Digital Versus Film Photography - Image Quality - Effects of Sensor Size

Effects of Sensor Size

Almost all compact digital cameras, and most digital SLRs or ILCs, have sensors smaller than the 36 mm x 24 mm exposure-frame of "35 mm" film. The smaller sensors found in DSLR cameras affect:

1. Depth of field;
2. Light sensitivity and pixel noise;
3. Relative cropping of the field of view when using lenses designed for 35 mm camera;
4. Optimizing lens design for smaller sensor area;
5. Increased relative enlargement of the captured image.

Depth of field is often quoted as being greater for digital cameras than for film cameras. The maxim packages several counterintuitive aspects of photography into a single (largely correct) theorem. Depth of field, for a given lens focal length, at a given f-number will scale with sensor (film/chip) size. In effect, a smaller sensor will increase the apparent depth of field because it magnifies the portion of the image that is in focus.

Manufacturers are increasingly using (especially in the budget digital camera market) "35 millimeter equivalent" focal lengths for lenses. This gives rise to the "depth of field is greater for digital cameras" myth: the shorter the focal length of a lens, the greater is its depth of field (at fixed F-stop). Therefore, if a sensor that is one-fourth the width and height of a 24 x 36 mm frame of film is exposed to an image through a lens that is correspondingly one-fourth the focal length, the depth of field increases 16x (scaling per the square of focal length) on an absolute scale, but 4x from a comparison-of-images perspective (the imaging dimension is 4x smaller).

This increase in relative depth-of-field may have advantages for taking snapshots; more image will be in focus than with a larger sensor and autofocus system accuracy is less critical for producing an acceptable image. Contrarily, photographers wishing to decrease depth of field to create certain effects, such as isolating subjects from their background need to increase aperature for sensors smaller than 36 mm x 24 mm to achieve the same degree of selective focusing. Depth of field can be minimized by use of large format cameras, which are very rarely digital.

Light sensitivity and pixel noise are both related to pixel size, which is in turn related to sensor size and resolution. As the resolution of sensors of a specific format increases, the size of the individual pixels naturally has to decrease. This smaller pixel size means that each pixel collects less light and the resulting signal must be amplified more to produce the final value. Noise is also amplified and the signal-to-noise ratio decreases, and the higher noise floor means that less useful information is extracted from the darker parts of the image. Countering these effects of digital-signal noise are advances being made in sensor technology itself. As of 2012, the top-end of digital sensor sensitivity is at ISO 204,800 (in both Canon and Nikon DSLRs), whereas less expensive prosumer DSLR and ILC cameras offer sensitivities up to ISO 6400 or even higher, often with good noise performance at one-quarter maximum sensitivity. In recent years larger sensor digital compacts have become available. However, they still are bigger and heavier than the smallest 35mm cameras and are not full frame.

Some digital SLRs use lens mounts originally designed for film cameras. If the camera has a smaller imaging area than the lens' intended film frame, its field of view is cropped. This crop factor is often called a "focal length multiplier" because the effect can be calculated by multiplying the focal length of the lens. For lenses that are not designed for a smaller imaging area whilst using the 35 mm-compatible lens mount, this has the beneficial side effect of only using the centre part of the lens, where the image quality is in some aspects higher. Only expensive digital SLRs and very rarely expensive 'compacts' have 36mm × 24 mm sensors, eliminating depth of field and crop factor problems when compared to 35 mm film cameras.

In compact cameras, the size of the sensor is often several times smaller than the standard 36 mm x 24 mm film, with the area being typically 20 to 40 times less than that of a frame of film. This difference gives film compacts a substantial advantage when it comes to image quality and the ability to take pleasing portraits. In the standard consumer market film's advantage over digital in the compact market is often negated by operator error, the generally poor quality of the cameras or because of poor quality processing of films. The smaller sensor size of digital compact cameras means that prints are extreme enlargements of the focused image, and that the lens must perform well in order to provide enough resolution to match the tiny pixels on the sensor. Most digital compacts have sensors that exceed the maximum resolution that the lens is capable of delivering.