Various alternative names exist – including: Compact System Camera (CSC), Mirrorless System Camera (MSC), Digital Single Lens Mirrorless (DSLM), Digital Interchangeable-Lens System camera, and – finally – Electronic Viewfinder with Interchangeable Lens (EVIL); this latter term not applying to cameras with an optical viewfinder.
Benefits
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. Thus 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.
Noise on shutter activation is quieter as there is no moving mirror. In August 2011 prices of MILCs were higher than the cheapest entry-level DSLRs[citation needed], but decreased sharply and, as of November 2011, some models sold for less than high-end compact, non-system cameras from the same manufacturers.
Drawbacks
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 taking lens to viewfinder, hence an SLR design.
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[9], often significantly, until July 2011 when the Olympus Pen E-P3 surpassed top range DSLRs in focusing speed for 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 (Olympus are soon to use 240 fps in some focus modes), 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.
Nikon's "One" system incorporates phase focusing together with contrast-detection autofocus, and Nikon claim it is as fast focusing for sport as their high end 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 uses 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.
Sony has recently announced an adapter system for their NEX series EVIL cameras that allows their SLT mirror technology to be mounted to NEX cameras by way of adapter. This adapter will allow the E-Mount camera to use A-Mount lenses and bring real time phase detection auto focus for both still and video photography.
Incompatibility with existing lenses
Most MILC camera systems use a new lens mount, which is somewhat incompatible with existing lenses – Micro Four Thirds (Panasonic and Olympus), NX-mount (Samsung), E-mount (Sony), 1-mount (Nikon) and EF-M mount (Canon). 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. The only exception is the Pentax K-01, a mirrorless camera that accepts all legacy K-mount lenses without any adapter, but the consequence is that the K-01 is not as slim as the Sony NEX-7 or the Nikon 1 V1, although slimmer than one of the smallest SLRs on the market, the Pentax K-5.
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 although most do not support autofocus on MILC bodies. 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, Canon FD, Leica M, M42, Nikon, Olympus OM, Minolta, Pentax K, and C mounts. The Nikon 1 series has an adapter for the company's F-mount, and the Canon EOS M will be introduced with an adapter for that company's EF and EF-S mounts. However, part of the benefit of MILCs is that newer, smaller lenses can be used; to realize these benefits, either new lenses or lenses for short flange distance legacy mounts, such as those used on rangefinder cameras, are required.
This can be compared with the situation for APS-C sized DSLRs, where the Canon EF-S and Nikon DX lenses are specifically designed to cover only the smaller imaging circle required for the smaller sensor, reducing lens size and manufacturing cost. However, they maintain the same mount distance to the sensor, providing compatibility with lenses designed for the larger full 35mm sensor size.
This drawback, however, is somewhat balanced by the fact that most MIL cameras are aimed at the "point-and-shoot" market where users rarely build a large system around their cameras and are usually satisfied with the lens supplied with the camera or, at best, purchase an all-around "super-zoom" lens to cover all possible shooting situations.
For manufacturers, this strategy eliminates price competition for their new lenses from second-hand legacy lenses.
source: wikipedia
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