Large-Sensor Camcorders and DSLRs
When Canon announced its EOS C300 camcorder, which has a 4K2K sensor but does not record 4K2K video, the company also announced it was developing a DSLR that will record 4K2K video. Although it may seem a bit strange that a camcorder designed to employ high-quality cinema lenses is limited to full HD video recording yet a still camera will be able to record 4K2K video, it's not strange given the history of DSLRs.
When large CCD and CMOS chips replaced an SLR's 35mm film, the next logical step was to place an LCD on the digital camera so one could review shots in the field. The next logical step was a “live view” mode that allowed one to view what was being recorded. It was only a small step to compress live view images and record them as video.
Large-sensor digital camcorders have evolved from DSLRs. It is primarily a marketing decision whether to release digital motion picture technology in a still camera package, a camcorder package or both. However, one clear advantage of a camcorder package is space for a mic jack (even XLRs), a headphone jack and manual audio controls.
When a potential buyer who is in the process of learning about 4K2K production and post production encounters the same technology in two different packages, it may prove confusing.
When a videographer shoots with a still camera, he or she will find expected camcorder functions missing. For example, every professional camcorder has some form of ND filtration; DSLRs do not.
A primary differentiator of DSLRs and traditional camcorders is their optical system. This is true for current HD and future 4K2K products.
Frame Size
While video cameras have frame sizes that relate directly to sensor size, such as 2/3in, DSLR frame size relates to 35mm film — in particular, 35mm still film. When shooting 35mm slide or negative film, each 36mm × 24mm image is placed with perforations above and below the frame.
DSLRs with 36mm × 24mm sensors are called full-frame cameras. The Canon EOS-1D X, announced for March 2012, employs an 18-megapixel 28.7mm × 19.1mm sensor. Canon calls it an APS-H sensor.
There are small variations in APS frame size: Canon APS-C (22.2mm × 14.8mm), and Nikon/Sony-C (23.4mm × 15.6mm). Both full-frame and APS sensors, when taking photos, have a 3:2 (1.50:1) aspect ratio. Panasonic uses a slightly smaller sensor for its AF100 camcorder and GH2 still camera called Micro Four Thirds (M43), which has a 1.33:1 aspect ratio and a frame size of 17.3mm × 13mm.
Sensors smaller than a full-frame sensor reduce the potential minimum DOF. Minimum DOF, of course, is a function of the maximum aperture size. A large-sensor camera does not directly provide a shallow DOF.
When a lens designed for a full-frame camera is mounted on a camera with a smaller sensor, the lens' focal length is multiplied by the lens crop factor. (Crop factor equals the ratio of a 35mm frame's 43.3mm diagonal to the diagonal of the image sensor.) A Sony APS-C camera, for example, has a crop factor of 1.5. A 50mm “normal” lens becomes a 75mm tele lens.
When shooting video, a 16:9 window on the sensor is employed. This has three ramifications. First, the viewfinder image will shrink when switching a DSLR to video mode. (This shift can be minimized by shooting 16:9 photos.) Second, the number of pixels read out will be reduced, which is a positive. Third, the lens crop factor will slightly increase. For example, when a Sony APS-C camera is switched to video mode, the crop factor increases to 1.8, thus a 50mm lens acts as a 90mm lens.
The earliest 35mm movie film had a 22mm × 18mm image, with perforations on the sides of each frame. In 1929, the Academy ratio was established. It has a 21mm × 15mm image that has a 1.37:1 aspect ratio. To obtain wide-screen, but not anamorphic, images, a Super 35 frame can be employed.
A 24.9mm × 13.9mm Super 35 frame has a native aspect ratio of 1.79:1 — a perfect match to 1.78 (16:9) HD. It also matches Quad-HD (3840 × 2160 pixels) and almost matches 4K2K, which is 4096 × 2160 pixels — a 1.90:1 aspect ratio. Not surprisingly, frame sizes that come from cinema cameras do not require the use of a 16:9 window when shooting video.
