Digital Camera Guide
What is a Megapixel? How Many Megapixels does my Digital Camera Need?
The term megapixel means one million pixels. Digital images are made of pixels,
or picture elements. A pixel is a "square dot" in your picture.
One of the first questions many people ask when shopping for a new digital camera is
"How many megapixels do I need?" Many people are aware that the number of megapixels the
camera is able to capture in a single image has a direct effect on the quality of
the photos the camera is able to output. However, because the number of megapixels also
has a direct effect on the price of the camera, most people also want to know how many
megapixels they really need to get good quality prints without spending too much
money on a camera.
It's important to note that there are many factors in choosing a digital camera, many of
which may be increasingly more important than the number of megapixels. A camera with a
high megapixel count but poor optics will take high resolution, poor looking pictures. You
will be able to see close up just how bad the picture looks. Since that's undoubtedly not
what you want, it's worth shopping carefully when choosing a new digital camera rather
than simply choosing the unit off the shelf with the highest megapixel count.
However, that being said, the megapixel count is still an important factor
in choosing the right camera for you.
Many people want to know how many megapixels it takes to replace the 35mm camera they are using now.
So, how many megapixels does it take to be equivalent to 35mm film? Unfortunately,
this is a difficult question to answer, partly because the answer depends largely on what kind of
35mm film you are using. Film works by chemically reacting when exposed to light. Some kinds of film
are more sensitive to light than others, meaning that it takes less light - or the same amount of light for
less time - to make the chemicals in the film react. This is known as the film's ISO (formerly ASA)
rating or "speed". Film with a higher ISO rating, such as ISO 800, needs less light to react than
film with a lower ISO rating, such as ISO 200. This is why it is better to use a higher ISO rated
film on a rainy day or when taking fast-action shots. However, there is a drawback to using film
with a high ISO rating: the higher the speed of the film, the larger the clumps of chemical particles
tend to be. When the particles are large enough, they produce a visible texture in the final image.
This texture is known as a film's grain.
As judged against an average 35mm photo taken on good quality, typical ISO-range film printed at a
standard size, most manufacturers seem to suggest that a 6 to 9 megapixel digital camera will give
you comparable image quality. Some originally devised this number by scanning film photographs at
different resolutions and determining at what point increasing the scanner resolution made the
film grain look worse instead of making the image look better. Once they had determined what
scanning resolution was a reasonable maximum for an average 35mm photo, they were able to
determine the number of megapixels needed to match this resolution. Of course, the 6 to 9 megapixels
number only holds true for a 35mm image with a typical film grain. To equal the quality of some
grades of film, such as the ultra fine grain film sometimes used for taking portraits or
landscape photography, you would need many more megapixels to even come close.
Does this mean we all need a million megapixel camera to get good prints? Of course not!
The truth is film grain is only one part of the equation. The other part of the equation is the
size of the print. Professional photographers care about film grain, in part, because it
determines how big the photo can be enlarged before the grain becomes obvious or the
photo loses detail, and therefore begins to look bad. Likewise, for a digital photo, the
important part for most of us is how large it can be printed before you can see the individual pixels.
An inkjet print, or even a glossy print in a magazine or on a poster, is made up of lots of
individual dots of ink. These dots are very tiny, and when printed close together our eyes
perceive them as continuous color images. The number of dots in the image is determined by
the dpi: for each inch of printed material, it takes so many
dots - or dots per inch - to make the picture.
As we learned earlier, pixels are just dots. And, as we just mentioned, printer
dots are also, well... dots. If you hadn't guessed already, there is a relationship between the
number of pixels per inch (or ppi) in the digital image and
the number of dots per inch (or dpi) when the photo is printed.
There are several different algorithms for determining the optimal ppi for your image based on your
printer's dpi, but it's a safe rule of thumb that:
- Acceptable quality prints can be made from 150 ppi images
- High quality prints can be made from 300 ppi images
A 300 ppi picture looks very good with most print methods, including those used by places that will
make photo prints for you. It's rarely necessary for most people to print at a higher resolution
than that. [On a side note, don't confuse your printer's dpi with your picture's ppi. Leave your
printer at the best possible settings when you print your photos.]
How many megapixels do you need? First, determine how big you want to print
your picture. Next multiply the number of inches by the desired ppi to calculate the total number
of pixels needed. For example, let's say you want to make a typical 3 x 5-inch print:
3 inches x 300 ppi = 900 pixels
5 inches x 300 ppi = 1500 pixels
900 x 1500 = 1350000 pixels, or 1.35 megapixels
Or if you wanted to make an 8 x 10 portrait instead:
8 inches x 300 ppi = 2400 pixels
10 inches x 300 ppi = 3000 pixels
2400 x 3000 = 7200000 pixels, or 7.2 megapixels
Of course, if you don't mind a drop in quality, you can print your portrait at 200 ppi:
8 inches x 200 ppi = 1600 pixels
10 inches x 200 ppi = 2000 pixels
1600 x 2000 = 3200000 pixels, or 3.2 megapixels
If you try to print your image at too low a resolution, it may turn out either blocky or
blurry depending on how your imaging software scales up the pixels. Some software attempts to
improve the appearance of oversized pixels by blending them with their neighbors, and while
this keeps the image from looking blocky or "jaggy", after a point the image ends
up looking fuzzy instead.
For most people, a digital camera capable of at least 3 or 4 megapixels seems to be a
popular choice. This provides enough resolution to print a pretty nice 4 x 6 snapshot
without breaking the bank. Of course, as technology improves and prices come down, a growing
number of people are experimenting with 5 megapixel and up. At the same time, some sub-megapixel
mini cameras can be found for $20-30, and while they're more suitable for sending pictures
via e-mail than making prints, they can still be great fun for kids or for hanging on
your keychain for those unexpected moments. Whatever your needs, there are many great
choices out there, so don't be afraid to shop around!