The megapixel myth has treated camera manufacturers well over the years, those ever increasing, and often meaningless, numbers have sold millions of cameras. But consumers are getting wise to it. We've all seen dodgy images from high megapixel cameras and know that after a point, megapixels don't matter for most people – a 16 MP compact isn't ever going to be as good as a 12 MP Full Frame DSLR. What does matter is sensor size!

Why is camera image sensor size important?

The size of sensor that a camera has ultimately determines how much light it uses to create an image. In very simple terms, image sensors (the digital equivalent of the film your father might have used in his camera) consist of millions of light-sensitive spots called photosites which are used to record information about what is seen through the lens. Therefore, it stands to reason that a bigger sensor can gain more information than a smaller one and produce better images.

Think about it this way, if you had a compact camera with a typically small image sensor, its photosites would be dwarfed by those of a DSLR with the same number of megapixels, but a much bigger sensor. Able to gain more information, the large DSLR photosites would be capable of turning out photos with better dynamic range, less noise and improved low light performance than its smaller-sensored sibling. Which as we know, makes for happy photographers.

Larger sensors also allow manufacturers to increase the resolution of their cameras – meaning they're able to produce more detailed images – without sacrificing too much in terms of other image quality attributes. For example, a Full Frame camera with 36 megapixels would have very similar sized pixels to an APS-C camera with 16 megapixels.

But I thought megapixels didn't matter!

Megapixels are a passionate issue for photographers; they're up there with the "which is better, Canon or Nikon?" debate. Some argue that no-one needs more than 16 megapixels (a couple of years ago it was eight) while others are of the opinion that the added detail is worth the trade off in terms of noise and the computer processing power needed to handle the extra large files.

The truth is that it's always going to be a balancing act between the efficiency of sensor technology, lens quality, image sensor size and ultimately what you want to do with your photographs. If you're going to heavily crop images or print them very large, extra resolution could be useful, if you're only sharing them online or producing normal prints, not so much. What we can conclusively say is that you can only make a call on megapixels in conjunction with considering sensor size.

What are the other characteristics of bigger sensors?

So larger sensors can help you capture better quality images, but they bring with them a number of other characteristics, some good and some bad. The first, and most obvious impact of a bigger camera sensor is that of size; not only will the sensor take up more room in your device, but it will also need a bigger lens to cast an image over it.

This is why smartphone makers generally stick with very small sensors, they want to keep devices pocketable and not deal with the bulk of larger lenses. It also explains why professional photography gear is still so big and heavy. The cost of producing bigger sensors also means that devices packing them also have a bigger price-tag.

If the HTC One had a Full Frame sensor rather than a 1/3-inch sensor, it wouldn't fit in your pocket, as this comparison with a Canon 5D Mark III shows

Bigger sensors can also be better for isolating a subject in focus while having the rest of the image blurred. Cameras with smaller sensors struggle to do this because they need to be moved further away from a subject, or use a wider angle (and much faster) lens, to take the same photo. Replicating a Full Frame 28 mm f/2.8 shot on a mobile phone-sized 1/3-inch sensor would take a 4 mm f/0.4 lens!

Angle of view is also something to consider when looking at cameras with different-sized sensors, particularly if using the same lenses between them. Cameras with smaller sensors than Full Frame 35 mm format (seen as the standard) have what's described as a crop factor. So an APS-C DSLR has a crop factor of 1.5x1.6x meaning that it crops into the Full Frame image – using a 28 mm lens on an APS-C giving a view similar to a 45 mm lens on Full Frame.

The image above shows what smaller sensors would have captured if using the same lens to take this photo. You can see why devices with smaller sensors use much wider angle lenses, especially by the time you reach smartphones. The lenses on these cameras are often detailed by their 35 mm format equivalent focal length to give a better idea of the angle of view they give.

The trend for cameras with bigger sensors

In recent years, camera manufactures have realized that more and more photographers are wanting the sort of better quality images that only come from having a bigger sensor. As such, we've seen devices (from smartphones to DSLRs) being sold with bigger sensors than in the past.

Within the smartphone market, Nokia has led the way with larger sensors – currently peaking with the Nokia 808 Pureview, which has a 1/1.2-inch sensor and can produce images to rival many compact cameras. In terms of point-and-shoot cameras, the Sony RX100 brings a 1-inch type sensor to the party, and Canon has released the not-quite-a-compact G1 X with a 1.5-inch sensor.

Mirrorless interchangeable lens systems have also seen small-bodied cameras fitted with larger sensors, typically ranging from Micro Four Thirds to APS-C … which have also made it to enthusiast compacts like the Fuji X100 (now the X100S) and the Nikon COOLPIX A. At the same time, the price of Full Frame DSLRs has also fallen, with the likes of the Nikon D600 and Canon 6D, bringing the affordability of big sensor shooting to a much wider market.

What do different sensor measurements mean?

Manufacturers can sometimes be strangely coy about revealing exactly how big a camera's image sensor is. And even when they do volunteer this information, it's often in a hard-to-understand naming convention … as the last section may have proved. Seriously, how many people would be able to tell you exactly how big a 1/1.2-inch or Micro Four Thirds sensor is without consulting the internet?

