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Digital Image Sensors -
Megapixels and Quality


 
Digital Image Sensors - Megapixels and Quality

As each new camera is introduced, it seems the number of megapixels increases. In the early days of digital photography, a 1 megapixel camera was extraordinary! But now, we're routinely seeing 24 (and higher) megapixel cameras. Phone cameras are now up to 41 megapixels (Nokia Lumia 1020)! Is this important? Is bigger really better? Hopefully, this article will help explain.


A little History
Digital imaging had its origins in 1961. While working at NASA’s Jet Propulsion Laboratory, Eugene F Lally first digitized images from an array of sensors. The goal was onboard navigation data for NASA. The first digital camera with a sensor was introduced by Fuji in 1988, although it was never marketed. The first commercial digital camera appeared in the early 1990s: the $800 Logitech Fotoman, a 380k pixel (1/3 of a megapixel) camera - for a bit of nostalgia, check out this video .

As you can see, the history of digital photography is only a little over 50 years - but, it has come a long way since in that time!


Sensor Types
There are two common sensor types being used today:
  • CCD (Charge Coupled Device)
  • CMOS (Complimentary Metal Oxide Semiconductor)
CCD sensors require more power to operate than CMOS sensors (about 100 times as much!) and generate a lot more heat. They were used in the early digital cameras because they were more sensitive to light. CMOS sensors have improved so much that they've now pretty much replaced CCD sensors. CMOS sensors use less power (better camera battery life) and don't get as hot as CCD sensors - and heat is a great destoryer of digital images. CMOS sensors are increasingly becoming more popular in consumer and professional digital cameras to the point where it's difficult to find any digital cameras today with a CCD sensor. The ability to shoot movies with DSLRs is due primarily to CMOS sensors.


Picture Elements (Pixels)
At the core of every digital image is the picture element, or as it is abbreviated, the pixel. A pixel is the smallest element in a digital image. Each digital image is made up of a matrix of pixels. The images your digital camera takes are called "Raster Images" - as they are a pattern of closely spaced rows of pixels that form the image. Raster images are more commonly called "Bitmap" images.

Each pixel on a digital camera sensor represents ONE color - either red, green or blue - as well as a brightness value (this is different than the final image, but more on this later).
Pixels



Sensor Sizes
Sensors are semiconductor devices that convert optical images into electrical signals. Inside of your camera or phone, there is a sensor that detects light and provides the data for the image. A relatively firm rule in digital photography is that the bigger the sensor, the higher the quality of the image. This is because if you fit a given number of pixels on a sensor, a bigger sensor permits each pixel to be bigger. Bigger pixels yield better quality.

Here is a relative size comparason of some common sensor sizes - you can see how dramatically different they are. Imagine cramming rows of 6,000 pixels in each of these sizes - you could fit bigger pixels on the larger sensor, but the smaller sensors would require that each pixel be much, much smaller. Thus, you can get HIGHER quality images when the resolution is LOWER, especially on smaller sensors.



Note: Some of the most popular point-and-shoot cameras have the smallest sensor size indicated above.

In the "old" days, camera manufacturers updated their cameras every 3,  4 or 5 years. The technology didn't change very frequently, and it was the FILM that made the big difference in image quality. But with digital cameras, the technology is changing VERY fast, and cameras are often replaced every year. Often, but not always, newer cameras have improved sensors and produce better quality images.



More on Pixels (technical stuff - skip if you like)
Each pixel on the sensor responds to ONE specific color - either red, green or blue. Each pixel records the intensity of that color along with a brightness level (luminosity). There are typically 2 green pixels for each red and blue pixel, which emulates how the human eye sees, as our eyes are more sensitive to green than any other color (See this article). This pattern is called the "Bayer Pattern", and is used in most camera sensors.


This is a sample of what the Bayer Pattern looks like.
Each pixel is sensitive to a specific color and there are
2 green pixels for each red and blue pixel. The sensor's
output has to be interpolated to create the image that
your camera ultimately provides you with.



The pixel data (one color per pixel) must be interpolated using a "demosaicing algorithm" to determine what the color of each pixel should be. Each pixel's color is calculated based on the relative intensity of the red, green and blue channel data from all the neighboring pixels. The final data that is output from your camera will have 3 parameters for each pixel: Red, Green and Blue intensity.

