This Topic will focus on digital sensors, which are at the heart of modern cameras, by following the path of image formation. It will show the benefits and limitations of digital photography technology and help decipher the true meaning of announcements made by manufacturers about the performance of their devices
There are sensors of very different sizes, which are adapted to the size of the devices and the price range in which the different models are located.
Size can be expressed as the value in inches (or more accurately in fractions of an inch) or by the length/width values in millimetres, which is more telling. The value in inches is supposed to be a diagonal value, but this value is based on an old convention and does not correspond to the actual diagonal of the sensor. Moreover, it is difficult to find two tables giving exactly the same correspondence between the conventional values in inches and the dimensions in millimeters!
Sensor size and image quality
Currently, most compacts have a 1/2.5′ (1/2.5 inch) sensor or a real diagonal of approximately 1 cm. The length/width ratio being 4/3, this corresponds, according to one of the sources consulted (a choice had to be made), to a dimension of 5.1×3.8 millimeters; but it can go up to 1/1.8′ (7.5×5.3 mm). It is obvious that a 1/1.8′ sensor being larger than a 1/2.5′ sensor, we can expect better image quality… at least for devices of the same generation. Some brands even now offer full frame sensors (so close to 24×36 mm) for professional compacts (hybrids).
Bridges are very heterogeneous devices by the size of the sensors. It ranges from 1/2.5” to 2/3”, a few devices even having a sensor comparable to that of reflex cameras.
The standard size of the entry or mid-range SLR sensors corresponds approximately to the APS-C format, or 23.4×15.7 mm, giving a 3/2 length/width ratio identical to that of the 24×36 format. The most expensive devices have a format sensor almost equal to the 24×36 format of the analog devices, called full frame.
Although the size of the sensor is a very important criterion, it is seldom given in the presentation sheets of the general public compacts, the commercial arguments focusing on the number of pixels which is expressed in millions (megapixels), which is much more spectacular. However, we will see that this argument is partly spurious.
Sensor size and objectives
The small size of almost all the sensors makes it possible to use short-focus lenses and thus obtain compact, space-saving devices. For example, with a 1/2.5′ sensor, a lens with a focal length of 7.4 mm is equivalent to a 50 mm in 24×36 (the 50 mm focal length is traditionally considered to give a rendering roughly comparable to the human eye in 24×36).
The lenses of the bridges and reflex reflex are more bulky according to two criteria: the size of the sensor and, above all, better image quality as well as a more powerful zoom (in particular a bright lens must have a large lens diameter and a good zoom is always cumbersome due to the number of lenses that make it).
Reflex cameras with an APS-C sensor (or a similar format) can be used with conventional lenses for 24×36. However, since they only use the central part of the image, a correction factor of the order of 1.5 to 1.63 must be applied to them, which means that a 50 mm focal length lens is approximately equivalent, for the reproduced field, to a 75 mm on a 24×36. The advantage of not taking the edges of the image is to eliminate these less well-corrected areas. Indeed, the sharpness of the lenses tends to be less good at the border and we often observe vignetting (the edges of the image are a little darker, especially in the corners) with strong apertures of diaphragm and wide angle. On the other hand, since the image needs to be larger in print than a 24×36 image due to the size of the sensor, a medium but very acceptable 24×36 lens will be able to show its limits on a digital camera in APS/C format.
We have therefore developed a series of lenses specially adapted to DSLR cameras using an APS-C sensor. These lenses are less bulky than their counterparts for the full frame format (an 18-85 mm zoom is roughly equivalent to a 28-135 mm for the 24×36 format). This smaller size is an advantage for the handling of the camera, a reflex being bulky by nature and quite heavy, especially with very open (bright) optics or powerful zooms. The digital lenses are corrected against the risk of reflection that can occur between the sensor and the rear lens, a problem that we had much less with the film. Of course, you can’t use the APS-C lenses with a 24×36 sensor because they project an image of too small a diameter.
It should be added, to conclude this section, that the price of the devices increases dramatically with the size of the sensors. The cost of the sensor itself is added to the cost of other improvements, since we are in the category of professional devices.
3. The useful size of a sensor varies slightly from manufacturer to manufacturer and even model to model. This explains that according to the devices the ratio is in the order of 1.5 to 1.6.