Bizarrely, the mostly fractional measurements used to detail sensor size date back to the age when vacuum tubes were used in video and television cameras. But the size designation is still nothing like as simple as the diagonal measurement of the sensor. Instead, it's the outer diameter measurement of a tube needed to produce an image, when the usable image takes up two thirds of the circle. Yes, it's that crazy.

It also doesn't help that different manufacturers use the same title to refer to different sizes, such as APS-C. While a Canon APS-C sensor measures 22.2 x 14.8 mm, the offerings from Sony, Pentax, Fujifilm and Nikon (DX) vary from 23.5 x 15.6 mm to 23.7 x 15.6 mm.

While we'd like to see all camera manufacturers listing the size of their sensors in millimeters, we can't see it happening any time soon. So, in the mean time, here's a couple of graphics showing some of the most common sensor sizes in relation to a Full Frame one.

Obviously there are also Medium Format cameras with even bigger sensors than those shown here, but if you're in the market for one of those, hopefully you already know how they differ.

What sized sensor is typically used in different cameras?

Smartphone Cameras – Most smartphones, including the iPhone 5 use a tiny 1/3.2-inch image sensor. In real terms this measures just 4.54 x 3.42 mm and explains how they are able to keep devices so slim and light, but also why image quality and low light performance suffers, especially when they can have as many as 12 megapixels. The HTC One uses a slightly larger 1/3-inch sensor (4.8 x 3.6 mm) and fewer pixels to combat this. The undisputed king of smartphone sensors, the Nokia 808, has a 1/1.2-inch sensor (10.67 x 8 mm).

Compact Cameras – With sensors starting as small as 1/2.7-inch (5.37 x 4.04 mm), it's easy to see why smartphones are making many compact cameras redundant. Budget compacts simply don't have sensors big enough to produce significantly better images. Typical compact cameras such as the Canon IXUS 255 HS and the Samsung Galaxy Camera use 1/2.3-inch sensors (6.17 x 4.55 mm) while more competent ones like the Canon S110, Panasonic DMC-LX7 and the Nikon P7000 come in at a larger 1/1.7-inches (7.6 x 5.7 mm).

Higher-end Compacts – With demand growing and the price of producing larger sensors falling, there are a growing number of higher-end compact cameras with larger sensors. For example the Fujifilm X20 has a 2/3-inch (8.8 x 6.6 mm) sensor while the Sony RX100 has an even bigger 1-inch sensor (12.8 x 9.6 mm). The Canon G1 X even boasts a 1.5-inch sensor (18.7 x 14 mm).

Ultra High-End Compacts – Increasing sensor size again are the growing range of ultra high end compacts. These are cameras such as the Leica X2, Fuji X100S and Nikon COOLPIX A, which all feature an ASP-C sensor (23.7 x 15.6 mm) along with a fixed-focal-length lens. There's also the Sony RX1 which does the same, but with a Full Frame sensor (36 x 24 mm).

Mirrorless Camera Systems – Within the mirrorless camera market, there is a wide range of sensor sizes. The smaller ones include the 1/2.3-inch (6.17 x 4.55 mm) sensor seen in the Pentax Q, and the 1-inch (12.8 x 9.6 mm) sensor used in the Nikon 1 Series. Panasonic cameras such as the LUMIX GF5 and offerings from Olympus (including the PEN series and the OMD EM-5) use a Micro Four Thirds 4/3-inch (17.3 x 13 mm) sensor.

Getting larger still are the APS-C offerings which include the Canon EOS M (22.2 x 14.8 mm) along with the NEX range from Sony and those those from Fujifilm (23.5 x 15.6 mm) … yes, not all APS-C sensors are the same size. Leica rangefinders such as the Leica M have a Full Frame (36 x 24 mm) sensor.

DSLRs – By the time you get up to DSLRs and other professional cameras, the sensor size has obviously increased. Most DSLRs whether from Canon, Nikon, Pentax or Sony use either an APS-C (22.2 x 14.8 mm Canon and 23.5-23.7 x 15.6 mm for others) or a Full Frame (36 x 24 mm) sensor. While Full Frame DSLRs have been the reserve of professionals for a number of years, more consumer-focused models such as the Nikon D600 and Canon 6D are now being released.

Summing Up ...

It's clear that more people are realizing that bigger image sensors mean better quality photographs (at least as much as, if not more than, megapixels) and thankfully manufacturers are beginning to cater to this demand with cameras like the Sony RX100 and Nikon COOLPIX A, which are presumably just the beginning.

That said, we'd like to see camera and smartphone makers being a bit more transparent about what size sensor is used in different devices and not hiding it away on some spec sheet in a hard-to-decipher format, or omitting it altogether. Retailers also need to step up and start publishing details on sensor size. It's only knowing (and understanding) this information that will allow consumers to make an informed decision on what they are purchasing.

Obviously, not every device can pack a considerably bigger sensor – as other issues such as form-factor and cost come into play – but do the sensors in smartphones and most compact cameras still need to be so tiny? Yes, the bigger sensor on the Nokia 808 added a significant bump, but few users seem to mind when looking back at their photographs, and a bigger sensor doesn't necessarily mean jumping to those proportions.

As sensor technology improves, we're seeing much better performance out of smaller sensors, but bigger will always be better. Does improved image quality justify the bigger device and price for you? Only you know the answer … but we hope that this guide will help you better understand the importance of sensor size when making your next camera purchase.