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With the above as background, you can see why we were surprised to find contrast-detect AF the only autofocus option available on the D90, and that they've carried that limitation forward on the Nikon D3100. The Nikon D300 for instance can focus and capture a shot in 0.465 second in Live view mode using phase-detect AF. While it takes some time for the mirror to drop and re-open for phase-detect AF in Live view mode, the overall result can still be faster than when relying on contrast-detect autofocus. This increases to an average of 1.49 seconds in Live view mode via contrast-detect, all other settings being the same. For example, the Nikon D3100 has a shutter lag of only 0.279 second when using the optical viewfinder and a single AF point. This accounts for the rather lengthy (and noisy) AF cycles in most Live view SLRs: To focus the camera the mirror has to be dropped, focus determined, and the mirror raised again, adding several tenths of a second to the normal non-Live view shutter lag. When the mirror is raised in Live view mode, though, light from the lens can't get to the separate AF sensor. This is fine in an SLR, when the mirror is down between exposures, as part of the mirror is typically partially transmissive, with the light passing through it deflected by a secondary mirror down to the AF sensor, usually located in the bottom of the mirror box. The catch with phase-detect AF though, is that it requires some of the light passing through the lens to be diverted to the focus sensor. As a result, phase-detect AF systems are generally much faster than contrast-detect ones. The camera can then adjust the focus setting to exactly the position needed in a single step. By contrast (no pun intended), phase-detect AF uses a system of prisms, lenses, and a secondary sensor to determine not only whether the image is in focus or not, but by how much it's out of focus and in which direction. Achieving focus this way necessarily involves some back-and-forth hunting, which can take a while to accomplish. The point of ideal focus is found by moving the lens elements back and forth and determining whether the contrast signal gets stronger or weaker. If an image is soft and fuzzy, brightness changes between adjacent pixels will be relatively slight, but if it's sharply focused, they'll be much greater. Contrast-detection autofocus involves looking at the image from a camera's main image sensor and evaluating it to see how abruptly brightness values change from one pixel to the next. Phase-detect Autofocusīy way of explanation, the phase-detect/contrast-detect distinction is one of the fundamental differences that separates digicams from digital SLRs, and is the core reason that SLRs focus more quickly. This is a fairly radical difference, compared to other camera makers. ![]() The D3100's only option for autofocus in Live view mode is contrast detection. Like the D5000, the Nikon D3100 has only one type of autofocus method in Live view, and it's not the traditional phase-detect AF. One thing we miss in the Nikon D3100, though, is the live histogram display that's an option on the Nikon D3 and on some competing SLRs from Nikon's arch-rival Canon. There's also an optional grid overlay mode, as well as a second info overlay that only shows basic exposure variables. #USE POINT AND SHOOT LIVE MODE DRAGONFRAME FUJIFILM MOVIE#It also shows microphone status and minutes/seconds of movie recording available at the current resolution and quality settings. #USE POINT AND SHOOT LIVE MODE DRAGONFRAME FUJIFILM ISO#Information displayed includes current exposure mode, flash mode, AF mode, AF-area mode, Active D-Lighting mode, image size, image quality, white balance mode, metering mode, shutter speed, aperture, exposure compensation, ISO setting, shots remaining, face detection status, battery status, and self-timer mode. The Live Mode shooting info display shows you a lot of what you'd normally see looking through the viewfinder, yet manages to keep most of the information out of the way of the live image area. #USE POINT AND SHOOT LIVE MODE DRAGONFRAME FUJIFILM FULL#Another nice feature in the Nikon D3100's Live view mode is the full information display optionally available while working in that mode. ![]()
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