It all seemed so easy: you bought a nice camera, you took great pictures, and you met like-minded photographic folk. Then those photographic folk started asking awkward questions about your camera. What is the dynamic range? How’s the noise? What is the sensor size? This last question is something that a photographer needs to know and understand, because it is highly relevant to the way our cameras take images. This article will take a look at understanding sensor size and how it affects your camera.
A history lesson
To understand sensor size, let’s go back a little in time to the days of film. To most people, 35mm film seemed ubiquitous, but there were in fact many different sizes of film. Professionals often used 120 film. This created negatives with a height of 60mm, as opposed to 24mm on a 35mm film. There were also a number of formats smaller than 35mm—120, 110 and Kodak Disk springing to mind. As a rule of thumb, the bigger the negative, the better the quality. The same remains true to a certain extent with digital sensors.
Our concept of full frame comes from 35mm film
Image by Tonystl via Flickr
Because of the almost ubiquitous use of 35mm film, the holy grail of digital photography has often been seen as a sensor with the same dimensions. This is known as a full frame sensor. Today these sensors are readily available in higher-end cameras, but in the early days of digital photography, sensors were incredibly expensive to produce, and hence were often a lot smaller than full frame. When the first DSLRs started to hit the market in the early 2000s, they generally used what’s known as an APS-C sensor. These are smaller than full frame sensors, and introduced us to an important piece of information about our camera’s sensor: its crop factor.
The Canon 30D was an early APS-C sensor camera
Image by Vox Efx via Flickr
What is crop factor?
As with sensor sizes, classifications for the focal lengths of our lenses are based on the 35mm standard. Hence, anything less than a focal length of 50mm on a 35mm camera is seen as a wide angle, 50-70mm is seen as standard or normal, and 70mm and longer is seen as telephoto. This is because 50mm is generally regarded to show the same field of view as the human eye. However, if we were to use those lenses on an APS-C sensor, we get what’s known as a “crop”.
APS-C is a cropped full frame. This gives you the “crop factor”
The way to imagine this is to think of the light shining through the lens onto the sensor. That lens is designed to cover the area of a 35mm film, i.e. 36mmx24mm. Put an APS-C sensor in the middle of that beam of light and, because of its smaller size, the image that is produced is going to be cropped.
Typically, APS-C sensors have a crop factor of 1.5x. This means that if we use a 50mm lens on an APS-C camera, our field of view is the same as a 75mm lens on a 35mm camera. 50 x 1.5=75.
It is important to note that it is only the field of view that has changed. The magnification of the lens is still the same; it’s just that the actual image is cropped in. The resulting image is a little like cropping a full frame image in Photoshop.
What are the pros and cons of a smaller sensor?
There are certain advantages to having a camera with an APS-C sensor. Firstly, that crop factor is increasing the zoom of your telephoto lenses. Your 300mm f2.8 will become the equivalent of a 450mm f2.8, bringing you closer to the action. Using a crop sensor is a good way for wildlife and sports photographers to increase the focal length of a lens whilst maintaining the same quality.
The downside is that, whilst the aperture remains the same, the depth of field changes. For example, if you are using a 300mm f2.8 on a 1.5x sensor, your depth of field at 2.8 will be the equivalent of f4.
Another advantage to a smaller sensor, specifically when using lenses designed for full frame cameras, is the image sharpness. Because you are only using a portion of the center of the lens, you lose the edge softness sometimes seen on full frame cameras, giving you edge-to-edge sharpness.
A crop sensor increases the range of your telephoto lenses
Image by Marco Hebing via Flickr
Whilst your telephoto range is extended using a cropped sensor, one of the disadvantages is that your wide-angle range is also extended. This means that the beautiful 18mm that you use for landscapes will now give you a field of view equivalent to a 27mm, meaning you may need to invest in some dedicated cropped sensor lenses to get back the extreme wide angles. However, most manufacturers of both APS-C cameras (1.5x crop factor) and Four Thirds cameras (2x crop factor) now produce excellent lenses dedicated to these cameras.
The Nikon 12-24 is a wide angle designed for APS-C cameras
Image by Thor via Flickr
The next thing we’ll look at is pixel density
Let’s imagine that you have three 18mp sensors. One is full frame, one APS-C, and the last is Micro Four Thirds. To fit 18 million pixels on the much smaller APS and Four Thirds sensors, the pixels have to be much closer together. The relevance of this is that pixels that are closer together tend to produce more noise when you increase the ISO. This is because, to increase the sensitivity of a sensor, more power is sent to it. The power generates increased electromagnetic fields, which in turn manifests itself as noise. Whilst modern cameras are superb at minimizing noise, the fact still remains that a larger sensor will have less noise for the same pixel count.
Sensor size can be a complicated subject, but by understanding the topics detailed above such as crop factor and pixel density, you can gain a better understanding of how your images are affected by sensor size.