Lens Zoom System
While the videographer likely knows that DSLR lenses do not have power zoom, he or she may not know that photo lenses have other issues. For example, the zoom ring may have high friction because of the need to significantly extend the lens when zooming. Pressure exerted to start a zoom while shooting can easily cause a visible disturbance.
Better Sony lenses, such as A-mount lenses that use micro ball bearings, may cause noise that will be picked up by an on-camera mic.
AF System
Photographers are used to trusting auto-focus — even on action shots where the shooter is following a moving subject. When a DSLR's mirror is in the 45-degree-position in order for the shooter to see the subject, AF is possible. A portion of the image passes through a semitransparent area of the mirror, reflects off a small mirror mounted on the back of the mirror and is cast onto a small sensor at the bottom of the camera. The sensor, in conjunction with a processor, sends commands to the lens' AF motor to move to a position calculated to be correct for precise focus. This system is called phase detection AF.
and a processor to calculate precise focus.
DSLRs employ a different AF system when shooting video because the mirror must be continuously up. The processor, therefore, obtains information from the CMOS image sensor, which is why it is called contrast detection AF.
Mirrorless cameras such as the Panasonic GH2 and Sony NEX-5N must use contrast detection AF. (Strictly speaking, digital cameras without a mirror do not have a reflex system and, therefore, are not DSLRs.)
Contrast detection AF systems work by having a microprocessor rapidly command the lens servomotor to step forward and backward by a tiny amount. The processor notes whether contrast increases or decreases. If contrast increases, then current focus is not perfect. Therefore, stepping forward and backward continues. When there is no change in contrast, the current focus is the best possible.
Contrast detection tends to be slower than phase detection and becomes slower at low light levels. And, unless the lens is designed to be quiet, AF noise may be recorded.
Aperture System
Photography lenses are designed to click into key f-stops: f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22. Cinema and video lenses are designed so the aperture changes in a continuous manner. One solution is to use camera lenses designed by the camera's manufacturer for video shooting. The other solution is to use cinema lenses.
ND Capability
To obtain a shallow DOF with a large-chip camera under bright light — at the slow shutter speed required for the correct amount of video motion blur — an ND is a must. (ND filtration also will be required to keep the aperture under f/11 to minimize diffraction.) When a camcorder does not have a built-in filter, a shooter has three choices: mount the camera on rails on which a matte box is mounted, attach one of several ND filters to the lens or employ a vario-ND filter.
Lens Mount Type
Both cameras and camcorders that employ large sensors use a lens mount designed to work with their brand of lenses. For example, Sony's NEX family — including the FS100 and VG20 camcorders — uses Sony's E-mount. Sony markets the LA-EA2 adaptor, which enables the use of Sony and Minolta A-mount lenses. The LA-EA2 has a translucent mirror system that provides phase detection AF to many A-mount lenses.
For most interchangeable lens cameras, third-party adaptors are available. These enable you to use your favorite photo lenses on a new camera or camcorder. For example, a Sony NEX camera can use Nikon F, Canon 5D, Leica M, Leica R, Pentax, Konica Minolta MD, Olympus and Contax/Yashica lenses by using an E-mount adaptor.
Only a few adaptors, such as the LA-EA2, provide electrical signals to a lens. Without electrical connections, in-lens optical stabilization, AF and aperture control cannot function and no information from the lens is received by the camera. Therefore, modern photo lenses that send the aperture ring's setting to the AE system cannot do so.
Solutions to these issues include working with still camera and cinema lenses in a fully manual way (which may be a camera operator's first choice) or using a manufacturer's lenses that have electrical contacts.
Bringing it Home
No matter whether you shoot with a still camera or camcorder, images from the sensor must be compressed and recorded. Currently, two codecs are used for recording 4K2K: the Sony F65RAW (16-bit RAW) codec to a docking SRMaster field recorder that records to SRMemory cards or the RED R3D wavelet codec to a REDMAG solid-state drive.
Future 4K2K codec options include H.264 (as a single stream or as four HD streams) and 4K2K versions of current HD formats.
By Steve Mullen, Broadcast Engineering
