Digital Image Files - Megapixels, Megabytes, or DPI?

When I promised readers that I was going to do an article on this topic I was scared. For two reasons - firstly, it's a HUGE subject. I get lots of questions about it, and I see a fair bit of misunderstanding about it. Secondly, there are already a lot of good articles about it on the web, like this one on luminous-landscape.com.

But I know that trawling the internet for technical information is not your idea of fun. That's my job! So here's my attempt at summing this up quickly for you:

DPI - Dots Per Inch

The most common question I get on this topic is, "My client / boss / nephew has asked me to send an image at a size of 300 dpi. What does that mean"?

The answer: Not much.

You see, DPI stands for Dots Per Inch. It's a useful measure of image resolution (in other words, how much information is resolved in the picture). But if you don't know the image size in inches (or feet, miles, centimetres, millimitres, or some other measure of size), then the amount of dots per inch doesn't mean much.

Using DPI to measure size is like using km/h to measure distance: "How far is it from here to the beach?" "Oh, about 60 miles per hour". For this to make sense the answer would need to be "about 10 minutes at 60 miles per hour".

Likewise, the size of an image needs to be expressed as, say, "six by six inches at 300dpi".

Different resolutions are used for different purposes. The most common are 72 or 75 dpi for screen viewing (Web use or PowerPoint presentations) and 300 dpi for printing.

OK, so to give an example - 1 inch by 1 inch, 300 dpi image would be 300 pixels by 300 pixels in size. A 2 by 2 inch image at 300 dpi would be 600 by 600 pixels in size. Here's where megapixels and megabytes come into it. Mega!

Megapixels

The term megapixels is usually used to describe the output size of digital camera images. For example, the Canon Ixus 50 produces images which are 2592 x 1944 pixels in size. Multiply these numbers together and you get 5,038,848 - just over 5 million. Hence this is described as a "5 megapixel" camera.

The last byte

On a couple of occasions, I've sent an image of a certain size to someone and they've said, "that's no good, we need a 10 megabyte file". Now, this I'm sure they were well-intentioned but they were also a little misguided.

The size in bytes (or megabytes - millions of bytes) represents how much storage the image takes up on your computer. This depends on all sorts of things, mainly the bit depth of the image and the file format - for example TIFF or JPEG.

So what should I do?

To avoid confusion, when specifying the file size you need, use pixels.

How do you work out how many pixels you need? Well, that's why I started this discussion with DPI. Work out the largest size you're going to want to reproduce the image, in inches; and the resolution - for example 72 dpi for or 300dpi for most print applications. Then just multiply the size in inches by the DPI figure you came up with.

Example: I want to reproduce the image A4 size in a printed magazine. A4 is 210mm x 297mm, or about 8.3 x 11.7 inches. The magazine needs artwork at 300dpi, so:

8.3 x 300 = 2490 and 11.7 x 300 = 3510 so I need an image sized about 2490 x 3510 pixels (about 8.7 megapixels)

By the way: 1 inch = 2.54 centimetres. Did you know you can also do conversions on google? Try it yourself.

Happy pixelling!

Steven Pam is a digital commercial photographer based in Melbourne, Australia, specializing in people, aviation and music photography. http://www.stevenpam.com.au

Los Angeles Times

Capturing the world through photography, video and multimedia Los Angeles Times PHOTOGRAPH BY: Niranjan Shrestha / Associated Press New Delhi — A Tibetan activist in exile takes part in a protest denouncing clashes between Chinese authorities and Tibetans. Bayrischzell, Germany — Skiers walk beside frost-covered Wendelstein ... and more »

Photographers: Chances Are, You Suck Wired News There are plenty of things photography-wise that I'm not very good at. I'm not great at creating images, but I'm pretty good at finding them. I'm terrible at self-promoting, marketing and the business stuff makes me squirm. Yet I'ma decent journalist, ... and more »

Opening Japan, Through Photography New York Times VENICE — Both Katsushika Hokusai and Ando Hiroshige, celebrated masters of the woodblock print, were still alive when the Western technology of photography made its first tentative inroads into Japan. Hiroshige died the year after the first ...

Take a Closer Look at Google Business Photos PCWorld (blog) To that end, the search giant is pairing its own certified "Trusted Photographers" with businesses that want to show off their interiors to the public. While including these photos in your Google Maps listing and Google Places page is extremely helpful ... and more »

New York Times (blog)

A Depot and a Fight to Photograph the City New York Times (blog) By DAVID W. DUNLAP David W. Dunlap/The New York Times Moments after taking this picture of the 126th Street Bus Depot on Second Avenue last week, David Dunlap was told by an MTA security officer that photography of transit facilities was prohibited. and more »

Amateur Photographer

Photographer Simon Marsden dies (update includes tributes) Amateur Photographer Renowned photographer Simon Marsden, who specialised in gritty b&w images of subjects including mystical landscapes, gothic graveyards and old ruins, has died aged 63. Simon first developed an interest in photography when his father, a keen landscape ... and more »

The Guardian

'Photographer Photoshops image' shock The Guardian All of us media consumers should applaud the management of the Sacramento Bee, which this weekend courageously fired photographer Bryan Patrick for high crimes against journalism. Patrick, or as he shall forever be known, the Great Satan, ... and more »

New York Times (blog)

Capturing Dinosaurs and Whales, Without Seeing Them New York Times (blog) By AIDAN GARDINER Glenna Gordon for The New York TimesVictorine Floyd-Fludd, a blind photographer, and Donal Martinez, a sighted volunteer with Visions, an organization that helps the visually impaired, at the American Museum of Natural History. and more »

BBC News

Photographer to "REVEAL" new exhibition isleofman.com Andi, whose interest in photography started around 10 years ago, said he primarily saw himself as a landscape photographer until something happened on a bus which made him change his path. He explained: "I noticed a guy sitting on his own and something ... Isle of Man faces star in new photography exhibitionBBC News all 2 news articles »

Los Angeles Times (blog)

Street photography exhibit at Hatakeyama Gallery Los Angeles Times (blog) It might come as a surprise to some that LA, a city stereotyped for its car culture, has a small but vibrant street photography scene. After all, it's pretty hard to take photos while driving, right? (Hey, we can't all be Lee Friedlander.) ...