How many pixels do I need?

Digital cameras come with more and more pixels. A couple of years ago, three megapixels (three million pixels) was a lot, now you need three times as many to have a lot of pixels. Some pro models now feature more than 20 million pixels.

But pixels don’t come for free. The more pixels you want, the more cash you will have to cough up. So, how much pixels do you really need?

The purpose of your digital photos

First of all, you need to decide what you want to do with your digital photos. Printing posters at 100 x 70 cm requires much more pixels than publishing digital photos on the web.

Pixels needed for web publishing

To display a photograph on a web page, you rarely need more than, say, 800 x 600 pixels, that is, about 0.5 megapixel. Any camera on the market will achieve this easily.

Pixels for fine printing

On the other hand, to print a picture with high resolution you need to deliver something like 300 pixels per inch, or at least 250 (that is, around 100 pixels per cm). For a print in A4 (29 x 21 cm, 11 x 8 inches), you’d need about 2800 x 2000 pixels, that is, around six megapixels.

Pixels for printing posters

However, when printing posters you may choose a lower resolution, for example 150 pixels per inch (60 pixels per cm). You’ll not be watching the poster at the usual reading distance, but more likely from a couple of meters’ distance. Because of the longer viewing distance, the pixels can be printed farther apart without decreasing the apparent image quality.

Often, 6 megapixel are good enough to produce fine posters at 100 x 70 cm.

Good and bad pixels

Another thing to consider is the quality of the individual pixels. It’s not only the number of pixels that matters. Each pixel in the picture corresponds to a sensor element in the camera. The light-sensitive sensors register the incoming light, and each one of them forms a point, or pixel, in the resulting image.

When digital camera manufacturers squeeze in more and more pixels on sensor chips of a certain size, the individual picture elements necessarily become smaller and smaller. We’re now at a point where individual elements are so tiny that some pretty weird physical limitations come into play. The sensors start losing their quality.

For example, a very small sensor may not be able to collect all the light energy that hits it, so that light (or electrical charge) spills over to heighbouring pixels. Of course, this will distort the image in unforseeable ways, and this kind of fault cannot be adjusted in image processing programs such as .

Fewer good pixels will often give you a higher image quality than more pixels that behave badly.

A comparison of two actual image sensors

Let’s compare a 12 megapixel compact camera such as Casio EXILIM EX-Z1200 with a 6 megapixel prosumer model such as the Nikon D40. The sensor in the EX-Z1200 measures 7.6 x 5.6 mm, which means that each individual picture element has a size of 3.5 10-6 square millimeters. The sensor of the Nikon D40 is 23.7 x 15.6 mm large, making the area of an individual pixel 61.6 10-6 square millimeters. With a smaller number of pixels on a larger sensor, the area of each pixel is 17 times larger!

It’s not hard to imagine that they have different physical behaviour than their smaller colleagues - and a different pixel quality.

Conclusion

To summarize, there’s no easy answer as to how many pixels you need. It depends on the purpose of your digital photos. But it should be clear: more isn’t always better.

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