How DSLR cameras work

Viewfinder Information Projection

A DSLR viewfinder includes various information systems that keep the photographer informed about the camera settings used, autofocus status, and more.

Viewfinder Side LCDs

In most digital single-lens reflex cameras, the viewfinder does not only display the preview of the scene but also additional information on the status and settings of the camera. Of course, all configurations can always be displayed on the main display on the back of the camera, but on-viewfinder screens practically summarize the current settings so that a photographer does not have to take his eye from the optical viewfinder in order to check the configurations. Most entry-level cameras only have one horizontal display strip placed directly underneath the preview screen. Conversely, for most pro-level cameras, a second display strip is applied vertically next to the preview screen. The image illustrates the viewfinder of a Canon EOS 1D X DSLR camera. The two displays mentioned are colored in cyan.

Looking at the technical implementation of these viewfinder screens, these usually consist of liquid crystal display (LCD) panels applied to the lower part of the viewfinder system. The LCD panels produce a negative image of the desired information and get backlit with LEDs. Although the LCDs have to be in the focal plane for a sharp and focused projection of the information, these LCDs cannot be directly adjacent to the focusing screen due to space limitations. In order to place them in a position equivalent to the focal plane, they are applied to the pentaprism sideways at a short distance. The image shows the position of both the vertical LCD (V-LCD) and horizontal LCD (H-LCD).

The LCD panels are optically connected to the viewfinder system by tiny prisms, reflecting light from the LCDs to the lower pentaprism surface, directed towards the eyepiece. Both side LCDs produce images that are flipped as they get corrected automatically when travelling through the pentaprism. The figure illustrates the projections of light from the side LCD panels towards the eyepiece.

Autofocus (AF) Points

Probably the most useful piece of viewfinder information is a visual confirmation on whether the image is in focus or not. Virtually any digital camera has some kind of autofocus indicator. The most common method is to display individual AF points or small rectangular AF frames (or a combination of both) on the viewfinder's preview image. The positions of the AF points are typically pre-defined and depend on the camera model. Entry-level DSLR cameras usually only provide a limited amount of AF points (up to 10) while professional DSLR models provide a larger set of indicator spots spread over the entire preview screen (over 60 focus indicators on the Canon EOS 7D Mark II, released in Sep 14).

It is important to understand that these are only indicators and no autofocus detectors. Each AF point indicates which part of the image can be used for autofocus detection of the camera. The exact position of AF points is predetermined by the autofocus system of the camera. The photographer can select single AF points or an entire group of AF points to be active when using the autofocus. If the camera has detected correct focus for a certain area of the image, it generates some form of feedback for the photographer to confirm the in-focus condition. On some DSLR cameras, this feedback is produced by flashing up the the corresponding AF point. Over the past decades, DSLR manufacturers have developed a variety of different methods for AF point illumination:

Rear Illumination

The rear illumination looks back on a relatively long history. This technology has already been used for analog cameras in the 1980s and was later replaced by the Superimpose LCD. In a viewfinder with rear illumination design, the superimpose array is applied to the rear surface of the pentaprism. This superimpose array consists of light emitting diodes (SI LEDs) and a prism (SI Prism). Light from an LED is guided through the SI Prism to the rear surface of the pentaprism. As light continues to travel through the pentaprism and through the condenser lens, it will eventually illuminate the AF point that is engraved into the focusing screen. The AF point in turn will reflect the light towards the pentaprism where it will be directed towards the eyepiece. This effect is what the photographer will perceive as the AF point lighting up.

In viewfinders of these early cameras (for example Canon EOS 1N, 1994), the AF points are usually arranged in a horizontal line in the middle of the focusing screen. Due to this simple layout, the SI LEDs can also be arranged in a vertical strip. A pinhole mask is applied in front of the LEDs to minimize their beam angles in order to make sure that each LED only illuminates its intended AF point. The illustration shows the rear illumination array of the Canon EOS 1N analog SLR that includes five AF points in a row. It should be noted that this lateral cross section can only display the center LED illuminating the center AF point. The remaining LEDs as well as the remaining AF points are located outside (2) and inside (2) the drawing plane.

Superimpose LCD System

A slightly newer technology is the use of a Superimpose LCD System. This technology also requires the AF points or AF frames to be engraved in the ground glass. In contrast to the rear illumination, the idea of the Superimpose LCD System is not to have the AF points reflect light from a light source behind the pentaprism, but to create the impression to the photographer as if they lit up directly.

The Superimpose LCD System is located in close proximity to the pentaprism and includes a light source (SI-LED) and a transmissive liquid crystal display (SI-LCD). The LCD acts like a digital mask that can either block all light from the LED or allow certain areas to become transparent. When a particular AF point needs to light up in order to confirm correct focus, the LED emits light and the LCD partially turns off to let a small beam of light through. A mirror reflects light beams from the LCD into a downward direction where a light combining unit will again reflect them towards the eyepiece, effectively superimposing the viewfinder image with the image of the LCD. The illustration clarifies the configuration of a Superimpose LCD System showing the cross section of a Canon EOS 1D Mark IV viewfinder.

