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).
Background
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 camera with an LCD for autogain framing live preview was the Casio QV-10 in 1995. The first prosumer camera to use live view for both exposure simulation live preview ES-LV control and live preview framing was the fixed-lens Canon PowerShot G1 from 2000 (possibly the first was Canon Powershot Pro70 in 1998), although this was still in the line of compact cameras. The first DSLR to use live view for framing preview only, like early other live view but non-mirrored digicams, was the fixed-lens Olympus E-10 from 2000. The first interchangeable-lens DSLR to use a live preview for framing was the Fujifilm FinePix S3 Pro, which was launched in October 2004. Its "Live Image" mode could display a live, black-and-white framing preview of the subject that could be magnified for manual focusing purposes, although the preview was limited to a duration of thirty seconds. The first interchangeable-lens DSLR following ES-LV capabilities of Canon's Powershot G1 (maybe, Canon Powershot Pro70) was in early 2005 with the Canon EOS 20Da, a special version of the Canon EOS 20D with exposure simulation live view modifications for astrophotography, which included a similar focus preview feature, plus first ever for any DSLR the new exposure simulation live view allowed capture of star exposures to selectable brightness 'effects' (Canon ExpSim LV). The first general-use interchangeable-lens DSLRs with live view for both exposure simulated live preview (ExpSim LV) and framing livepreview were the Canon 20Da of 2005, followed by Canon EOS 1Ds Mark III and Canon EOS 40D of 2007. The first general-use interchangeable-lens DSLR with live view for framing preview only like most fixed lens digicams was the Olympus E-330 of 2006.
Types 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." The same technology was used by Olympus prior to Sony in the Olympus E-10.
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