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Live preview is a feature that allows a digital camera's display screen to be used as a viewfinder. This provides a means of previewing framing and other exposure before taking the photograph. In most such cameras, the preview is generated by means of continuously and directly projecting the image formed by the lens onto the main image sensor. This in turn feeds the electronic screen with the live preview image. The electronic screen can be either a liquid crystal display (LCD) or an electronic viewfinder (EVF).
The concept for cameras with live preview largely derives from electronic (video) TV cameras. Until 1995 most digital cameras did not have live preview, and it was more than ten years after this that the higher end digital single-lens reflex cameras (DSLR) adopted this feature, as it is fundamentally incompatible with the swinging-mirror single-lens reflex mechanism.
The first digital still cameras with an LCD and live preview were the Casio QV-10 and Ricoh RDC-1 in 1995. The first prosumer camera to use live view for both exposure control and preview framing was the fixed-lens Canon PowerShot G1 from 2000, although this was still in the line of compact cameras.
The first DSLR to use live view for framing preview only was the fixed-lens Olympus E-10 from 2000. The first interchangeable-lens DSLR to use a live preview was the Fujifilm FinePix S3 Pro, which was launched in October 2004. Its "Live Image" mode could display a live, black-and-white preview of the subject that could be magnified for manual focusing purposes, although the preview was limited to a duration of thirty seconds. It was followed in early 2005 by the Canon EOS 20Da, a special version of the Canon EOS 20D with modifications for astrophotography, which included a similar focus preview feature. The first general-use interchangeable-lens DSLR with live view for framing preview only was the Olympus E-330 of 2006. The first general-use interchangeable-lens DSLRs with live view for both exposure simulated preview and framing preview were the Canon EOS 1Ds Mark III and Canon EOS 40D of 2007.
There are two distinct modes of live-preview, with only a few manufacturers offering both in their digital cameras.
The first is a more rudimentary type of live preview that displays the overall framing on an electronic display and allows a preview of what the camera's sensor will detect before the photograph is taken. This can be particularly helpful when the lighting conditions are too dark for an optical viewfinder. This type of live preview is the autogain/framing live view type (or framing priority display). This requires traditional means of exposure determination requiring metering of the light coming through the lens, and interpreting the light intensity indication on an automatic light meter, and then adjusting exposure parameters for a desired effect.
The second is a more sophisticated type of live preview that displays the exact exposure 'look' on an electronic display and allows the photographer to alter the exposure look via adjustments to parameters such as shutter speed, film speed or ISO, and aperture, before the photograph is taken. This second type of live preview is the exposure simulation live preview type (or exposure priority display). This type of live preview eliminates reliance on traditional light metering usage.
Many modern bridge and compact cameras with movie mode have only an automatic exposure and limited exposure compensation control, and live view that is primarily for framing only.
Live preview only cameras 
Live-preview-only cameras, that is cameras without an optical viewfinder, include four different categories: Compact digital cameras, bridge digital cameras, and the newer mirrorless interchangeable lens cameras and Sony SLT cameras.
Bridge cameras in general are higher-end, that is they contain more advanced features, better build quality, larger size, and are more expensive than compacts, but retain a small digital sensor. Mirrorless cameras feature a larger sensor and interchangeable lenses, like DSLRs, but sacrifice the SLR mirror mechanism and viewfinder to save size and cost, and hence only offer live preview.
The SLT, developed by Sony, uses a mirror similar to that found in a DSLR. However, unlike the moving and fully reflective mirror of DSLRs, the SLT mirror is fixed and semi-reflective. New Zealand-based British photographer Gordon Laing describes the technology:
"[A] fixed semi-reflective mirror allows around 70% of the light to pass through to the sensor for full-time live view composition, but reflects the remaining 30% or so to the phase change AF system, allowing quick and continuous autofocusing in Live View and movie modes. 30% is however too little for a traditional optical viewfinder, so Sony doesn't bother, instead using the main sensor to deliver a live image to an electronic viewfinder in addition to the main rear screen for composition."