Liquid Crystal Display (LCD): The liquid crystal display is primarily responsible for the superimposing effect. The display itself is a thin structure of various transmissive sheets of glass with a layer of liquid crystals inbetween. An additional layer of glass substrate directly adjacent to the liquid crystals includes electrodes that can turn the liquid crystals into an opaque surface by applying an electric field. The electrodes on the substrate are applied exactly in the same shape and configuration as all the AF points or AF frames on the focus screen of the individual camera, just reduced in size. In order to light up a particular AF point in the viewfinder, the LED flashes up and a beam of light that is condensed through the dome lenses illuminates the backside of the liquid crystal panel. At the same time, the LCD is turned to an opaque state and is therefore blocking all light from the light source. However, the LCD is activated in a way that the AF point or AF frame that is representing the in-focus situation is left transparent. The LCD is now a negative image of a single focus point on the ground glass. This now projects a beam of light in the shape of an AF point towards the light combining unit.

Superimposing Lenses: The beam of superimposing light travels through an array of various lenses (SI lens I, SI lens II). These are configured in a way so that the LCD display appears to be at the same distance from the photographer's eye than the focusing screen. As the LCD is a lot smaller than the camera’s focus screen, the condensing power of the superimposing lenses and that of the eyepiece lenses are combined to let the LCD appear similar in size with the focusing screen.

Light Combining Unit: A light combining unit is either composed of a dichroic mirror or a dichroic prism. The dichroic mirror is roughly 90% transparent for most wavelengths but is completely reflective for 700 nanometer (red color) light. It is located where the light rays coming from the Superimpose LCD System and the ones exiting the pentaprism intersect. In this position, the light combining unit reflects light rays from the Superimpose LCD System towards the eyepiece, completely superimposing light from the pentaprism. This arrangement creates the illusion of the Superimpose LCD Screen to be exactly in the position of the focus screen. Light from the Superimpose LCD is virtually combined with light from the viewfinder image, hence the name.

The Intelligent Viewfinder Display

Intelligent Viewfinder Display

An intelligent viewfinder display represents a more recent concept used to display both photographic information and AF points. It consists of an LCD screen that can display countless different shapes and indicators such as AF points or grid lines on demand. Information on the LCD is normally turned off so that it does not obstruct the preview image.
The Canon EOS 7D was the first DSLR camera to be released with an intelligent viewfinder to provide a customizable overlay screen. The 7D has a set of 19 AF frames but does not display all of them simultaneously as they would cover a large portion of the viewfinder image. The 7D only displays the AF frame that has been selected for autofocus, and the surrounding AF expansion points if these are used. The 7D's intelligent viewfinder can also show architectural grid lines, but these are not active by default. Its successor, the Canon EOS 7D Mark II used the intelligent viewfinder to display a lot more information including a digital level and other photo-taking information (flicker warning, file format, battery charge status, etc.).
In order for the intelligent viewfinder's information to be focused and clearly visible, the LCD unit is installed directly above the focusing screen where the preview image is formed. In general, this LCD has the same structure than the LCD used for the superimpose LCD system except that the intelligent viewfinder LCD is configured to display a positive image of the information. This means that the LCD blocks light from passing through the LCD where information is supposed to be displayed. Also, the intelligent viewfinder LCD is much larger in size than the superimpose LCD as it needs to cover the size of the ground glass. The image shows the layout of an intelligent viewfinder as used in Canon DSLR cameras.

With this concept, AF points or AF frames are not illuminated separately. An in-focus situation is simply confirmed with a short beeping sound, and therefore it is not required to have features to light up individually. However, the intelligent viewfinder flashes up all information currently shown on the screen when the camera's autofocus system has measured a difficult low-light condition. This red-colored flash both enhances visibility of information and informs the photographer about the low-light condition that might lead to a possible reduction in autofocus capability.
To be able to illuminate features displayed on the intelligent viewfinder, two arrays of red illumination LEDs are attached to opposite sides of the LCD screen. Light from these LEDs is led into the sides of the LCD (edge lit LCD). With no piece of information displayed on the LCD, red light from the illumination LEDs bounces off the large LCD surfaces due to internal reflection, and ultimately exits the panel on the opposite side. By contrast, as soon as the LCD panel produces some information, light introduced from the illumination LEDs will now interact with the liquid crystals and light them up. These glowing LCD crystals in turn will emit light perpendicularly towards the large LCD surface, allowing it to exit the LCD on the large surface. This light travels through the viewfinder system in the usual way and will appear as the intelligent viewfinder flashing up its information in red. The image summarizes the general concept of an intelligent viewfinder.