Megapixel Count


The fascination with megapixel count is, to a great degree, unwarranted.  Most people will never print a photograph larger than 8 x 10″, and a decent 5 MP image file will handle that to the standards of all but the crustiest perfectionist.  Being one of those critters, I’m sure of that.

Megapixel count is a way of expressing the number of tiny sensors on the chip that takes the place of film.  We’ll call them pixels (although that’s not technically accurate — they’re actually transistors). 

Pixels gather information, expressed in computer language.  This information includes the relative levels of three different colors in the light, the brightness, and the information’s exact location in the picture.  The more pixels, the more detail the chip can gather — in theory.  In a very general way, it is like differences in film size: the bigger the film negative, the more detail in the photograph.

Now here’s the catch.  The smaller the pixel, the less light it can gather.  As you add pixels to a sensor of a given size, they have to be smaller.  A standard 1/18, 10 MP sensor in a small camera has crowded on it twice as many pixels as there are on a 5 MP chip the same size.  There is more detail available, but as it gets darker, less light falls on each individual pixel, and they begin to have trouble delivering the necessary information to the processor. The signals get scrambled, and we have the “noise” that we associate with digital images taken in dim light.  The same thing is true of the shadow areas of a brightly-lit photo.  If we try to bring out detail in the shadows, we end up with noise.  If we “smooth out” the noise, we lose detail.

You can see where we’re going, here.  A sensor with fewer pixels will deliver less data overall, but all else being equal will perform better in dim light.  As long as we can “get by” with 6 MP, we will get overall better results than we are likely to get with a 13 MP chip, over the widest range of conditions.  Some of this is corrected by sophisticated software in the camera’s processor, but the basic equation remains:  poor data, poor image.

One of the reasons digital SLRs deliver better performance than smaller digital cameras is because their sensor chips are much larger.  This means that a 10 MP chip in an SLR, while having the same number of pixels as a 10 MP camera with a smaller chip, will have much larger pixels, able to gather more light.  To get pixels the same size in the smaller camera, we would have to drastically reduce the total number, to have room for the bigger pixels.  It’s like using a camera with — you guessed it — bigger film negatives.

The good news is, most consumer grade cameras perform extremely well for what they cost and the uses they are put to.  But don’t be carried away by Megapixels.  The other features of the camera are far more important.  In fact, all else being equal, fewer MP’s may be a better choice.  Even knowing this, the marketers will insist on more pixels, and the manufacturers will keep adding them for people who are impressed by numbers. 

There have been recent developments in small sensors for phone cameras that may change this picture somewhat. Nokia have released (July 2013) a 42 megapixel camera that combines the data from seven sensors into one pixel in the image. This produces, a five megapixel result, but with far more detail than a normal image of that size, and much better low light performance. Allegedly this permits extremely high quality zooming as well. Initial reviews are excellent, but it remains to be seen if this is a major breakthrough, or if it will carry over to consumer cameras in general.

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