Image Resolution, megapixels, megabytes, PPI and DPI explained.

As our title suggests, a digital image's size can be described in a number of ways. This can lead to confusion when it comes to ordering prints online.

It does not help by the fact that all of these terms can be used individually or combined when describing an image's size.

So let's take a look at each of them in turn.

Image Resolution

The resolution of the digital image describes how much colour information there is stored within the image. The higher the image's resolution, then the more information there is.

When you take a photograph with a digital camera, the light travels through the lens and hits the camera's sensor, the sensor is covered in thousands of photo-electric cells. These cells, often referred to as pixels, react to the wavelength of the light's colour.

As all visible colours in a digital image can be made up from a combination Red, Green and Blue, the pixel's reaction to the light is recorded as electronic values of how much Red Green and Blue information there is in the light.

The range of these values is from 0 - 255. For example: if we had pure red light the values would be (Red 255, Green 0, Blue 0)

The amount of pixels on the sensor determines how much colour information can be recorded about the image.

The resolution of an image can be measured by counting the number of pixels per inch (PPI), not to confused with dots per inch (DPI) which is a print measurement.

So if we break the image down into pixels of information, then the level of resolution in the image would tell us how large each individual pixel would be.

For example, if we look at a high resolution image that is 10" wide with 300 PPI (pixels per inch) there would be 3000 pixels across the image. If we look at a low resolution image that is 10" wide with only 72 PPI (pixels per inch) there would be 720 pixels across the image.

Why is this so important? In the high resolution image, there are a lot more pixels recording information about subtle smooth colour transitions, whereas the low resolution image would miss out some of this information and look blocky.

Megabytes

The size of the image file is measured in megabytes, this determines how much space the file will take up on your hard drive, and also how long it will take to transfer to us to print.

As stated above each pixel records the colour information of the light hitting it, this takes the form of electronic data or bytes. So the more pixels that you have on the sensor, the more data will be recorded and the bigger your file will be.

Megapixels

The pixels are arranged in horizontal and vertical lines on the camera's sensor. A 2 megapixel camera will have a sensor 1600 pixels wide and 1200 pixels high, 1600 x 1200 = 1,920,000 pixels which is rounded up to 2 megapixels. You will note that this is 4:3 ratio.

It has been generally observed that the human eye cannot differentiate detail over 300 PPI, so if we use 300 PPI as our benchmark, a 2 megapixel camera's default image size would be 5.33" x 4", we arrive at this by dividing the amount of pixels across the sensor by the PPI benchmark.

If we compare this to a 8 megapixel camera (3264 pixels x 2468 pixels), then using the same calculation the default image size would be 10.88" x 8.23". So the more megapixels your camera has the bigger the default image size will be.

So how does this affect the size of the print that can be produce from your image? The answer is this: If you send us an image file that is 300 PPI from a 2 megapixel camera, the default print size as discussed would be 5.33" x 4" or 1600 pixels x 1200 pixels, but we only need 300 PPI if you are going to view the image at arm's length, due to the normal viewing distance of a large print on the wall, we can lower the PPI to as little as 72 PPI without any perceived visual loss of quality.

We can interpolate the pixel information into a larger print. For instance we know that 1600 pixels ÷ 300 PPI = 5.33", it follows that 1600 pixels ÷ 72 PPI = 22.22". Therefore we can produce a print sized at 22.22" x 16.66". If we apply the same principle to an 8 megapixel image we can produce a print sized at 45.33" x 33.99" without any perceived visual loss of quality.

PPI and DPI

As discussed earlier PPI (pixels per inch) is the unit that can be used to measure image resolution.

DPI (dots per inch) is a print measurement and relates to how many dots of ink a printer will use per inch when it prints your image.

This terms are often confused and mistakenly used to describe the other. The difference between PPI and DPI is as follows:

  • With PPI the pixels in a digital image are square with no gaps between them, all the colours including black and white are made up from the Red, Green and Blue values, black having a value of (Red 0, Green 0, Blue 0,) white having a value of (Red 255, Green 255, Blue 255).
  • With DPI, when an inkjet printer prints an image, the inks (Cyan, Magenta, Yellow, Black, and more depending on the make of the printer) are sprayed onto the medium (paper or canvas) in the form of layers of circular dots set out in patterns dependent on the desired colour, each dot pattern being produced by varying levels of each ink, which creates the illusion of many more colours. To produce white during the process, no ink is used.

When a digital image is ‘sent’ to the printer, the pixels are converted into the ink dot patterns, due to the circular nature of the dots, there is space left between them. However as each dot is so small (up to 2400 dots per inch on some commercial printers) you cannot visibly see the spaces with the naked eye.

 


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