Why do digital cameras still use an SLR mirror and optical viewfinder?
Asked 8/10/2012
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If a digital camera can show a live preview from its sensor on the rear LCD, why is an SLR mirror/viewfinder system still used? Wasn’t the SLR design mainly for seeing exactly what the lens sees on film, and doesn’t live view make that unnecessary in digital cameras?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
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I suspect the major reason this is true for DSLRs is to get the lightning-fast focus times that point-and-shoot cameras don't have. The autofocus mechanism is not actually part of the main CCD/CMOS sensor, but a separate device in the camera body, and the mirror splits the light coming through your lens so that half goes to the viewfinder and half goes to the autofocus sensor. See, for example, this site describing the differences between a mirrorless camera and a DSLR; note in Figure 1 the autofocus module below the mirror.
This autofocus module is doing a phase detect autofocus, which is extremely fast. Without a mirror, you have to do contrast-detect autofocus, which is slower. Recent cameras (e.g., the Sony a55), have gotten their autofocus fast enough that they no longer need a mirror, but it's taken quite a long time for the technology to get there. So I suspect the trend will be towards cameras with DSLR quality and focusing speeds, and no mirror (perhaps with an electronic viewfinder, instead). But it's only been recently that such things have become possible.
Originally by user9104. Source · Licensed CC BY-SA 4.0
user9104
14y ago
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Mostly, yes: live view means a digital camera does not strictly need an SLR mirror and optical viewfinder. That’s essentially the idea behind mirrorless cameras.
DSLRs kept the SLR design for a few practical reasons:
- Fast autofocus: traditional DSLRs use a separate phase-detect AF module fed by the mirror system, which has historically focused faster than contrast-detect systems used by many early live-view cameras.
- Optical viewfinder: some photographers prefer a direct optical view through the lens rather than an electronic display.
- Power use: an optical finder works without continuously powering the sensor and screen/EVF, which can save battery life.
Mirrorless cameras remove the mirror and use the sensor itself for preview and focusing. This can make cameras smaller and lighter, and modern systems have become much better at autofocus, reducing the old advantage of DSLRs.
So your thinking is basically correct: digital cameras don’t inherently require an SLR mechanism, but DSLRs persisted because the mirror system offered advantages in autofocus, viewing, and power efficiency.
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