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Digital Camera Guide

What is a Megapixel? How Many Megapixels does my Digital Camera Need?

[Digital images are made from individual pixels.]

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.

[Both the dog (left) and the baboon (right) were shot in 35mm and scanned on the same scanner at 600 dpi. However, due to differences in the ISO rating of the film, the image of the dog appears much grainier.]

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.

[The same image close up at 600, 300 and 150 ppi.]

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!