What is focus? Focus is concentrating so that we can see things sharper. If we lose concentration, we get confused and can’t see things properly. Same thing applies in Photography. An image that is completely sharp is considered to be in focus. If the focus is missed, the picture becomes blurry.
Lets understand how focusing in camera works.
Light passes through the lens and reaches the sensor. In order to create sharper image, the lens concentrates the light rays on the image sensor. The size of the aperture decides how focus the rays are once they reach the sensor. Smaller aperture hole does better job as compared to larger aperture hole.
Each lens has optical device that consists of curved material which allows passing light through it to the sensor. Depending on the design of the lens it consists one or more elements that both diverge and converge lights to focus into the sensor. The sensor is usually much smaller than the picture we try to capture therefore we need to bend the light to reduce the size of the image.
Manual Focus vs. Auto Focus
Majority of the manufactured today are capable of focusing automatically. However in almost all the autofocus lens has a mode of manual focus. There is no set guidelines when to use manual focus and when to use auto focus. It entirely depends on the style and comfort of the photographer. However there are some tricky situations like, Macro work, Low light situation, Portraits etc, when manual focus is recommended.
With the progress in the technology, the camera manufacturers discovered how to motorize the camera body and the lens focusing elements or focusing groups towards or away from the sensor. The autofocus system calculates focus electronically as the lens moves to and from the sensor. Autofocus system these days works almost accurate the majority of the time.
As we know the light enters through the lens of a DSLR, reflects from the mirror (which is in front of the sensor) to the pentaprism and then to our eyes. However most of the camera illustration does not show another mirror which is just behind the main/reflex mirror. A small portion of light goes through the main/reflex mirror gets reflected by the secondary mirror and reaches to the phase detect / AF sensor. The light is divided as it passes through that transparent part of the main mirror/reflex, where that area acts like a beam splitter. The two distinct images are directed downward to the aforementioned autofocus sensor. The sensor redirects the lights to the group of sensors. The camera then analyzes and compares these images from the sensor and if they do not look identical it instruct the lens to make proper adjustment.
Contrast detection system is used mostly in mirror less camera, point and shoots camera, smart phone cameras and DSLRs in live view mode. Unlike Phase detection, Contrast detection focusing system has rather simple mechanism. It uses the light falling on the main sensor to provide focus. The difference in the intensity between adjacent pixels of the sensor naturally increases with correct image focus. The maximum intensity indicates sharper focus. The optical system is adjusted until the maximum contrast is detected. The contrast detection system has one major advantage over phase detection that is the number of autofocus point. The camera can have almost unlimited number of autofocus point. This focusing system is also considered more accurate as this does not have back focus or front focus issue.
AF-S / One-Shot AF: In this mode the camera will focus only once when you press the shutter button half way. There is not continuous adjustment hence saves battery power. This mode is ideal for non moving subject.
AF-C/ AI Servo AF: This AF mode is for moving subject. Once you press the shutter button half way, the focus is achieved. However the camera adjusts refocus automatically along with the movement of subject. Since there is a consistent adjustment of focus, this mode uses more battery power then AF-S / One-Shot AF mode. This mode is ideal for sports, wildlife or any kind of photography where the subject moves continuously.
AF-A/ AI Focus AF: This is an automatic mode. It’s a relatively new feature which has turned out to be very successful. In this mode the camera shuffles between AF-S / One-Shot AF and AF-C/ AI Servo AF mode depending on the situation. This is a default focusing mode unless you change it.
Manual Focus: Nowadays there is usually an option to switch between auto and manual focus on the side of the actual lens. You will find this mode in some DSLRs maybe it’s in case you come across a lens that doesn’t give you that option.