CONTRAST
Hunt for the sharpest point
Contrast detection moves the lens until neighbouring pixels differ most. It can be very accurate, but a blur gives no clue which way to move, so it hunts back and forth.
Why it failsAUTOFOCUS, EXPLAINED
How a camera
finds focus.
Half-press the shutter and the lens jumps to sharp. Here is what happens in that fraction of a second.
Why won't it focus?
1977first mass-produced AF camera
A point is in focus when its light converges as nearly as possible on the sensor. Cameras judge that from contrast in the picture, either by hunting for the sharpest point or by comparing light from opposite sides of the lens.
At a glance
- Contrast hunts
- Phase knows the way
- Both need detail
- Give autofocus an edge to grip
⅓ of the depth of field
Typical professional AF accuracy
Depth of field is the span between the nearest and farthest things that look acceptably sharp. Autofocus accuracy within a third of it, at the lens's widest aperture, is common in professional autofocus SLR cameras.
ACCURACY
f/5.6, or f/8 on pro bodies
Widest aperture for reliable AF
Most consumer cameras autofocus reliably only with lenses whose widest aperture is at least f/5.6; professional models can often cope with f/8. Around f/2 to f/2.8 is generally best for speed and accuracy.
LENS APERTURE
TWO NUMBERS WORTH KNOWING
Accurate, if the lens
lets in enough light.
One figure on how precise good autofocus is,
one on the light a lens must pass for it to work.
Very fast lenses, such as f/1.4, have such shallow depth of field that reaching exact focus can take longer, despite the extra light.
A QUICK FOCUS CHECK
Why won't
it focus?
Pick a scene to see how hard it is for passive autofocus and why. The bar shows how likely the camera is to struggle.
General behaviour of passive autofocus; individual cameras and lenses vary.
THE SHORT ANSWER
Two ways
to judge sharp.
Passive autofocus reads the image itself.
The difference is whether it knows which way to go.
PHASE
Know the direction and distance
Phase detection compares light from opposite sides of the lens. The offset between the two images tells the camera which way to drive the lens and how far, which helps with moving subjects.
Help it lock on
HISTORY
From sound to sensors
The first mass-produced autofocus camera was a simple compact in 1977. The first autofocus SLR, in 1978, measured distance with ultrasonic sound. Today many cameras build phase-detect pixels into the image sensor.
Hybrid autofocus
FOUR HABITS FOR SHARPER FOCUS
Give autofocus
something to grip.
None of these needs new kit. They follow from what autofocus measures and where it struggles.
Put the point on an edge
Choose a focus point over a sharp edge or strong texture. For portraits, the eye is the usual choice, partly because it has good contrast.
Match the mode to the subject
Use single-shot mode for still subjects, which waits for focus lock, and continuous mode for moving ones, which predicts where they will be.
Pre-focus for action
Focus in advance at the spot where you expect a moving subject to pass, so the lens does not have to search a long range.
Turn the camera in poor light
Many focus sensors are line sensors. If the detail runs the same way as the sensor, rotating the camera 90° while focusing can help it lock.
HYBRID AUTOFOCUS
Fast first,
then fine.
Phase detection is very fast because it measures both the amount and the direction of the focus error. Contrast detection has no such limit on accuracy and even works with the lens stopped down, but it has to search.
Many mirrorless cameras put phase-detection points on the image sensor itself. Some systems use them for a quick, coarse move and let contrast measurement do the final fine focus; phase detection has become the norm on most mirrorless cameras.
Questions
Questions people ask
Why does my camera's focus go back and forth?
That is called focus hunting. Contrast-based autofocus searches for the point of highest contrast and, on a difficult subject, may give up without locking. Low light, low contrast and motion are the usual causes.
Which is better, phase or contrast detection?
Each has strengths. Phase detection is fast and knows the direction to move, which helps track moving subjects; contrast detection can be very precise. Many cameras combine the two.
Why does autofocus fail in the dark?
Passive autofocus needs visible contrast in the scene. In low light an assist lamp, often infrared or the flash, can light the subject so the camera can focus, at the cost of slower focusing.
Does my lens affect autofocus?
Yes. Most consumer cameras need a widest aperture of at least f/5.6 to focus reliably, and apertures around f/2 to f/2.8 are generally considered best for speed and accuracy.
When was autofocus invented?
Patents date back to the 1960s. The first mass-produced autofocus camera, a point-and-shoot, appeared in 1977, and the first autofocus single-lens reflex camera followed in 1978.
Sources
Where the facts come from
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