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

---

The Beauty of Nature Photography

Nature is all around us, and its beauties are unlimited.... Read More

The Benefits of Underwater Digital Cameras

Underwater digital cameras are fun and easy to use under... Read More

Is Film Better than Digital for Weddings?

As a professional wedding photographer I get this question asked... Read More

Digital Wedding Photography: Myth Vs. Reality

1. Myth: Digital photos are pixilated or fuzzy. Reality: most... Read More

Using Film Speed Effectively (Black & White Film Thoughts)

It's hard to find sometimes but it's making a resurgence,... Read More

Creating Personalized Photo Calendars

About Personalized Photo Calendars:Photo Calendars are 12-month, wall-hanging calendars that... Read More

How to Shoot Video of Your Kids Sports Team So That Anyone Else Will Watch It!

Break out that video camera, there is a game this... Read More

Digital Cameras & Resolution

While analyzing any camera, one of the most important qualities... Read More

The Photography Portfolio: Building Your Reputation

If you have hopes of becoming a professional photographer, or... Read More

Should You Print Your Digital Photos at Home or Use a Photo Printing Service?

There are lots of options available for getting your digital... Read More

Easy Tips for Digital Photo Touchups

Using a digital camera to take pictures is great. It's... Read More

Choosing the Right Digital Camera

Choosing the right digital camera for general use can be... Read More

Digital Cameras vs. Film Cameras- the debate goes on

One of the biggest debates in the photography world is... Read More

Camcorder Techniques: How To Make Home Movies Your Family And Friends Will Be Excited To Watch

Pre-Production:It's all got to start somewhere, so before you even... Read More

Digital Cameras and Digital Photography

Before you rush into buying your digital camera this holiday... Read More

Elephants in the Sky and Four Poems

Elephants in the Sky[1980s, Lee Evens in Mali, Timbuktu/Africa]Advance: Lee... Read More

The Venice Rookery

Love is in the air at the Venice Rookery. The... Read More

What Should You Look for in a New Camera?

These parameters are discussed at 2 levels: -Based on... Read More

Color Management, the Digital Darkroom, and Adobe Photoshop

PrefaceIn order to understand color management, it is important to... Read More

The Well-Dressed Photographer - Winter

Some of the best images can be captured during the... Read More

Picture Framing for Photographers - Part 1

Imagine being able to frame all your own photographs just... Read More

Proper Handling and Storage of Paper Photos

Youv'e taken that perfect photo in the perfect light at... Read More

Is Digital Photography Expensive?

Being such a fascinating device as the digital camera, this... Read More

Digital Cameras Demystified

Demystifying Digital Camera JargonWant to buy a digital camera but... Read More

Make Your Digital Camera Batteries Last Longer

One technology that hasn't kept up with the rapid pace... Read More

Getting Your Photos Ready for Sharing

One of the main reasons people buy digital cameras is... Read More

History of the Camera

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

Disposable Underwater Cameras

Disposable digital and film cameras (also called single-use cameras) provide... Read More

The Naked Truth on Shooting Nudes

There are major obstacles to overcome to get into nude... Read More

Travel and Scenic Photography 101

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

Where to Find Cheap Disposable Cameras

There are many kinds of disposable cameras. All disposable cameras... Read More

Black & White Photography

Photography is a diversified field of creating a spectral variety... Read More

The Benefits of Disposable Cameras

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