Photographer, Steven Pam, gives his take on the pixel and dpi
(dots per inch) conundrum, and also recommends an article at
The Luminous Landscape.
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.
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 commercial photographer and founder of Smartshots commercial photography. He is based in Melbourne,
Australia, and specializes in people, aviation and
music photography. Steven has over 10 years experience
working with clients in the UK, USA and Australia,
from small businesses to national publications and
publicly listed companies. For Steven's free 20-point
smartguide to organising a commercial photo shoot,
visit http://www.smartshots.com.au
