The Megapixel myth

Pick up any spec sheet for a camera, walk into a shop or peruse the internet and one of the first things you will read about is the number of megapixels cameras have. Recently these figures have jumped from an average of 4 - 5 MP to much higher numbers. 6MP is now considered “entry level”, 7 is common and 8 and even 10 are far from unknown. But what does the megapixel number tell you? Is more always better? How can you tell the good from the bad?

How many pixels is “enough”
OK, hang on tight because there's going to be some maths here.

The megapixel rating of a camera is simply the number of pixels in its sensor. Sensors are rectangular so to work out the megapixel number you just need to multiply the length by the width. A 3,000 x 2,000 sensor will be rated as 6MP. A 3,648 x 2,736 will be rated as 10MP. (yes, that actually comes to a little less than 10 million but it's close enough that the marketing team can call it 10).

Sensors also come in certain ratios. The ratio of the length to width is usually either 4:3 or 3:2. If you see a 6MP camera then it's either going to have a sensor that's about 2,800 x 2,100 or 3,000 x 2,000.

Most people agree that for printing images you need about 150 pixels per inch as a minimum. Lower numbers are sometimes used but these tend to be for lower quality printing or larger print sizes. So, if you want a print that's about 12 inches by 8 inches then you're going to need a file size that's about 1,800 x 1,200 pixels. That's a little bit over 2 million pixels. Higher print densities are usually better and lots of people like file sizes of 300 pixels per inch. On our 12 X 8 print that comes out to 3,600 x 2,400 or 8.6 MP.

In the table below I've listed some common print sizes and the megapixel rating you would need to print at both 150 and 300 pixels per inch. These are probably the two limits to keep in mind for “regular” size prints - below 150 ppi and the print is likely to look poor quality. Above 300 and you're unlikely to notice any improvement. Within this range you should get a decent print. Whether the 300 ppi print looks better than the 150 ppi print depends on a lot of things not least who is looking at it!

Note that this table assumes full, uncropped images and that the sensor is the same aspect ratio as the print which is not possible for all print sizes.

There are a couple of things to note straight away. First, a 10MP camera will give you huge prints - it should easily do A3 even if you need to crop a bit. Secondly, bear in mind that the bigger your print the lower the ppi you will probably need because the viewer tends to stand further away. 30 x 20 prints are not really produced from 54MP cameras!

One final very important point - look closely at the table. The ppi you print from is much more important than the MP rating of your camera. If you print at 150 ppi then a 4MP camera will almost give an A3 print. Print at 300 ppi and you won't go a lot bigger than 7 x 5! An acceptable ppi depends on a lot of factors but one of them is the quality of the image. A 4MP D2H capturing extremely high quality but small files really can produce better prints than an 8MP compact with a questionable lens.

How many pixels is “enough”
The short answer is “yes”. There are a couple of reasons for this.

Firstly, you're paying for them. Look at the table above - if you only ever want to produce 6 x 4s then you “need” at most 2.1 million pixels. Allow yourself a little margin for cropping and you'll easily get away with a 4MP camera. If you are just shooting to place your pictures on a website (at a typical resolution of 800 x 600) then that's less than 0.5MP. As a very (VERY) rough rule of thumb, the higher the MP of the camera the more it costs. If you really only need a 4MP camera then why pay for a 10MP one?

Secondly, you're paying for them. Not this time literally but in all the “hidden” costs. A 6MP camera turns out jpegs that are about 3 - 3.5 MB. A 10MP camera produces jpegs that are roughly 3.8 - 4.2MB. That's about 20 - 25% more storage space. More and bigger SD cards, a bigger hard drive and if you do a lot of editing a faster computer.

Thirdly, there are “technical reasons” - that's a way of saying it's a bit complicated. As a simplification - if you take two identical sensors, one with 6MP and one with 10MP then the 10MP one will produce more image noise than the 6MP one. That's because in general packing more pixels into the same space produces more noise. The Sony 828 was one of the first 8MP cameras and had many great innovations on it. Most of the reviews focused on the fact that its images were noisier than the 5MP 717 that it replaced. As pixel counts have gone up, in camera noise reduction has improved but it doesn't get away from the generalisation that more pixels on the same size sensor is usually noisier. (Of course more pixels on a larger chip would not be such a bad thing - compact chip sizes have been pretty standard for a while now. Maybe they are about to increase.)

All pixels are not equal
This should be obvious from the things we have already talked about - not all pixels are created equal. The overall quality of an image is based on a complex relationship between sensor, pixels, lens and processing software,

When you read the reviews here at ThinkCamera, you'll sometimes see us say things like “this will produce great A3s” or “at 800 ISO this would be adequate for 6X4s but no larger”. The fact is that any camera of at least 6MP should be capable of A3 prints (go back to our table and imagine you are printing at about 180dpi). With care, I've been able to produce much bigger prints from much lower megapixels.

In the real world many things get in the way. A lens may not deliver a decent quality image to the sensor - it doesn't matter how many pixels you have if the image is soft! One issue we see a lot here is over zealous image processing in the camera. Some manufacturers apply clever routines to the out of focus areas of the pictures. This can be to make up for a lower quality lens, a noisier chip or simply because their design team prefers the look. This can lead to a “smeary” or plastic look or simple a loss in critical resolution.

All of this means that the question “how large can I print my 8MP pictures?” is far from a simple one. When we test cameras here, we go out and take pictures with them. Then we scrutinise the pictures and look at the prints. This may lead to some strange results (6MP cameras that will produce larger prints than 8MP) but it takes into account the real world factors rather than a marketing number.

When 7MP is really 6MP
As a small aside, you might remember that the table at the start of this article assumes “the sensor is the same aspect ratio as the print which is not possible for all print sizes”. There have been a crop of 7MP compacts lately for which this is highly relevant.

A typical 7MP compact has a sensor that measures 3072 x 2304 pixels. That's in the familiar 4:3 ratio. It will therefore produce 6 x 4.5 inch prints from a full frame (or 12 x 9 or whatever size you like in that ratio). Many people don't like this ratio since it can be hard to get photo frames or albums for it. If you want to produce pictures in more familiar sizes, such as the classic 6X4 inches or the scales up 12 x 8 or 15 x 10 then you either need to crop the pictures or turn the camera to its “3:2” setting. The 3:2 setting simply masks off part of the sensor so that instead of it being 3072 x 2304, it is 3072 x 2048. As far as the advertising laws allow - that's pretty well 6MP.

So should I ignore MP rating?
From what you've read, you may be imagining that I think that a 5 year old 2MP compact will give better pictures than a brand new 30MP wonder camera. Far from it. There has been a HUGE increase in the quality of digital cameras in the last 2 - 3 years. New features such as image stabilisation, noise reduction and even high pixel counts that mean you can actually use digital zoom have all made their mark.

Megapixel counts can be a factor in deciding which camera to buy but reviews and your own tests are going to tell you much more. The advertising world loves numbers and so would love us to believe that 10MP is better than 8, and 8 is better than 6 and so on - really it comes down to deciding what you need and how the particular camera you are considering fulfils that need.