Ok… ok… even your absolutely basic and generically indistinguishable compact digital camera of questionable origin has Live View. So why do we need to talk about it? Well, the answer is simple. Although the concept of live view on both compacts and DSLRs are the same, the performance and limitations are much more different. In this article, we will let you know the differences between Live View in compact cameras, micro FourThirds as well as DSLRs and how to make sure that they don't get in the way of you taking great photos.
Live View - The Mechanics
• Compact Cameras
The mechanics of live view for a compact camera is simple. What you have is essentially light going through the lens and aperture and striking directly on a sensor which may have an electronic or mechanical shutter. During operation, the sensor receives the image and projects it on the LCD. Focusing is done using contrast detection technology, which is achieved by measuring contrast within a sensor field, through the lens. By measuring the intensity difference between adjacent pixels of the sensor, correct image focus can be achieved by adjusting the optical system until the maximum contrast is detected.
• Common DSLRs
In normal shooting, where the user uses the optical viewfinder, focusing is done with phase detection. However, with the use of Live View, the mirror needs to be lifted for the image to be displayed on the LCD via the sensor. With the mirror lifted, focusing with phase detection will not be possible without disrupting Live View with mirror dropping back down for the focusing. (The only exception to this is Sony DSLRs because they include a second sensor near the viewfinder for dedicated Live View display. This allows them to continue with Phase Detection AF.) In Live View settings, you would find that you can actually choose this method but the drawback is that you will get a black screen during focusing. However, this is only one of two options. Manufacturers have introduced Contrast Detection focusing for Live View so that you can achieve focusing without interrupting Live View.
• Micro FourThirds
As micro FourThirds are basically compact cameras with the ability to interchange lenses, the mechanics is pretty much the same as those of compact cameras.
Top-of-the-line DSLR is focusing really slow in Live View
If your photography is of the genre where fast response is essential, when using Live View, you may be rather disappointed. The main reason for this is because of the difference between phase detection AF and contrast detection AF. Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. It uses dedicated hardware (line and cross-type autofocus sensors) to measure relative focus by analysing them for similar light intensity patterns. This would allow the camera to determine instantly the exact direction of focusing and amount of focus ring's movement necessary to achieve correct focus.
Problem is that in common DSLRs, phase detection is not possible with the mirror up as it blocks the light going to the AF sensors. Instead, the camera switches to Contrast Detection AF. A disadvantage to this is that it can only determine that it's out of focus and not whether it's front or back focused. Therefore, it needs to take at least two measurements from different focus positions to determine the correct focus. In real life, it will take more than two measurements and probably more if you can't hold the camera still or if the action in the frame is rather busy. The worst case scenario is when you fail to attain correct focus but end up with see-sawing measurements while focus hunting. It also tends to take longer under dim lighting. However, it's not all bad. As it does not use a separate sensor, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate. Some application possible with Contrast Detection AF includes Face Detection technology being applicable.
But why is it even slower than my compact camera at times?
Before we talk about focusing on a compact camera, we must first consider the depth-of-field, that is, the zone of sharpness that extends in front and beyond the point you actually focus on. With a more extensive depth-of-field, you could expect most of your frame to be sharp. Conversely, the more shallow the depth-of-field, only the more immediate distance from the point of focus will be sharp. There's a simple, direct relationship between aperture and depth-of-field, where the smaller the aperture, the more extensive the depth-of-field. Hence, in compact cameras where the apertures tend to be small, the depth-of-field tends to be more extensive and thereby allowing focus to keep most of the frame sharp in most cases. As focusing on compacts is done with contrast detection technology, the extensive depth-of-field makes it easier and quicker for the camera to attain focus. Which is why most of our focusing with compact cameras, while not fast, could hardly be considered sluggish.
Using DSLRs or micro FourThirds cameras, we run into instances when we are using larger apertures, and so with a shallow depth-of-field, the contrast detection AF is put under more pressure to attain the correct focus. With the ongoing software developments though, we are likely to see more improvements in Contrast Detection AF technology whereby it would at least minimise the gap with Phase Detection, if not close it altogether.
