of time.
(Of course, taking aperture into account, the shutter speed could be the same
and a smaller aperture used, but the amplifying effect / gain will need to be
the same, since if the aperture and shutter are changed in step with each other,
the light hitting the sensor remains constant.)
With any electrical system, increasing the gain has a
byproduct of inducing noise into the system. In an audio system, increased gain
results in an increased “signal to noise ratio,” meaning that the
primary audio signal increased in amplitude, but so did any background noise
(typically in terms of a hiss or slightly audible static). Worse in audio is
total harmonic distortion, which is clearly audible when an audio amplifier
is turned up too loud and you can hear the warble, or you just notice that it
starts to sound bad.
Noise at high ISOs is similar to both of these audio
counterparts. Signal to noise is most easily correlated. As the gain is increased
for a photosite, both the signal and the noise (present in any electrical system)
is amplified. The noise at the circuit level should be fairly constant regardless
of the ISO, but what has changed is the amplification. If the noise is constant
but the light level is weaker, and the amplification is increased to compensate
for the weaker light level, the noise is inherently amplified as well. When
someone figures out how to amplify the primary signal and not the noise, it
will apply to digital cameras, stereos, TV, video recorders, cell phones, you
name it, and they will become very rich.
So how does this relate to a digital SLR having lower
noise than a pocket sized digicam? As you correctly state, the photosites are
larger and further apart. This has a two-fold effect. First, each photosite
is further away from its neighbor, and the neighbor is a large part of the electrical
noise generated at the circuit level. Second, a larger photosite will generate
a larger signal with respect to the noise, since due to its size it inherently
has a larger light gathering capacity, and thus the signal to noise ratio is
greater. So, compared to a pocket sized digital camera (and to use fictitious
numbers for the sake of clarity), the noise produced by the smaller digital
camera’s circuit might be measured at 0.1 volts. The signal produced by
the circuit at the time of exposure might be 1 volt, so the signal to noise
ratio is 10:1. On a digital SLR, there is less noise at the circuit level due
to the increased size and distance between photosites, so let’s say it
is measured at 0.05. Also, the sensor can collect more light so its output might
be 2 volts, producing a signal to noise ratio of 40:1, and hence a cleaner image.
(This is an oversimplified explanation and the numbers are bogus, but it hopefully
serves to illustrate a point.)
This also explains why low light images have a higher
noise content. The signal is smaller as there is less light to gather, but the
noise at the circuit level remains fairly constant. The signal is then amplified
heavily to produce the image from a low light situation, the electrical noise
is amplified in turn, and voila, you have a noisy final product.
Finally, remember that digital imaging noise is much
like static, and thus somewhat random in nature. The noise in the above example
is not a steady 0.1 or 0.05 volts, or we would easily be able to remove it.
It constantly fluctuates slightly and unpredictably, and thus the need for complex
noise reduction algorithms like Neat Image, which attempt to characterize the
noise to remove it.
From: Joerg Colberg
Regarding your "What is noise?" article I'm
not sure I buy the explanations on why digital imaging cells produce noise.
I used to work in digital image processing (I actually processed Hubble Space
Telescope images before it got its "glasses", right after it went
into orbit) about 10 years ago, and we did lots of tests of what we called "dark
currents". Basically, we tested the behaviour of CCD chips when no signal
was coming in from a light source. There are various physical sources for the
noise (incl. temperature and quantum effects) all of which are then amplified
depending on which ISO you choose - as Darren pointed out. I am not quite convinced
about the "leaking" theory (even though I might be wrong, of course)
because if leakage caused noise you'd expect it to be very uniform, wouldn't
you? In other words, cells would leak in all directions and the effect wouldn't
really generate noise but actually de-sharpen the image.
The chip sizes, however, do affect noise quite a bit because if you have a small
chip the effective size of the pixels is much smaller and this introduces larger
sampling errors - noise. In a sense, the chip size translates into noise in
a very similar fashion as film negative sizes translate into noise aka grain.
You can't expect the same resolution from a 6x6 and a Minox negative.
From: Esquid (Nov 25, 2003)
In the responses to the "what is noise" article,
Joerg Colberg expresses doubt about "current leakage" being a source
of noise, and the original article also states that extra distance between photosites
prevents signal leakage.
Actually, the "leakage" is a fundamental problem
with semiconductor memories - which the CCD of the camera was historically based
upon. (DRAM) memory in a computer must be "refreshed" - rewritten
- regularly to prevent it from eventually going blank. Local variations in the
manufacturing process of the CCD means that each photosite will have variations
in
leakage current, and the leakage changes with temperature also. Some photosites
may be so bad they are "hot" pixels and are actually mapped out of
the image even in a normal exposure.
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You
are here: 
The biggest
image sensor on a prosumer digital camera is 2/3 in. sized at 8.8 x 6.6 mm (though
most of them now use an improved 1/1.8 in. type). We wait for the day when an
APS-sized image sensor is used in a prosumer model!
The next size
down is 1/1.8 in. (sized at 7.2 x 5.3 mm) and is prevalent in most of the 5MP,
6MP and 7MP consumer digital cameras today.
The dSLRs
have image sensors that are much larger than 2/3 in. Some dSLRs have an APS-sized
(or, 'half-frame', approx. 23.7 x 15.6 mm) image sensor.
When
we talk about a 'full-frame' image sensor, it is in relation to a 35mm film
and is therefore sized at 36 x 24 mm. Compare these with the 2/3 in. image sensors
in prosumer digital cameras sized at 8.8 x 6.6 mm, and you'd agree that the
size difference is indeed substantial. No wonder dSLRs produce practically noise-free
images.
For
example, Camera A may be 3MP using a 1/2.7 in. image sensor (sized at 5.3
x 4.0 mm), while Camera B may also be 3MP but uses a larger 1/1.8 in. image
sensor. Camera B will probably produce images that have less noise than Camera
A.