Note: There are several other type of sensors that do not use the Bayer Pattern, but they are not very common - they include the Foveon and 3CCD sensors.

Each pixel can represent both color and brightness (luminosity). The Bits per (Color) Channel determines the total number of colors that can be represented by a pixel. This chart shows how the number of bits per channel affect luminosity and colors:

Bits per Red, Green
& Blue Channel
Total Bit DepthLevels of
Luminosity
Total Number of
Colors Possible
8 bits (JPEG images)24
(8 bits x 3 channels
256 ( 28) 16,777,216
(256 x 256 x 256)
12-bits
(Most RAW images)
36
(12 x 3 channels)
4096 ( 212) 68,719,476,736
(4,096 x 4,096 x 4,096)
14-bits
(Some RAW images)
42
(14 x 3 channels)
16,384 ( 214) 4,398,046,511,104
(16,384 x 16,384 x 16,384)
16-bits
(TIFF images)
48
(16 x 3 channels)
65,536 ( 216) 281,474,976,710,656
(65,536 x 65,536 x 65,536)

Many, but not all digital cameras, allow you to select the number of bits per color channel for RAW images - usually 12 or 14. JPEG images created by the camera are always 8 bits per channel.



Internal Filters
Digital camera sensors are very sensitive to infrared light, which would interfere with the normal photography by confusing the autofocus, softening the image or oversaturating the red channel. Digital cameras employ an optical low-pass filter (OLPF) mounted on top of the sensor to reduce the sensitivity of the sensor to infrared light.

Another filter, the Anti-aliasing filter is also sandwiched with the low-pass filter to help eliminate Moiré fringes.

The image below shows an example of what Moiré looks like.


Example of a Moire Pattern

Recently, some digital cameras have been introduced without the anti-aliasing filter, which helps increase sharpness, at the possible cost of causing Moiré patterns in some images that contain certain patterns.



Quality vs Size
Size matters. Pixel size matters when it comes to quality. Pixel size is measured by the "Pixel Pitch". Pixel pitch is the distance between the center of one pixel and the center of its closest neighbor. Let's compare a few cameras (sorted by pixel pitch) - remember, as a general rule, the larger the pixel pitch (last column), the higher the quality of the images:

CameraResolution in
Megapixels
Sensor Size
(square mm)
Pixel Pitch in micro-meters
(distance of adjacent pixel centers)
Nokia Lumia 1020 (phone)41331.1
Samsung Galaxy S5 (phone)16221.1
Apple iPhone 5/5S (phone)8151.5
Canon Powershot SX50 HS12281.5
Canon Powershot G1616421.8
Canon Powershot G1210422.0
Nikon Coolpix 790012382.3
Nikon 1 J4181162.5
Nikon D7100243663.9
Canon 70D203374.1
Canon 7D183324.3
Canon Rebel T5i/700D183324.3
Nikon D7000163684.7
Nikon D800368624.8
Canon EOS 5D Mark III228646.2
Nikon D4s168607.2
Nikon Df168607.2

Given the same level of technology, the larger the Pixel Pitch, the higher the quality of the image. You can see how the resolution (megapixels) and sensor size combine to determine the pixel pitch. For example, compare the Canon Powershot G12 and G16. Both have the same sensor size, but because the G16 has a higher resolution, the pixel pitch is lower.

Smaller pixel pitch means more noise (random colored pixels) and less sharp images.



Are Megapixels Important??
A higher megapixel count does impact image quality. Rarely do you ever need to use an image at its full size - you would only need to do this if you were making large prints or needed to severely crop. So, why do megapixels matter?

Well, higher resolutions can mean sharper, better defined, and more detailed images. Higher resolutions can capture smaller details in your images. So, higher resolution images can be a good thing. Most of the time, you will be down-sizing (reducing the number of pixels) of your images if you will be emailing them or using them on the web. When down-sized, high-resolution images retain their detail and can make much higher quality images than those taken at lower resolution. So don't discount megapixels completely, but at some point, more megapixels become meaningless. If you're not making large prints or cropping your images a lot, 16 megapixels is probably more than you need.



