![]() ![]() Achieving focus this way necessarily involves some back-and-forth hunting, which can take a while to accomplish, since there's no indication as to how close the camera is to focus nor which direction focus needs to be adjusted. 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. When focus is locked, the focus box changes from red to green.īy 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. One advantage of contrast-detect AF is that you can put the AF box wherever you like it. Nikon D300S Live View Contrast-Detect Autofocus Unlike the D90 and some of the competition, the Nikon D300S does not offer a face-detection focusing option. Tripod mode is more useful for static subjects with the camera mounted on a tripod, offering two main benefits: you can move the AF point anywhere you want within the frame area, and front- or back-focusing issues shouldn't come into play. As Nikon's naming clearly suggests, Handheld mode is the more useful of the pair for shooting handheld and/or moving subjects, since the AF operation itself is faster. Tripod mode, meanwhile, uses contrast detection on a feed from the image sensor. Handheld mode uses the camera's dedicated phase-detection sensor. ![]() Like the D300 before it, the Nikon D300S provides two modes for autofocus operation. Of course, the aperture and/or shutter speed values will update in response to the user changing them via a command dial. This means it is possible to capture improperly exposed images even though they looked fine in the preview, if you're not careful. Tripod mode assumes the lighting has not changed, so it really is designed to be used in a studio environment where lighting is controlled. But in Tripod mode, exposure values remain static even after focusing. In Hand-held mode, the exposure values are updated after autofocus, because the metering sensor is briefly active during AF. This implies the D300S always uses the dedicated 1,005-pixel RGB metering sensor for determining exposure regardless of Live View mode, and doesn't attempt to meter using image sensor data (except while recording video). The aperture and shutter speed shown at the bottom of the display are the values that were set when Live View mode was enabled they are not continuously updated in response to lighting changes like they are in the viewfinder or LCD panel when Live View mode is off (if Live View is active, the top Status LCD is also not updated with new automatic settings as lighting changes). Live View will show the effect of exposure compensation applied to Program, Shutter, and Aperture priority modes, but you get no preview of over or underexposed settings, nor is an analog exposure meter shown in the Live View display. ![]() A live histogram would have been especially useful because the D300S's Live View display does not actually simulate exposure. One thing we miss in the Nikon D300S, 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. ![]()
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