The digital sensor sitting behind the reflex mirror and the lens is the heart of the digital camera. It is here that the magic of photography technology meets light, the be all and end all of all photography. If light is the fundamental aspect of photography. The digital sensor comes in second on that list. It is the same thing as the film in film cameras. With advancement in digital technology and better production processes, it has been made possible to reduce the cost of manufacturing these digital sensors. Thus, we are able to see better, faster, more efficient and affordable digital sensors seeing the light of day. This article is not about the quality of the sensors though, but about their size factor and the associated pros and cons.
For this discussion we shall look at two basic digital sensor sizes – the larger full-frame sensor which mimic the size of the erstwhile 35mm film with a size roughly of 36mm x 24mm and the smaller and more common APS-C sensor. The name APS-C also comes from a film size that was discontinued years ago. Anyways, the APS-C sensor size varies from manufacturer to manufacturer. Nikon e.g., produces an APS-C sensor, typically used in the Nikon D7200, which measures 23.5mm x 15.6mm. Canon produces an APS-C sensor, typically used in a camera like the EOS 7D Mark II, which measures 22.4mm x 15mm.
There are advantages and disadvantages in the use of both these sensor sizes (full-frame and APS-C) and in this discussion we shall look at a few of them. The first and the most obvious advantage is that a larger sensor size increases the amount of light that is gathered. More light is better in a number of ways. More light ensures that a properly exposed image is possible even when shooting in low light situations. Thus, you could avoid shooting at the maximum aperture or at a high ISO at all times.
Let’s say that you are shooting after dark portraits and want to capture the twinkling street lights and the general ambience of the place using a small aperture. With a small sensor and therefore reduced light gathering, you cannot do that unless you increase the ISO. But increasing ISO would expose your images to noise. With a larger sensor, and therefore increased light gathering, you can shoot with a small aperture, get a large depth of field and yet avoid shooting at a small ISO.
Increased surface area of the sensor, with the resolution remaining the same, also means an enhanced physical size of the individual pixels that make up the sensor. Each pixel represents a photo-diode. The task of that photodiode is to capture light, convert the photons into electronic signals and then transfer these signals to the image processing unite inside the camera for further processing into a JPEG image. If the surface area of the sensor becomes small, the individual size of that pixel becomes small as well. Smaller the pixel, smaller is its light gathering capabilities and increased associated noise. A fundamental truth of sensor size, thus, is that if the resolution is the same, increasing the sensor size produces cleaner imagery.
Tonal range denotes the individual degrees of change in light from the brightest to the darkest areas of your image. That means an image with a greater tonal range translates into that you can see a wider variations of tones across the brightest and the darkest areas of the image. An image with a greater tonal range looks sharper and more contrasty than one where the histogram appears skewed either left, right or middle.
Smaller sensors have a lower tonal range compared to large sensors. Tonal range is often a touchy subject in photography. Now that we have cameras with built-in HDR modes. HDR stands for high dynamic range, a style of shooting where a composite image produced by merging two or more images that are shot using different exposures to show a dynamic range impossible in a single capture.
Lenses that are designed for a larger full-frame camera, when mounted on a smaller APS-C camera would produce an effect that is sometimes referred to as ‘crop factor. Let’s understand in deeper details how this really affects your images.
A full frame camera, as you can imagine, has a larger sensor, significantly larger than a crop sensor. Lenses which are designed for these sensors thus, have a larger image circle. When you mount the same lens on a smaller crop sensor camera a significant portion of the image coming through the lens is not recorded by the sensor at all. That portion of the ‘image’ which falls outside the area of the sensor is actually wasted. If you look at the resulting image, it would appear as if the image is slightly ‘zoomed-in’. In reality, however, it is not. The image simply have been cropped in camera because as explained above, the entire image is not being used by the smaller sensor.
This has some advantages. The prime advantage is that with smaller sensors you can multiply the focal length of your full-frame lens. Nikon APS-C (also referred to as DX cameras) have a crop factor of 1.5. Whereas Canon APS-C cameras have a crop factor of 1.6x. Thus, a sharp 50mm f/1.8 lens becomes a 75mm (or 80mm depending on the crop factor of the brand of camera that you are using) f/1.8 lens. The longer the focal length of the lens in question, the better it is. Thus, the focal length of a 70-200mm lens will become 105-300mm (or 112 – 320mm depending on the crop factor of the camera system).
Sensor sizes will affect the depth of field of the image produced. Larger the size of the sensor, shallower is the depth of field and vice versa. That means if you shoot with the same lens, from the same spot and the same camera settings, you are going to get a shallower depth of field with a larger sensor compared with a smaller sensor. The advantage is that with a small sensor camera you can get more of the scene to be acceptably in focus. On the other hand when you need to capture a lot of background blur the larger sensor is the better choice.
It is however, possible to capture the exact same image with the same depth of field with two different camera / lens systems. This is provided you know how much to tweak the settings so that the focal length and the aperture difference is equalized. Let’s say that you are shooting with an APS-C camera with a crop factor of 1.5x, with a 60mm lens set to f/8. To get the exact same depth of field on a full frame DSLR, you will need to shoot with a 40mm (60/1.5) lens set to around f/5 (8/1.5).
Most definitely, a reference to depth of field brings us to the next point of difference and that is field of view. With increasingly smaller sensor size, the field of view will become increasingly smaller. This is because smaller the sensor size, more of the image it will crop out.
A larger sensor can ‘see’ more of the scene than a smaller crop sensor. When making landscape, architecture and other scenes, a larger sensor is a much better proposition than a crop sensor. You can make full use of an ultra-wide angle lens mounted on a full-frame sensor than if you mount the same lens on a smaller crop sensor.