Demystifying Megapixels, Resolution, photo size, photo megab
Demystifying Megapixels, Resolution, photo size, photo megabyte
There are some common words you will see again and again as you delve into digital photography; Megapixel, Resolution and DPI megabyte.
What do each of these mean?
Megapixel basically means one million pixels. A pixel is a small dot that contains a piece of your finished picture, similar to a puzzle piece. When you put each of these pixels together, they form a picture. Like the picture of marigolds above, you can see that the first one is clear. In the second picture, we have shown how it is formed of millions of tiny pixels. Our eyes process an image as a whole, where as a computer and a digital camera break each image up into pixels.
The more megapixels per image, the clearer and sharper it becomes. Digital cameras today range anywhere from 2 megapixels to 20 megapixels for each picture taken.
These settings will affect the quality of your image and the file size of each image.
Resolution refers to the size of the image that your camera will produce. This is normally referred to as megapixels, which we discussed above. You will normally see cameras advertised as having a 1600 x 1200 resolution. To figure out the amount of megapixels that will give you, you multiply the dimensions. For example:
1600 times 1200 = 1.92 million pixels or 1.92 Megapixels. This would commonly be rounded up and referred to as a 2 megapixel camera.
As this amount goes higher, you will notice a much sharper image. We have provided the following examples below. Please look closely at them and you will be able to see the difference each level makes.
1024 x 768 resolution = .78 megapixels
1839 x 1461 resolution = 2.61 megapixels
3200 x 2400 resolution = 7.8 megapixels
DPI - Dots Per Inch
DPI means Dots Per Inch. This is what your printer will recognize when it prints your image. Your computer monitor is configured to view items at PPI, or Pixels Per Inch. This can cause a great disparity when trying to get what looks like a great image on your screen to come out looking just as good on your printer.
An image that has 72 PPI will look fine on your monitor. Each inch of the image will contain 72 pixels. However, when you print that image, it will not be as clear. In fact, it may look choppy and distorted. When you print an image, it will be sized at approximately 25% of what you see on your screen. You will need to have a larger PPI in order to get a printed photograph that is true to size.
The best way to get a true print is to have your image sized at 300 DPI. This will ensure that your image will be the correct size when you print it.
So what does this all mean in the final analysis? In order to get the best finished product, you will need to have a higher resolution, more megapixels and a high DPI. Unfortunately, this also means extremely large files and very large prints. How can you estimate the size of your images before you are ready to print?
Here is an easy reference guide to get you started:
Final Print Size Megapixels Image Size on Monitor
2.1" x 1.6" 0.3 640 x 480
4.25" x 3.2" 1.2 1280 x 960
5.3" x 4" 2.0 1600 x 1200
6.8" x 5.1" 3.0 2048 x 1536
10" x 6.5" 5.3 3008 x 1960
10.25" x 6.8" 6.3 3088 x 2056
13.5" x 9" 11.1 4064 x 2704
As you can see, if you wish to get an 8 x 10 print of a certain shot, you will need to have a higher megapixel amount to achieve good results. Otherwise, your finished print will have a coarse, or grainy look to it, instead of being a sharp image.
Each pixel of a megapixel camera is in fact a minute sensor and together they make up the charge couple device or CCD chip in your camera which captures the picture.
To capture and reproduce a color image, a way of measuring the specific amounts of red, green and blue that make up each segment or pixel of a scene is needed.
Knowing the exact amount of each primary color (red, green and blue) that makes up EVERY pixel segment of a scene allows the camera and computer to construct a full color image from a pallet of 16.7 million color possibilities. (More on this later.)
To separate and measure the amounts of red, green and blue in each element of the picture, most digital camera manufacturers place a Bayer filter array over the CCD sensor array with each sensor covered by a block of 4 colored filters two green one red and one blue. Each transparent red, green or blue filter allows only its own color to pass through and strike the picture element or pixel below. In this way, red, green and blue light components are segregated and recorded.
You will notice that there are two green for every one red and one blue filter per sensor, this is due to the human eye's bias for the color green. Our eyes are most sensitive to the color green. So to mimic our eyes and provide a bias to the color and intensity of light in the scene, camera manufacturers use 2 green filters to every 1 red and blue.
The color information gathered at each sensor is translated into a digital color code for each pixel. This code describes the color of that pixel in terms of an intensity of red, an intensity of green and an intensity of blue. Each color (r,g and b) has an intensity scale range of 0 to 255. So a specific pixel color code would look something like this red 102, green 153 and blue 204. This description is very specific color of blue and when you consider the number of possibilities with a range of 256 for each color. That's 256 times 256 times 256. Doing the math yields 16,777,216 possible colors at each pixel site -- usually rounded off in "photo-speak" to 16 million colors. The "number crunching" that takes place inside your camera is mind blowing when you consider 16 million possible colors at each of the millions of pixel locations on the sensor.
Note: Absolute photographic black is described as 0, 0, 0 -- no red, green or blue recorded. While pure white is assigned the values of 255, 255, 255 -- maximum and equal amounts of red, green and blue. Anytime the three colors are assigned the same numbers (e.g., 110, 110, 110) a shade of gray is the visual result.
These values are recorded by the camera as 8-bit values one 8-bit (or 1 byte) value for each red, green and blue, that hits a sensor or pixel
Megapixels make sense, but how does that relate to the size of the image file stored on the camera's data card?
These 3 digit values which describe the amount of red green and blue hitting each sensor or pixel are recorded by the camera as 3 bytes of data per sensor, one for red one for green and one for blue, the final image file size will therefore be three times the total number of sensors or megapixels used in your camera's CCD chip to record a picture scene.
In terms of a 10 megapixel camera, the final image file would be 30 megabytes in size (3 bytes per pixel x 10,000,000 pixel locations). This file size represents the UNCOMPRESSED information provided by combining the inputs from all the pixels on your camera's sensor.
Note: Technically, not all pixels on a camera's sensor are used to collect color information. Some serve other purposes in the recording process. So the 30 megabyte calculation above is approximate. You will often see two separate megapixel counts given for a specific camera's sensor. The first number is the actual number of pixels contained on the sensor, while the second is called the "effective" number of pixels. The second number (effective) is more representative of the number of pixels being used to measure and record the scene. Multiplying the "effective" number of pixels by 3 is closer to the actual uncompressed file size produced during exposure.
This is an important distinction because some form of compression will take place when you save the file. Saving files using the JPEG option on your camera compresses this data to save storage space. Depending on the amount of JPEG compression you specify (for example, "fine", "normal" and "basic" choices on a Nikon DSLR) the final file size will be progressively smaller. But when these files are opened on your computer, they will be expanded to the full, original rendition megabyte size of your image.