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Mirrorless interchangeable-lens camera

DSLR cameras are equipped with relatively large sensors, and are defined by having a through-the-lens (TTL) optical viewfinder – light enters through the lens, enters a light box, reflects off a mirror, then reflects off a pentaprism (or pentamirror) and exits through an optical viewfinder. When a picture is taken, the mirror flips out of the way, and the light instead hits the imaging surface (film or digital sensor).
Compact cameras are equipped with small sensors, do not have a TTL viewfinder and do not have interchangeable lenses: a small sensor can in fact be well-served by a single lens, which can even be a superzoom: see bridge cameras, some of which allow an additional, secondary lens. Small sensors, however, have relatively poor imaging in many situations, most notably low light, being unable to capture as much light as large ones. Though superzoom lenses exist for large sensors too, they suffer disadvantages in criteria such as optical quality and weight compared to more restricted lenses (prime or zoom). For this reason, virtually all modern cameras with large sensors, so-called system cameras, use interchangeable lenses.
MILCs' initial purpose was to provide DSLR-like quality imaging in a small body, to obtain which they kept a DSLR-like sensor, but replaced the TTL viewfinder with an electronic one. Recently, though, small-sensor MILCs (i.e. MILCs adopting small, compact-camera like sensors) have been introduced on the market. Current MILCs are therefore characterised just by having interchangeable lenses (like DSRLs) in the absence of a TTL view-finder. Versatility will therefore be DSLR-like, whilst image quality will either be compact-like (small sensor) or DSRL-like (large sensor).
An alternative design, hybrid between DSLRs and MILCs, is the SLT single-lens translucent camera, which features a translucent mirror (like a DSLR), particularly for focusing, but no prism or optical viewfinder, instead using live view or an electronic viewfinder (like a MILC). Due to having a (fixed) mirror and no prism, this category is intermediate in mechanical complexity and bulk between DSLRs and MILCs.

 MILC Types

Sony NEX-5
Situated between compact cameras and DSLRs, two main types of MILCs have developed: compact and DSLR-like. Compact-style ones are approximately the size of larger compact cameras and, particularly with pancake lenses, they are pocketable to some degree (can fit in a pocket). DSLR-style MILCs overlap with entry-level DSLRs, providing a contoured body and extensive features, like DSLRs, but still in a significantly smaller and lighter body.
Not all MILCs have a large sensor: Pentax Q(announced in June 2011) has a tiny 1/2.3" sensor (typical of compact cameras). In September 2011 a new sensor format has been announced by Nikon who introduced it to equip its first MILC: the CX format, with a sensor size halfway between 1/1.7" compact camera sensors and Micro Four Thirds sensors. Sony NEX looks like a compact camera with a zoom lens. 100% compatible lenses are growing in availability. Samsung NX10 and Panasonic Lumix DMC-G2 have larger bodies and a DSLR-like design (but they are still significantly smaller than entry-level DSLR).
Finally, there are some camera models on the market such as the Fuji X100 and Leica X1 which are close to MILCs in that they are mirrorless and have a large sensor. But they have a fixed lens.

 Lenses equipping MILCs

Sony has supplied 7 "E" lenses for its "NEX" system (adopting a large, APS-C sensor). Panasonic (who share the Micro Four Thirds standard with Olympus) has 11 lenses available for its "G" cameras. Panasonic lenses are also almost fully compatible with Olympus's CSC "Retro" "Pen" cameras. Likewise, Olympus's 8 micro four thirds lenses (not counting versions of the same lens; e.g., all three versions of the 14-42mm lens are counted together as one lens) are compatible with most Panasonic cameras, in addition to their own. Samsung has 6 different lenses available for its NX cameras (using an APS-C sensor)

Sensor size

There is some inevitable trade-off between sensor size and compactness of the camera, due to the size of the lens required. Sensor size varies among mirrorless interchangeable-lens cameras. The Micro Four Thirds system uses the same size sensor as the Four Thirds System (smallest among DSLRs but over nine times the area of typical compact camera 1/2.5" sensors), while the Samsung NX cameras and Sony NEX cameras use a 50% larger APS-C size sensor. The Nikon 1 series uses a smaller 1" type sensor (13.2mm × 8.8mm) with a 2.7 crop factor and the Pentax Q uses an even smaller compact camera 1/2.3" image sensor with a crop factor of 5.5, while APS-C and Micro Four Third MILCs have crop factors of 1.5 and 2.0, respectively.As of 2011, no company has announced or released a full-frame MILC, with the exception of the Leica M9 (which - being a rangefinder camera - having an optical viewfinder may be included in MILCs, but it is certainly not an EVIL).