Digital SLRs 
Live preview in DSLRs does not typically serve as their principal means of framing and previewing before taking a photograph, with this function still being mainly performed with optical viewfinder. The first DSLR to feature live preview was the 2004 Fujifilm FinePix S3 Pro.
While initially largely a novelty feature, live-preview functionality has become more common on DSLR cameras, and almost all new DSLRs have had the feature since mid-2008. This is particularly the case since the advent of movie mode features on these cameras.
The following is a partial list of DSLRs with live preview:
- Canon: 20Da, 40D, 50D, 450D, 500D, 550D, 600D, 650D, 1000D, 1100D, 1D Mark III, 1Ds Mark III, 5D Mark II, 7D, 60D, 1D Mark IV
- Fujifilm: FinePix S3 Pro, FinePix S5 Pro, FinePix IS Pro
- Leica: Digilux 3
- Nikon: D90, D300, D300s, D700, D5000, D5100, D3100, D3, D3x, D7000, D3s, D4, D800
- Olympus: E-10, E-20, E-330, E-410, E-420, E-450, E-510, E-520, E-620, E-30, E-3, E-5
- Panasonic: DMC-L1, DMC-L10
- Sony: A300, A330, A350, A380, A500, A55, and A77
- Pentax: K20D, K-7, K-x, K-r, K-5
- Samsung: GX-20
The principal function of live preview on Canon DSLRs from the 20Da onwards has been allow, via LCD viewing, rapid acquisition of consistent and predictable exposure selection before taking a photograph via its 'exposure simulation' (ExpSim) enabled mode, or exposure priority display. This mode replaces the shortcomings of traditional estimation involved in the through-the-lens (TTL) metering techniques of the older framing type live view DSLRs with autogain/framing only, DSLRs without live view, as well as the shortcomings of optical viewfinders when ambient lighting becomes too low.
Real time exposure simulation live preview (or exposure priority display) had been available in many prosumer cameras, such as the Canon PowerShot G series, before being available to in live view DSLRs. Some manufacturers of compact cameras, including prosumer models, still lack this exposure simulation live view feature, instead relying on the more rudimentary autogain/framing only live view, where slow focus and slow exposure estimation via TTL metering techniques are relied upon. Real time exposure simulation capable live preview digital cameras make achieving the desired exposures, especially in manual modes, more quickly available for both still and video photography.
Among the DSLRs that do manage to focus using the standard phase-detection sensors used by DSLRs, unlike some compact digital cameras none has managed to show 100% frame coverage. This removes one of the key advantages of live preview over optical viewfinders, especially on more budget cameras. Additionally, 100% coverage optical viewfinders have recently become more common and affordable with the appearance of the Canon EOS 7D, Nikon D300, Olympus E-3 and the Pentax K-7. Still more recently, the (upper) mid-range Nikon D7000 has brought the feature to a somewhat lower price point.
Some real time exposure simulated capable live preview digital cameras also offer live histogram graphs for tonal balance or color, where the graph changes instantaneously as exposure adjustments are made. Other features include live depth of field (DOF) preview, and live indication of overexposed areas of the image
Almost all modern bridge and compact cameras have a movie mode, while this feature only became available on DSLRs during 2008. The Nikon D90 with 720p video was announced in August 2008, and the Canon EOS 5D Mark II with 1080p video was announced in September 2008. The Canon EOS 5D Mark II was the first DSLR to offer full control over its real time exposure simulation live preview for video, rather than just autogain/framing only live preview.
See also 
- K. J. Kabza. "Evolution of the Live Preview in Digital Photography". DigitalCameraInfo.com.
- Digital Photography Review: Fujifilm S3 Pro final press release, 22 September 2004
- Digital Photography Review: Fujifilm FinePix S3 Pro Review
- Laing, Gordon (August 2011). "Sony Alpha SLT-A77 preview". CameraLabs.com. Retrieved 1 February 2012.
- Reichmann, Michael H. (2004) FujiFilm FinePix S3 Pro Luminous Landscape.
- Digital Camera and Photography: Tips, Information and News | Neocamera Blog » Blog Archive » Live-View Advancement
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