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 the announcements made by manufacturers about the performance of their devices.
First and foremost, four categories of digital devices should be briefly defined: compact, SLR, hybrid and bridge. Find out what their particularities are and make your choice.
Digital devices: the compact
By far the best-selling category is compact, the ultimate consumer device. There is the best and the worst in terms of the quality of the objectives or the electronic equipment. The ultra-compact are extra-thin compacts that can easily slip into the pocket.
This type of device derives from silver cameras of the same category except that the digital technology has made it possible to further reduce its size since we are no longer constrained by the need to house a 35 mm film cartridge in the case. It is also possible use less bulky lenses. These cameras are equipped with an LCD screen on the back which serves as a viewfinder and which also allows to view the photographs already taken. They may or may not have a more conventional optical viewfinder. They have a “central” shutter built into the lens. However, unlike silver compact cameras, this shutter remains constantly open to allow continuous viewing of the targeted image on the LCD screen. It only intervenes briefly at the time of the exhibition itself.
This category offers a very wide range of prices but it is losing speed in front of the generalization of smartphones that offer correct or even very good photographic possibilities for some.
Another type derived from traditional photography is reflex. Basically, it is a case on which you can set a wide range of objectives covering the most varied needs (and also budgets). The image formed by the lens is intercepted by a 45° mirror which projects it onto a horizontal frosted glass located at the top of the exposure chamber. This frosted glass is surmounted by a pentaprism (or a pentamirror in some entry-level models) which returns the image to the eye after straightening it. The frosted glass is seen through an optic that plays the role of a magnifying glass to enlarge the image.
The frosted glass, pentaprism and optical set forms the viewfinder. By convention, for a 24×36 reflex, it is said that the viewfinder has a magnification ratio of 1 if the image formed by a 50 mm lens has the same angular dimension as the same scene seen with the eye. Indeed, we consider that, for this format, a focal length of 50 mm provides a rendering of the perspective close to that of the eye (which is actually not very accurate). Due to the small size of most digital sensors, it is difficult to reach this ratio of 1 for the viewfinder of many DSLR’s. At the time of shooting, the mirror folds up so that the image forms on the sensitive surface (film for silver models, sensor for digital models) and then returns to its initial position.
In a SLR, the LCD screen is not used as a viewfinder but is used only for posteriori visualization of images taken and menus. However, 2007 saw the appearance of some rare reflex cameras that can also display the image in real time on the LCD screen (live view mode during which the mirror is raised) and this solution has since spread widely. Due to the technology of this type of device, this mode is subject to various constraints, such as the heating of the sensor, which means that its use is recommended only in a few specific cases where the conventional aim would be uncomfortable or impossible (for example, to avoid lying on the ground by taking a small flower in macro photography, or when it is necessary to stretch the camera at arm’s length to have a favorable angle of view).
The shutter is a focal plane, located directly in front of the sensor. It was formerly called a curtain shutter. In its initial form, it was indeed formed of two curtains of opaque fabric. Let’s take the example of an installation time of one sixtieth (1/60th) second. The first curtain, stretched in front of the film, unmasks it when pressing on the trigger by moving horizontally at high speed. Then, 1/60th of a second later, the second curtain starts to hide the film. Of course, for much shorter exposure times, the second curtain starts before the first one has finished its movement so that the sensitive surface is swept by a slit the narrower the exposure time is shorter. When the shutter is rearmed, the two curtains are simultaneously brought back to their initial position, without allowing light to pass through.
Various evolutions have improved this basic mechanism, replacing it with a vertical movement (faster since it has to sweep a smaller width) and replacing the old curtains of opaque fabric with metallic or very light plastic shutters. It is currently possible to reach 1/4.000th of a second, or even 1/8.000th on some devices. A restriction of this type of shutter is that flash can only be used for speeds where the entire sensitive surface is discovered because the flash flash is extremely short. With the old shutters, the shutter speed could not exceed 1/60th of a second. With current models, this speed (referred to as “synchro-flash speed” or “synchro-X”) varies from 1/125th to 1/250th of a second. The shorter this speed, the better the shutter.
Due to the complexity of the mechanism that ensures synchronous operation of the two curtains in a fraction of a second, as well as the one that allows the mirror to be quickly retracted, reflex cameras are marvels of mechanics.
It should be noted that entry-level or mid-range enclosures are generally sold as a kit with a “go-anywhere” zoom of medium quality (even for reputable brands), or even mediocre, in order to display an acceptable price. It is much better to buy the housing and the lens separately, even if it means taking a smaller housing and a good lens. However, we understand that this type of equipment should be reserved for knowledgeable amateurs, able to find and understand the technical information necessary to make this choice.
This category emerged and developed well only a few years ago. Basically, these are interchangeable lens compacts that have larger sensors than traditional compacts (APS-C and even full frame for some). These are high-quality and expensive cameras that have the same advantage as reflex cameras for optics but are much more manageable and discreet. They use an electronic viewfinder that allows a more natural and stable hold of the device than the sight on a screen on the back of it. Various professionals use them more and more in addition to reflex cameras, or even in replacement of them when maneuverability is necessary.
They are named like this because they bridge the gap between compact and reflex cameras. It is a fairly disparate intermediate category in terms of the quality of the optics, the definition and the possibilities of adjustments. Their case is more bulky than that of compact but less than that of reflex cameras.
Their lens is also more bulky, which is necessary to provide good optical quality and brightness. It is usually a zoom of high amplitude (zoom factor of the order of 1 to 30, unlike compacts where it is usually of the order of 1 to 3). This lens is not interchangeable unlike that of reflex cameras, except for a few models.
Like the compact ones, the bridges are equipped with a real-time display of the image on an LCD screen located on the back of the device, a device appreciated by amateurs, and often an electronic viewfinder. In some models, the screen can be steerable.
The choice of a bridge does not necessarily guarantee a better image quality compared to a compact. On the other hand, if you go to the trouble of reading unbiased reviews that may be found in specialized magazines or websites, or in the records established by some major resellers, it is possible to acquire a quality device. This type of model is therefore to be advised to all those who want to do more than what can offer a compact, without facing the complexity… and the price of a reflex. However, the boundaries have shifted with the appearance of hybrid compacts whose exterior appearance is comparable. The difference between the two categories becomes blurred except that, typically, a bridge has no interchangeable objective.
Note also that compacts and bridges allow to make videos. Currently, most models are in Full HD; those that only offer 720p mode are to be avoided. It is also necessary to check that they operate at a frequency of 50 or 60 frames per second. The video mode appeared later on the reflex cameras, to the great displeasure of the purists for whom a reflex should only make photos!
It should be pointed out that a classic defect in digital cameras (except reflex cameras) is the considerable delay between pressing on the trigger and taking the photo. For moving subjects (for example children photographed unexpectedly), this disadvantage can lead to catastrophic results. This can be a criterion for separating two aircraft, especially since it is easy to control. The same defect is also generally found with smartphones and worsens for many models if the brightness is low.
This difference in speed is due to the fact that compacts and smartphones focus on finding the maximum contrast while in a reflex camera, a small auxiliary mirror diverts part of the light to specialized sensors with phase detection, much faster.