MILCs combine some of the benefits of both compact cameras and DSLRs.
Compared to compact cameras, they offer the versatility allowed by interchangeable lenses.In addition to this, those MILCs which are equipped with a large sensor also offer all the advantages associated with it.
Compared to DSLRs, MILCs are smaller (due to fewer parts) and sturdier (due to fewer moving parts). Due to the lack of the mirror system, MILCs equipped by a large, DSLR-like sensor, can place lenses considerably closer to it (flange back distance) when compared to DSLRs. This way high-quality lenses can be made smaller, cheaper, and lighter (wide-angle lenses in particular). However, current lens selection, though growing, is still relatively limited and expensive compared with the very well-developed DSLR lens market.
Compact-style MILCs fitted with a thin "pancake" lens are pocketable, hence as portable as larger compact cameras, but when fitted with larger lenses they are less portable and not in general pocketable.
As of August 2011, prices of MILCs are higher than the cheapest entry-level DSLRs. Drawbacks
Conversely, MILCs share many of the limitations of both compact cameras and DSLRs. These include:
No TTL optical viewfinder
The lack of through-the-lens optical viewfinder (TTL OVF) is a defining feature of MILCs, and also found on compact cameras – a TTL optical viewfinder requires an optical path from lens to viewfinder, hence an SLR design or similar. If a TTL OVF is desired or required, DSLRs are the only viable option.
MILCs primarily use a rear LCD display for arm-level shooting, but some also feature an electronic viewfinder (EVF) for eye-level shooting, or an optical viewfinder that is not TTL (as in a rangefinder), which hence suffers from parallax, particularly at short distances.
Contrast detection autofocus, rather than phase detection autofocus system
Contrast-based AF has generally been slower than the phase-based AF systems found in DSLRs, often significantly so until July 2011 when the Olympus PEN E-P3 surpassed top range DSLRs in focusing speed when shooting still shots.The improvement in speed has been achieved by reducing the time taken for the contrast-detection autofocus system to begin operation after half-pressing the shutter button, doubling the sensor readout speed to 120 frames per second, and increasing the speed with which contrast detection routines operate. Although micros from Olympus and other manufacturers also have closed or leapfrogged this gap, there is still a gap in continuous autofocus accuracy and speed, and thus MILCs are still not as good at photographing moving objects, notably in sports, as DSLRs. One advantage of contrast detection autofocus is that for still subjects, autofocus accuracy tends to be higher than with phase detect systems, as the camera is using the actual sensor output to determine focus. Therefore, CDAF systems are not prone to calibration issues such as front or back focus as can occur with phase detect systems.
Incompatibility with existing lenses
All extant MILC formats use a new lens mount, which is somewhat incompatible with existing lenses – Micro Four Thirds (Panasonic and Olympus), NX-mount (Samsung), E-mount (Sony) and 1-mount (Nikon). This means both that existing lenses cannot be used without an adapter, and that relatively few native lenses exist for these cameras at the time of their introduction, as new lenses must be designed and manufactured for the new mount.
As the largest investment in a system camera is the lenses, not the body, and lenses often last decades, changing a mount and rebuilding a lens collection is a significant investment.
Adapters exist for legacy lenses. Micro Four Thirds has adapters with Four Thirds, Canon FD, Leica M, M42, Nikon, Olympus OM, Minolta, Pentax K, and C mounts. The Sony E-mount has an adapter for the older Minolta A mount, Four Thirds, Leica M, M42, Nikon, Olympus OM, Minolta, Pentax K, and C mounts.. However, part of the benefit of MILCs is that newer, smaller lenses can be used. Thus, to realize these benefits, either new lenses or lenses for short flange distance legacy mounts, such as those used on rangefinder cameras, are required. Additionally, adapted legacy lenses may not be able to autofocus on MILC bodies.
This can be compared with the situation for APS-C sized DSLRs, where the smaller Canon EF-S lens mount and Nikon DX lenses are specifically designed with a smaller imaging circle for the smaller sensor. However, they maintain the same mount distance to the sensor, providing compatibility with lenses designed for full 35mm sensor size.



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