Sensor Noise
We won't cover Sensor Noise here, as that was covered in another etip (see here). But suffice it to say, that there is a very direct correlation between sensor sizes and noise: with smaller sensors, you can expect significantly more noise - and noise is a quality-killer!



Crop Factor or Focal Length Multiplier
The size of a digital camera's sensor affects how much of an image is captured by your camera. Sensors are usually compared to 35mm film. A sensor that is the same size as 35mm film (36 mm x 24 mm) is called a "full-frame" sensor. This size sensor captures what's considered to be 100% of an scene. Smaller sensors capture less of a scene. This difference is quantified by the "Crop Factor". The following chart shows different sensor sizes and their respective crop factors:




What this means is that with a crop factor of 1.5, you need to multiply your lens focal length by 1.5 to get its effective focal length. Thus, a 50mm lens with a sensor that has a crop factor of 1.5, has an effective focal length of 75mm. That means that a camera with a crop factor of 1.5 only captures 50% of the scene compared to a camera with a full-frame sized sensor.

This illustrates why a smaller sensor captures less image.


So, the sensor size not only makes a difference in image quality, it also affects how much your camera lens "sees".



Dynamic Range
Dynamic range is the measure of how much detail can be captured in the lightest and darkest areas of an image. Low dynamic range causes shadow and highlight areas of an image to lose detail. Larger sensor pixels can capture better shadow detail than smaller sensor pixels, thus, larger pixel pitch provides better dynamic range.

This image was taken with a point-and shoot camera. It has very poor dynamic range. The sky has no detail in it, although it was a sunset with lots of color. The foreground has very little detail, with a lot of muddy-dark areas.





This very-high contrast image was taken with a full-frame DSLR - you can see that there is still detail in the brightest and darkest areas of the image.




Depth of Field (DOF)
Although the lens effective focal length is increased with sensors smaller than full-frame, the depth of field, which is affected by the actual lens focal length, isn't affected by the crop factor. But, to capture the same image area, a different actual focal length is needed, so, when the actual focal length is changed, so is the DOF.

Consider the following:



To capture the same area as a DSLR camera with a full-frame sensor, the point and shoot camera had to shoot with a very short focal length. Although the EFFECTIVE focal lengths are identical in the above two images, the ACTUAL focal lengths are very different: 105mm vs 23mm, thus the DOF is greatly affected. If both cameras shot with an ACTUAL focal length that were the same, there would be no difference in the DOF, although the point-and-shoot camera would have captured MUCH MUCH less of the scene.



In Summary
What all this means to you, is that you should have a basic understanding of the relationship between sensor size, resolution and quality.

The word "Quality" can be very subjective. Some people find that camera-phones have enough quality - while others consider these images to be quite poor. It's a matter of what you plan on doing with your image, and who you plan on showing it to. As a professional photographer, quality is of the utmost importance when coming to providing images to clients. But not everyone needs to provide the best quality.

Here are some benefits and drawbacks to consider when purchasing a digital camera:

 BenefitsDrawbacks
Larger
Sensor
Less noise, especially at higher ISO speedsLarger camera required
Sharper images, especially at smaller lens aperturesLarger lenses required
Better Dynamic Range 
Better low-light performance 
Better overall quality 
More control over Depth of Field 

Smaller
Sensor
Smaller and lighter camerasMore noise in images
Smaller and lighter lenses Poorer dynamic range
More depth of field but less control over DOF as the lenses will have
shorter focal lengths (both a benefit and a drawback)
 Poor low-light performance

More
Megapixels
More details captured in imagesSmaller pixel pitch:
Ability to make better prints       Reduced low-light performance 
Ability to crop without losing a lot of detail       Reduced dynamic range 
        More noise

Less
Megapixels
Larger pixel pitch on larger sensors:Less detailed images
       Better low-light performance Lower quality prints
       Better dynamic range Inability to crop without losing detail
       Less noise 

Higher megapixel counts are only good with larger sensors. You have to look at BOTH the megapixel count and the sensor size to understand what kind of images a camera is capable of.

When purchasing your next camera, you should conciously be looking at these attributes. Hopefully the information in this eTip will provide you with the knowledge to make an informed decision.



Happy shooting!!
 

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