Tuesday, May 11, 2010
SLR vs. EVF
A new approach to interchangeable-lens camera systems gives photographers the option of smaller bodies. How do they stack up?
The Panasonic G2 offers HD video and a cool touch-screen LCD—activate the AF system by touching your subject. The LCD tilts and swivels, too. This 12-megapixel model can shoot still photos as fast as 3.2 fps.
Which design is better? It depends a lot on your shooting style and the types of subjects you typically photograph. One consideration is whether eye-level viewing is more important than compact size, or vice versa. SLR viewfinders are clearer, making it easier to compose and focus, especially in dimmer light. EVFs make for much smaller cameras, but are less clear, especially in dimmer light. Electronic viewfinders also cost much less to produce than an SLR optical system with its mirror box, precision moving mirror assembly, focusing screen and pentaprism or pentamirror viewfinder.
Early EVFs had slow refresh rates, making them poor for action subjects. Newer EVFs have much faster refresh rates and are much improved for action and in dim light, though still not as good as an SLR optical TTL finder. Another factor for speed is the type of AF system these cameras use.
Autofocusing. DSLRs normally use phase-detection autofocusing, which is extremely fast and accurate. With phase-detection, a beam-splitter directs some of the light coming through the lens to a dedicated AF sensor. The AF system compares the split images and can determine from that single measurement whether the image is in focus, and if not, in which direction it’s out of focus and by how much. Because of its speed and precision, phase-detection AF is especially well suited to action photography.
However, SLRs with live-view capability will behave differently when in live-view mode. The SLR’s mirror has to flip up out of the light path so the light can reach the image sensor. This prevents light from reaching the phase-detection AF sensor. Most DSLRs with live-view capability deal with this by providing two live-view AF modes. In phase-detection mode, the camera’s standard phase-detection AF is used, but the live view is briefly disrupted when the SLR mirror drops down into the light path so the phase-detection AF sensor can receive light. In contrast-based AF mode, the camera determines focus by reading differences between adjacent pixels right off the image sensor, so the mirror can stay in the up position and the live view isn’t disrupted during focusing. The drawback is that contrast-based AF is slower than phase-detection AF, especially in dim light, and less suitable for action shooting. (Sony’s live-view DSLRs provide only phase-detection AF, but use a second sensor in the viewfinder, and thus avoid disruption of the live image.) Many digital cameras, including newer AF DSLRs, now also offer face-detection AF, a contrast-based system that recognizes and focuses on human faces in a scene.
The EVF cameras all use contrast-based AF, but it’s typically quicker than the contrast-based AF in DSLRs and compact digital cameras, and better capable of handling action than those contrast-based AF systems.
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