How many of you have seen adverts for digital cameras announcing “12 Megapixel sensor for unprecedented image quality” ? The number of megapixels is often the only thing people enquire about a new camera, and the higher the number, the more impressed they are. That, as well as how much it costs, how it looks, and how many times it can zoom.

While I also think that cost and looks are important, I think the “megapixel” (MP) race has reached ridiculous levels. A few years ago, when 2 MP cameras were the norm, more was indeed better. But that was then. In fact, that shiny new 12 megapixel compact camera you bought will almost certainly take worse pictures than your neighbour’s 6 megapixel digital SLR which he bought 4 years ago. Let me try to shed some light on the situation…

When looking at your photographs (on a computer monitor, high definition plasma TV, or prints), you won’t see the detail-advantage of a 12 MP photograph over a 6 MP photograph because:

• Your computer monitor can probably display somewhere between 1 and 2 MP
  (1920×1200 is pretty high-res, but still only 2.3MP)
• An A4-size print on photo paper has less than 9 MP resolution, and 13x18cm less than 4MP

There *are* differences between good and bad photographs, but it is not a function of megapixels:

• Your shiny National Geographic or Cosmopolitan magazines have limited resolution on a “full page” pic (probably < 5 MP), and yet those photographs look awesome!
• Pictures on facebook are only about 0.27 MP, and yet you can often see the difference between a good and a bad (e.g. cell phone) camera. This is mostly due to differences in dynamic range and the quality of the lens, not megapixel count!

Having more megapixels is actually often a disadvantage:

• A camera can only take pixel-level sharp pictures if the lens is able to form a sharp image on the sensor. The higher the pixel density, the sharper the lens needs to be. Very few cameras have lenses which can consistently provide detail to a sensor with 12 or more MP
• A sensor pixel can only hold a certain amount of charge before “overflowing”. The smaller the pixel, the less charge it can hold. This directly influences dynamic range (see images below)
• Every pixel needs light. Adding more pixels means each pixel will receive less light. This leads to “noisy” (speckled) images, since the camera needs to amplify the image signal more.
• The size of the sensor directly influences the price and size and weight of the camera, and therefore small cheap cameras invariably have small sensors. The only way to keep pixel density low is by keeping the number of megapixels reasonable.

Relative sizes of sensors used in most current digital cameras. (click for detail)

DPReview recently started listing the pixel density (MP/square cm) of all the cameras in their database. Because cameras have different size sensors the megapixel count on its own doesn’t tell you how densely they’re packed. Notice the enormous range in numbers! (a high pixel count combined with a low pixel density is best).

*prices (including a basic lens for DSLRs) based on Amazon.com quotes for November 2008

“Dynamic Range” is the ratio between the brightest and the darkest values a camera can record at the same time. Large pixels generally have more dynamic range than small pixels. And that is something you notice even on small pictures on facebook. Cell phone cameras are terrible in this regard.

Having an excessive pixel density can lead to images with “blown highlights”, where regions of the photo are overexposed (just white), or underexposed (just black).

A well exposed photograph taken with sufficient dynamic range (click for detail)	Blown highlights and underexposed shadows, due to low dynamic range (click for detail)

Also, since the signal from each pixel is weaker, it needs to be amplified more. Amplifying the signal also amplifies the noise, which leads to random speckles and reduced detail. The noise can be suppressed with “noise reduction”, but this blurs the image and removes even more detail!

Low-Noise Image (click for detail)	Noisy Image (click for detail)	Blurring due to “noise reduction” (click for detail)

One fact, however, is that an image file with more pixels will take up more of you hard drive space. So, with your new little camera you might just end up with huge files showing millions and millions of noisy, blurry pixels which can’t cope with the really dark or really bright pictures.

In fact, when comparing the brand new semi-professional Canon 50D (15MP) against its predecessor the 40D (which had 10MP), the DPreview team concluded: “in terms of per-pixel sharpness the 50D cannot quite keep up with the better 10 MP cameras on the market … It appears that Canon has reached the limit of what is sensible, in terms of megapixels … Considering the disadvantages that come with higher pixel densities such as diffraction issues, increased sensitivity towards camera shake, reduced dynamic range, reduced high ISO performance and the need to store, move and process larger amounts of data, one could be forgiven for coming to the conclusion that at this point the megapixel race should probably stop.”

Today’s new cameras can take better videos than ever before, have nice large displays, funky colours, are small and light, cost very little and can even detect faces and smiles. And these are all really cool. But we don’t need more megapixels for this!

To conclude: Beyond a certain point more pixels don’t improve picture quality – in fact, often the opposite. Sadly, it’s unlikely that manufacturers will stop the so-called “megapixel race” anytime soon. Why? Because numbers sell.