What’s Passive Auto Focusing?

Understanding Passive Autofocusing

Auto-focusing is the process by which a camera locks focus on a subject on its own. It is a complicated system that looks for contrast and is dependent on the amount of light, contrast and other aspects.

Have you noticed that when you are shooting in low light conditions the flash on your camera tend to fire a beam of light just before the exposure is made? Not the full power flash though just a burst of low-intensity light. Ever wondered what that beam of light actually does?

It assists in the optical system of the camera to lock focus because there is not much ambient light to go with. When you are shooting in broad daylight there is sufficient light around for the camera so this beam is not required to be fired.

There are two different types of auto-focusing in modern digital cameras:

  1. active and
  2. passive.

The most popular method and the ones that you are likely to see more often in your cameras is the passive auto-focusing modes. We shall be dealing with passive auto-focusing in this article.

We shall be dealing with passive auto-focusing in this article.

Related Post: Exposure Triangle Explained

Passive Auto-Focusing

As the name suggests passive auto-focusing denotes when the camera uses a non-intrusive method to ascertain focus. Here it is closely associated with the optical system, unlike in active auto-focusing system where it is independent. Passive auto-focusing mainly works by detecting contrast in the scene. It tends to

Passive auto-focusing mainly works by detecting contrast in the scene. It tends to lock focus on the spot closest to the camera where maximum contrast is found.

There are two different types of passive auto-focusing:

  1.  contrast detect and
  2. phase detect.

Contrast Detect Autofocusing

In this mode, the camera looks for contrast in the field of view to lock focus on. This comes from the logic that a higher contrast is an indication of sharpness. A major drawback of this system is when you aim towards a subject that does not have sufficient contrast. It can be a really smooth surface or a uniform color with no texture.

Another problem of contrast detection is it fails when there is not enough ambient light to go around. Contrast detection auto-focusing is the predominant form of auto-focusing in digital cameras.

Some cameras like the smaller (and cheaper) Point & Shoot systems have only this type. DSLR cameras also have contrast detection auto-focusing but it is used only in live-view mode.

Phase Detection Autofocusing

Phase detection auto-focusing uses the light coming through the lens and using dedicated phase detection sensors compares two sets of images to ascertain whether it is in focus. The technique is similar to what rangefinder cameras use but in this case, the onboard sensors do what the two lenses do on rangefinder cameras. Phase detection system is predominantly found on DSLR cameras.

AF-Assist Lamp

In the opening paragraph, I referred to the beam of light firing just before the exposure is made. This is not infra-red light but a normal light that simply triggers the auto-focusing system. It is also known as an AF-assist lamp. This is just the same passive auto-focusing mechanism that is triggered when the light makes it possible to ‘see’.

This is just the same passive auto-focusing mechanism that is triggered when the light makes it possible to ‘see’.

A brief word on Active Autofocusing

Active autofocusing in contrast to passive autofocusing uses infra-red light or sound waves which are fired to detect the distance between the subject and the camera. Just like a sonar, it bounces off the subject and the AF system of the camera calculates the distance to the subject independent of the optical system. Information is passed on to the optical system and focus is locked.

Just like a sonar, it bounces off the subject and the AF system of the camera calculates the distance to the subject independent of the optical system. Information is passed on to the optical system and focus is locked.

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