Like compact digital cameras, digital single lens reflex cameras (DSLRs) use digital data storage instead of film. Single lens means the camera is not a rangefinder (an older camera type, with a different focusing system). Reflex means the light entering the lens can be directed either to the sensor or the viewfinder.
DSLRs have a number of highly sophisticated features not found on most point-and-shoot digital cameras. Here is a summary of their main features.
Most DSLRs’ lenses sport an aperture ring, which can be used to manually select the desired aperture. When the ring is placed in the auto position, the aperture value can be controlled by the camera (either automatically or user-selected).
Some DSLRs can accommodate this accessory which serves two purposes. The first is to increase the available power by adding batteries to the camera; the second is to create a convenient hand grip that can be used to shoot in portrait (vertical) orientation.
Most battery grips are equipped with basic controls and dials, as well as a shutter button, and duplicate the disposition of the controls near the main shutter.
Some DSLRs (mainly high-end) still use a top or rear monochrome control panel displaying the most important camera settings (aperture, shutter speed, ISO, shots remaining). Other cameras do not have such a control panel and rely on the back LCD to display this data.
Dust removal system
DSLR cameras often offer a dust removal system. These systems work by shaking the sensor very rapidly (the sensor is actually vibrating more than moving) in order remove the dust particles.
In addition, many manufacturers now put anti-static coatings over their sensors. These systems work with relative efficiency. One last resort, careful manual cleaning of the sensor can be required in order to remove dust.
Hot shoe for external flash
Like point-and-shoot digital cameras, DSLRs usually have a small, built-in flash. However, DSLRs can be equipped with external flashes. These flashes are more powerful and carry more advanced features.
They often can be tilted or swiveled, can zoom to accommodate the focal length of the camera’s lens and they can help with the autofocus system of the camera. These flashes communicate with the camera via connectors in the hot shoe and can work in either manual (the user selects settings), automatic (the flash calculates the desired light intensity) or through-the-lens (the camera and flash work together to select settings) modes.
DSLRs have more than one focus area usually between three and 11, though professional level cameras will have many more. The camera can automatically select one or more focus areas, or the user can manually change and lock the focus system on one specific area.
DSLR lenses almost always have a ring for manual focusing. When the camera is in manual focus mode, the ring can be rotated by the user to achieve focus. Some lenses let the user override the autofocus system at any time by using the focus ring, but most lenses don’t allow this.
Older, manual focus-only lenses usually have rings with a large “travel” distance, meaning that fine tuning of the focus is easier. More recent autofocus lenses often have a much shorter travel distance. Some autofocus lenses’ focus ring will rotate when focusing, especially entry-level lenses.
Some newer DSLR models (mainly four-thirds systems) offer a live preview of picture taken with the LCD. This is especially useful for photographers using a tripod or working at odd angles. Because a DSLR is heavy, particularly with longs lenses attached, it can difficult to hand hold the camera while using Live view.
DSLRs have a tilted mirror in front of the sensor, after the lens. This mirror is used to display the preview image in the viewfinder (see TTL). When the user presses the shutter, the mirror flips up for a brief instant, allowing light to hit the sensor. This is why DSLRs make a characteristic sound when a picture is being taken. This is also why DSLRs are typically thicker than point-and-shoot cameras, even without a lens mounted.
A feature present in some — though not all — DSLRs. Mirror lock-up means that the user can force the mirror to remain lifted. This serves two purposes:
- First, it allows access to the sensor for cleaning without risks of damaging the mirror with a cleaning tool.
- Second, it prevents camera shake created by vibrations when taking long exposures, especially in low light.
The mount is the physical connection between the lens and the camera body. Mounts nowadays are bayonet-style, but in the past some mounts have been designed with screwing parts. In addition to holding the lens and body together, the mount is also where communication between the two parts occurs.
In general, the lens informs the camera of its focal length, apertures range and sometimes physical characteristics (such as the sharpest settings, for instance). The camera can set the aperture value and control the focus mechanism, among other things.
Lenses mounts and communication protocols are brand specific, meaning that one cannot use a Canon lens on a Nikon camera. The only exception is the standardized four-thirds system.
Lenses with a fixed focal length, which means that they cannot zoom. Typically, these lenses offer the best image quality for a given price, since it’s much easier to design a lens with one focal length value than one spanning a large range of values.
It is also often easier to find prime lenses with larger apertures than the equivalent zoom lenses, and prime lenses are often (but not always) more compact than zoom lenses. However, they are often less convenient for casual use. In particular, Pentax has always offered a large range of primes.
DSLRs sensors are either CCD (charge-coupled device) or CMOS (complementary metal-oxyde semi-conductor). Both are different in design but similar in use. Currently, all point-and-shoot cameras use CCDs.
The main difference between a point-and-shoot and a DSLR is the size of the sensor, the DSLR’s sensor being as much as 14 times as large as the point-and-shoot’s. This means that more light will actually hit the sensor for a given exposure time. This, in turn, means that the sensor will be more accurate in its measurement (a CCD or CMOS is basically a device that measures the intensity of light on each of its pixels). In practice, a larger sensor will exhibit less electronic noise, will display a larger dynamic range and will perform much better in low light.
TTL (Through The Lens)
The viewfinder does not show a small and inaccurate tunnel view like on point-and-shoot cameras, but displays the actual image that will be formed on the sensor. Light travels through the lens, then hits a mirror and is eventually redirected in the viewfinder. The result is a much brighter and sharper image in the viewfinder.
The user will also directly see the effect of any filter placed in front of the lens. One downsize is that, as with most tunnel viewfinders, the TTL viewfinder generally does not show the whole scene, but between 80-95 percent of it.
Ultra-sonic motor (USM)
A faster, more accurate and quieter focusing system used in higher-end lenses. Instead of using rotating screw-like regular lenses, USM lenses use vibrations from a tiny element to create rotation and movement. Known by others names depending on the company (USM for Canon, SDM for Pentax, SWM for Nikon, HSM for Sigma, etc).
Unlike prime lenses, a zoom lens can cover a range of focal lengths, helping the user to frame an image more easily.
As with point-and-shoot cameras, DSLR zoom lenses rarely keep a constant maximum aperture over their whole range, with the aperture getting narrower as the focal length increases. Also, image quality is not always consistent with focal length.
Most lenses having a “sweet spot,” usually somewhere near the middle of the range. This is also true for point-and-shoot cameras, but much less visible because of the smaller sensor. Generally speaking, the larger the focal range of a zoom lens, the more compromises have to be made in its conception, yielding lenses with less-than-ideal image quality.
Long-zoom lenses can still please many users, however, and the added convenience of having one lens to cover a broad range means less lens swapping, a clear advantage to many users.
Usually, in addition to the image preview, a DSLR viewfinder will display the various focus areas superimposed over the image, and the most important image parameters at the bottom or side of the frame.
In order to zoom with a DSLR lens, the user doesn’t press a button as with a point-and-shoot, but rotates a ring around the lens barrel. This allows easier fine-tuning of the zoom position and means the zooming action is mechanical, not electrical (there are exceptions).