One of my email subscribers asked me for a post on this subject which often confuses people - especially when it comes to talk of crop factors and ratios. Here I hope to explain simply the differences between sensors and what that means to your photos.
Firstly - the bigger the sensor the more visual information it can record - that's fairly self explanatory but what does that actually look like?
To help demonstrate it I've shot exactly the same photo on a number of my cameras to demonstrate how much of the scene each can capture:
Fuji X100T - APS-C sensor: 23.5 x 15.6mm
Canon 7D- APS-C sensor: 22.3 x14.9mm
Canon 5D MK iii - Full frame sensor: 36 x 24mm
All three cameras were set to the same focal length and placed on a tripod.
As you can see the bigger the sensor the more of the scene is captured.
Full frame cameras shoot the equivalent to the old 35mm film whereas the other cameras have what are usually called cropped sensors.
It's also worth noting that whilst the Canon 7D and the Fuji X100T both have APS-C sensors they are different. So the actual size of the sensor in a camera may be important if comparing models.
Secondly - there is a difference in the quality of what a bigger sensor can capture. In essence the bigger the sensor the larger the photosites (think of them as squares) on the sensor. Each square is captures a pixel of visual information. This will mean the camera is better in low light and the images will suffer less from digital noise.
On a cropped smaller sensor these photosites are obviously smaller.
Thirdly - there is a difference in the depth of field effects. Going full frame will enable you to get those shallow DOF effects whilst being much closer to your subject than when using a smaller sensor camera.
Lastly - if upgrading from a cropped sensor to a full frame body you will need to check whether your existing lenses can operate on a full frame body. For example my wide angle lens (which is a third party lens from Sigma) is not compatible with my full frame camera.
Of course the quality of an image is not just down to the sensor size - lenses , settings and the photographer's skill are also factors!
Do you need a full frame camera? It depends on what you are trying to shoot. I upgraded to full frame mainly to get low light capabilities that my cropped sensor camera didn't have. There are certainly lots of benefits but it is a much heavier camera!
There are more and more coming onto the market especially in the compact system camera category so there is now plenty of choice - even at entry level prices.
A quick note about smartphone cameras and sensors. Obviously the smartphone's sensor has to be tiny in comparison. In recent years there has been a significant increase in the size of sensors in smartphones but also significant developments in the technology that enables us to get great results despite the size of the sensor. Size of sensor, therefore, may become a more significant factor when choosing your next handset.
There's a great article here if you want more information.