What is the meaning of megapixel, dpi, ppi, file size and how do they relate to print size?

Among the most common questions I am asked by students, clients and fellow photographers is how many pixels do I really need for a project, print or email? The answers are both simple and complex.

Here is a bit of a primer:

A pixel (or picture element) is the smallest unit or dot in an image. If you zoom into your digital photograph on your computer monitor, you will see individual dots, which are usually represented as squares. These correspond to the individual recording pixels on your digital camera's sensor.

Sensors on digital cameras and scanners have rows and rows of pixels. These pixels are measured by how many pixels there are in one row (the width) multiplied by how many rows there are (the height). Canon's Rebel T3i camera has 5,184 pixels in each row and 3,456 rows. If you multiply these two numbers, 5,184 x 3,456, the total will be 17,915, 904 or 17.9 million. A million pixels is called a megapixel. thus, the Canon T3i has about 18 megapixels in its sensor.

How do megapixels compare to megabytes?

Megapixels tell us how many pixels there are in an image. A megabyte tells us how much space the file will take up on a memory card, flash drive or computer. Huh?

If I take a photo with a Rebel T3i, I would probably shoot in jpeg. The jpeg file can be opened by all computers and is easily shared via the web, which makes it very attractive. Jpeg files are also compressed which makes them take up less space on a memory card. Compression identifies pixels of similar color, saturation and brightness and stores their data together rather than separately. Using moderate compression, my jpeg takes up about 6 megabytes (6 Mb) on the memory card.

My camera also offers two levels of compression. The second level compresses my 18 megapixels into a smaller space. I think of this like stuffing my pixels into the overhead carry-on bin of an airplane. I still have the same 18 megapixels; I am just stuffing them into a smaller space. Using this greater compression, the same jpeg file takes up about 2 Mb on the card.

When I download and open the photo on my computer it decompresses and opens to its full size. An uncompressed 18 megapixel file is about 51 Mb!

Which file has the most megapixels? They all have the same!

  • 5,184 x 3,456 + moderate jpeg compression = 6 Mb
  • 5,184 x 3,456 + significant jpeg compression = 2 Mb
  • 5,184 x 3,456 = no compression = 51 Mb
The pixels remain the same; it is only the size of the container and degree of compression that are changing.

What is the significance of this?
It is impossible to judge either the quality of an image or the number of pixels by looking at the number of megabytes (Mb). Megabytes will only tell you the amount of bandwidth or disk space required to transport the file.

How do I determine the quality of an image and how 'big' I can make it?
In order to calculate a print size, you need to know how many pixels your image contains. Here are several ways of finding your pixel count:
  • iphoto - click on any individual image and click Photo then Get Info
  • Adobe Bridge, Photoshop and Photoshop Elements - Highlight or open an image and click File then File Info. Pixel dimensions are found under Camera Data.
  • Windows Explorer - Highlight an image; right click and choose properties. Click on the summary tab and choose advanced.
  • Windows Picture Gallery - Highlight an image. Look in the panel at the bottom for pixel information.
  •  Picasa - Highlight a photo; right click and choose properties. Pixel info will show in a panel at the right.
Once you know how many pixels you have, you are ready to figure out print size.

Many competitions, submissions and designers request images at 300 dpi. DPI stands for dots per inch. Dots per inch is a printing term that refers to how many individual dots of ink fit into a one inch square. If you took a super magnifying glass to a commercially printed 300 dpi print, you would be able to count 300 individual dots of ink in a one inch row.

The significance of 300 dpi is that it is the threshold point where almost everyone is unable to see the dots in an image and thus perceives the image as seamless and natural. With pixels larger than 300 dpi (100, 150, 200), it is possible that some individuals might see the dots.

When looking at the resolution of a digital file, you will see it rated in pixels per inch or ppi, rather than dpi. While the terms are generally considered interchangeable, pixels are always uniform in size, rectangular or hexagonal and always butt up against on another. Printing ink can vary in size, overlap and is usually rounded.

Dividing pixels by ppi

In our example of the Rebel T3i, we have a file that has 5,184 x 3,456 pixels. In order to determine how large we can make it at best quality, we divide each of the dimensions by 300.

5184 divided by 300 = 17.28 inches
3456 divided by 300 = 11.52 inches
The largest standard size I could expect to make in best quality would be 12 x 18 inches.

However, in most cases, an image can be printed at 150 ppi and still appear photographic.

5184 divided by 150 = 34.56 inches
3456 divided by 150 = 23.04 inches
Therefore, many photos made with a Rebel T3i can be enlarged to 24 x 36 inches without major quality loss.

My Canon Rebel T3i shoots at 72 dpi - do I need to convert to 300 dpi to get the best print?

NO. A file that has 5,184 x 3,456 pixels will print the same quality regardless of ppi/dpi count. It is the total quantity of pixels that counts, not the configuration. I think of it like bringing home milk from the grocery store. One gallon of milk is the same amount whether it comes in a gallon container, two half gallons or four quarts.

One last question - I crop all my photos to 4 x 6 before I email them. How come they are still gigantic in the email?

Most programs on your computer automatically scale images to fit your screen. Thus, when you put a photo in PowerPoint or Word, it usually fits on the slide or page.

Web applications, such as email programs, blogs, and websites usually can't scale images. Instead, they divide the pixel dimensions (our Rebel's 5,184 x 3,456) by the maximum screen dimension (how many pixels there are on your screen) and display the image accordingly. Many new wide screen monitors have a 1920 x 1080 resolution.

5184 divided by 1920 = 2.7
3456 divided by 1080 = 3.2

This means that a full sized image from a Rebel fills a space 2.7x the width and 3.2x the height of the average monitor. To avoid this, please downsize your images before emailing or posting to the web.

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