Size comparison of the most common sensor formats. Full frame (white); APS-C (blue); APS-H (red); and Four Thirds (green).
Full Frame. Full-frame sensors measure 36x24mm, same as a full 35mm film image frame. Full-frame sensors are the largest in the category and thus have room for more pixels of a given size, or larger pixels for a given pixel count, both of which tend to increase image quality, especially at higher ISO settings. These sensors also provide the same angle of view as 35mm film SLRs with any given lens and the same depth of field. The drawbacks? Full-frame sensors are costly and result in bulkier camera bodies.
Smaller sensors crop further into the full image produced by a given lens, resulting in a “telephoto” effect. In the early days of DSLRs, this made for problems for wide-angle shooters: a 28mm wide-angle lens on a 35mm SLR framed like a 42mm “normal” lens when used on a typical DSLR. Today, the camera and lens manufacturers produce very short-focal-length lenses designed specifically for the smaller sensors, making wide-angle shooting no problem.
APS. APS-C sensors are about the size of an Advanced Photo System C-format image, a little less than half the size of full-frame sensors. There are three basic sizes. Nikon, Pentax, Samsung, Sony and the new Sigma SD1 feature sensors measuring around 23.7x15.7mm, with a 1.5x focal-length factor. A 1.5x focal-length factor means a 100mm lens used on that camera frames like a 150mm lens on a full-frame camera.
Canon uses a size of around 22.3x14.9mm, with a 1.6x focal-length factor. A 1.6x focal-length factor means the 100mm lens frames like a 160mm lens on a full-frame camera. Canon also offers an APS-H sensor in its non-full-frame pro DSLR, currently, the EOS-1D Mark IV. It measures around 27.9x18.6mm and has a 1.3x focal-length factor.
Four Thirds. While most SLR camera manufacturers based their digital SLRs around their 35mm film SLRs, the Four Thirds System (introduced by Olympus in 2003) was built around an all-new, smaller image sensor design measuring 17.3x13mm, with a 2x focal-length factor. The recent Micro Four Thirds System is based on the same sensor size; the “Micro” refers to the smaller camera bodies made possible by eliminating the SLR’s mirror, mirror box and prism viewfinder. By designing the camera bodies and lenses specifically for the Four Thirds sensor, Olympus was able to eliminate some of the problems that can occur when adapting lenses designed for 24x36mm film frames to use with smaller digital image sensors.
Foveon. Sigma’s unique Foveon sensors, which records all three primary colors at every pixel site, are APS-C format (1.7x factor for the SD15 and previous models, 1.5x for the new SD1). While conventional DSLR image sensors record only red, green or blue at any single pixel site, acquiring the missing color data from neighboring pixels using complex proprietary algorithms, the Foveon sensors “stack” three pixel layers, taking advantage of the fact that light wavelengths penetrate silicon to different depths depending on wavelength (color)—the top layer records blue, the middle layer, green, and the bottom layer, red. Thus all three primary colors are recorded at every pixel site, no “demosaicing” (deriving the missing colors) or image-blurring, anti-aliasing filters are needed.
What Is “Mirrorless?”
“SLR” is short for “single-lens reflex.” The “reflex” part refers to the reflex mirror that these cameras contain. The mirror normally sits at a 45° angle, reflecting the image from the lens up to a focusing screen and then to the pentaprism (pentamirror in lower-priced cameras) where the image is switched to a laterally correct and upright orientation and sent to the viewfinder eyepiece. When you fully depress the shutter button to take a photo, the mirror flips up out of the light path so light can reach the image sensor. The mirror then automatically drops back down to the viewing position after the exposure has been made. The viewfinder image is briefly interrupted while the exposure is made—the viewfinder momentarily “blacks out.”
Mirrorless interchangeable-lens cameras, as their name suggests, don’t have this mirror. Instead, they rely on full-time live viewing with their large external LCD monitors to provide the image used for composing shots and confirming focus. Doing away with the SLR’s moving mirror, bulky mirror box, focusing screen and pentaprism or pentamirror viewfinder allows manufacturers to produce much smaller camera bodies (and eliminates the cost of these items).
Some mirrorless cameras also have an eye-level electronic viewfinder, which allows them to be used like SLRs, held up to the eye. Electronic viewfinders aren’t as clear as true SLR finders, especially for action subjects and in dim light, but they provide convenient eye-level viewing, are much more compact and cost less. With some such cameras, the electronic viewfinder is built-in (the Panasonic Lumix DMC-GH2, DMC-G2 and DMC-G10 and the Samsung NX10, for example); these models resemble very compact DSLRs. Some other mirrorless models offer a clip-on EVF as an accessory, while others don’t offer an electronic viewfinder. If eye-level viewing is important to you, make sure the camera you choose has one.
Sony’s new SLT cameras (the SLT-A55 and SLT-A33) feature an SLR design, but use a translucent mirror that doesn’t move. This permits quick phase-detection AF during live view and video operation, and does away with mirror vibration and viewfinder blackout during exposure. Pellicle mirrors tend to produce dim SLR viewfinder images, so Sony uses eye-level electronic viewfinders in the SLT cameras. These cameras provide convenient eye-level operation when shooting videos—a nice feature.