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

---

Taking Panoramic Landscapes - The Easy Solution

I love panoramas. There's something very appealing about their shape.... Read More

History of the Camera

Early cameras of the 16th and 17th century were able... Read More

Video Camcorder Formats and Media

There are way too many tape, disk and stick formats... Read More

Transfer The Digital Camera Images To Your Computer

There are a few very important tasks associated with using... Read More

Magic and the Photograph

It was widely reported in the press (15 September 1997)... Read More

How to Buy a Digital Camera

Are you buying your first digital camera? Well, you must... Read More

Modeling Portfolio Webites

In the age of home phones and 8x10's we all... Read More

How To Make A Time-Lapse Video With Your Digital Video Camera

Getting the most out of your digital video camera can... Read More

Tips for Taking Better Vacation Photos

Photos are a great way to share your travel experiences... Read More

Make More of Your Photos

We all want to make the most of our wedding... Read More

Travel and Scenic Photography 101

When you're driving through the mountains somewhere, and you... Read More

The Mysterious Powerful Element of a Picture That Sells

A photo buyer calls for a picture of pigs. It... Read More

Things You Should Know Before Buying A Digital Camera

In a traditional camera, the lens was king. If the... Read More

Getting Rid of Red Eye

The photos you took of the party are really great... Read More

How a Digital Camera Works

The digital camera is not something of a magic box... Read More

Digital Camera Disc Formats

As it is a fact that digital cameras are comprised... Read More

Photo Equipment Lists

You arrive at your photo shoot excited about the images... Read More

Extreme Digital Photography: Beyond Point-and-shoot

When my old Olympus digital camera got broke while caving... Read More

Megapixel Cameras: How Many Megapixels Do You Need?

Knowing which megapixel camera to buy can be intimidating. There... Read More

Photography 101 Part 2

Basic CompositionNow that you are beginning to understand how your... Read More

Photography Courses Online

There are also some excellent online courses available and some... Read More

Landscape Photography - Capture the Beauty That is All Around

There are many different types of photography. You can take... Read More

Home Run Baseball Photography Tips

Strike one! Strike two! Strike three!Baseball! America's Pastime, and a... Read More

Portrait Photography: Tips and Methods

Portrait is defined as, "A likeness of a person, especially... Read More

Market Your Photography Business

Setting up your own photography business it easy, you don't... Read More

Buying Your Child A Digital Camera - What You Need To Know

Purchasing any gift item for your child is a mind-numbing... Read More

Kodak CX7300 Digital Camera

Quality digital cameras are becoming affordable for just about everyone... Read More

The Benefits of Disposable Cameras

Disposable cameras are cheap and easy to use. If you... Read More

Canon-Powershot-A80 Review

Last month we bought the Canon Powershot A80 online for... Read More

HP Digital Cameras - History and Beyond

HP has started making digital cameras only recently. However, the... Read More

Kodak Digital Cameras - The Giant Comes To Digital

George Eastman, founder of Kodak is world renowned today. Digital... Read More

Diffusion & Softening of Digital Photography Images

Like many people who've made the switch from film cameras... Read More

How to Make Your Production Look a Lot Slicker for £40 More

Tight BudgetsTight budgets are common these days. But the demand... Read More