Many recently introduced digital single lens reflex (DSLR) cameras have a new shooting mode called Live View. Live View mode is nothing new for digital cameras. Actually, almost all compact (point and shoot) digital cameras have them. Most people know how slow the auto focus (AF) on P&S camera is. Is Live View really useful for DSLR cameras?
To answer this question, it is necessary to understand two auto focus detection methods: contrast detection AF and phase detection AF.
Contrast detection auto focus (AF) works by acquiring a series of images from the main image sensor while turning the focus motor until the subject is in focus, which is determined by the examining the image contrast through many iterations. The speed of focus is limited by how fast the camera acquire the frames and analyze them to determine focus. This mode of auto focus can focus anywhere in the field of view. It can be vary accurate but is typically very slow for fast moving subjects and predictive AF tracking is very difficult if not impossible.
In comparison, the auto focus operation in SLR cameras is far more advanced especially for its speed. Some light is diverted from the main optical path to a special AF module, which has a pair of sensor arrays to look at the images via two prisms. This works very much like some old film SLR cameras (e.g. Minolta X-700) that has a split prism focusing screen in the center of viewfinder. When the focus is off, the two images will not be on the same locations on the two sensor arrays. The AF module is able to determine based on the shift of the images on two array readouts if it is front focus or back focus and off by how much. It can then drive the AF motor a certain distance to acquire focus.
In spite of the advantages, there are some drawbacks for the phase detection AF system.
- The focusing and actual photo-taking occur at two different locations. If the two optical paths are not precisely calibrated or mismatched due to different lens characteristics, mechanical tolerances, etc, front or back focusing will occur. Even though many late DSLR cameras offers the abilities to fine tune the AF, contrast detection AF will save you all the trouble and gives you the AF accuracy you can see.
- There are only limited number of focus points available. To focus on where user wants, the user will sometimes need to do focus and recompose, which may cause focus errors. It is technically very challenging to place AF points close to the outer edge of the field especially for DSLR cameras with full frame sensor. Contrast detection AF will allow focusing anywhere with detectable contrast.
- The AF sensor indicators in the camera is not the same size as the actual AF sensor. As a result, focus error can happen.
As we can see now, the contrast detection AF in Live View mode does offer quite a few benefits mostly for the accuracy of auto focus.
Besides the differences in auto focus, there are other benefits of Live View as well.
- 100% frame coverage. Some very high end DSLR cameras do offer this feature but you pay a lot more $$$.
- Accurate depth of field. This may not be obvious, but the view finder optics seem to alter the DOF!
- No mirror slaps to cause vibration and blurry images.
- Allow shooting at difficult or impossible shooting angles. LCD screens allow very wide viewing angles. It is also possible to connect the camera to a TV monitor for even better shooting experience.
- Good focus accuracy when zoomed-in with manual focus. The focus screen in modern DSLR cameras are no longer optimized for manual focus.
Keywords: AF, auto focus, Focus, Live View, SLR