How does autofocus really work??? at DVinfo.net

K.C. Kennedy · March 31st, 2010, 04:26 PM

In modern days photo cameras you choose the point of focus and let the camera focus then release the shutter. Point and shoot photo cameras with video mode constantly lose POF and ghost back and fourth all the time. Please, explain how it works in a semi-pro or pro cameras with autofocus feature. For example, if I shoot a concert and a person is 2 feet away from a microphone and the camera can't figure out either to focus on the mic or person's face (in second case face will be out of focus assuming that it's dark and iris is open all the way). Thank you in advance and sorry about sounding uneducated on the subject.
KC.

Chris Hurd · March 31st, 2010, 04:56 PM

There are two primary methods; phase detection AF (high end broadcast) and contrast detection AF (lower end semi-pro and consumer). I'm not a huge fan of Wikipedia, but here's a good place to start:

Autofocus - Wikipedia, the free encyclopedia

K.C. Kennedy · March 31st, 2010, 05:08 PM

How do I avoid focus ghosting in the above scenario using auto focus? Assuming camera set up to the right side of stage and mic is on the way between the camera and performer's face? Manual focus will make me refocus once I change the subject and move camera onto another performer on stage :(

Chris Soucy · April 4th, 2010, 08:28 PM

If I understand the question correctly, you're trying to get the AF system to think for you.

Unfortunately, AF systems have a hard enough time thinking for themselves.

In order for you to do the utmost to help it do it's thing correctly, you need to do some testing, no other way around it.

Every camcorder AF system I've ever used has a minimum object size below which it simply will not lock. In low light that gets even worse.

Seeing as how you're talking of performers, get some white cardboard and cut out a very rough outline of a human head and upper shoulders, life size.

Nail or pin it to a stick and park it about 15 feet in front of a wall (a dark one, preferably, most stage backdrops are dark).

Set you camera lens to full wide and stand about 5 feet in front of your cutout.

Will AF lock on the cutout at that range, every time?

Keep repeating this test, moving 5 feet further away each time.

At some point AF will fail and keep haring off into the background.

Zoom in till it will lock every time again.

Keep testing whilst walking backwards at 5 foot intervals.

Sooner or later, you'll run out of zoom and AF will fail again.

You've just established the limits of you AF system in good light, and the minmum size object in your viewfinder required to lock consistantly.

If you can't keep within these parameters in the venue, AF will fail, simple as that.

CS

March 31st, 2010, 04:26 PM	#1
K.C. Kennedy Regular Crew Join Date: Sep 2007 Location: New York Posts: 80	How does autofocus really work??? In modern days photo cameras you choose the point of focus and let the camera focus then release the shutter. Point and shoot photo cameras with video mode constantly lose POF and ghost back and fourth all the time. Please, explain how it works in a semi-pro or pro cameras with autofocus feature. For example, if I shoot a concert and a person is 2 feet away from a microphone and the camera can't figure out either to focus on the mic or person's face (in second case face will be out of focus assuming that it's dark and iris is open all the way). Thank you in advance and sorry about sounding uneducated on the subject. KC.

March 31st, 2010, 04:56 PM	#2
Chris Hurd Obstreperous Rex Join Date: Jan 2001 Location: San Marcos, TX Posts: 27,368 Images: 513	There are two primary methods; phase detection AF (high end broadcast) and contrast detection AF (lower end semi-pro and consumer). I'm not a huge fan of Wikipedia, but here's a good place to start: Autofocus - Wikipedia, the free encyclopedia __________________ CH Search DV Info Net \| 20 years of DVi \| ...Tuesday is Soylent Green Day!

April 4th, 2010, 08:28 PM	#4
Chris Soucy Inner Circle Join Date: Feb 2007 Location: Fairfield, Dunedin, New Zealand Posts: 3,691 Images: 18	Hi K. C............................ If I understand the question correctly, you're trying to get the AF system to think for you. Unfortunately, AF systems have a hard enough time thinking for themselves. In order for you to do the utmost to help it do it's thing correctly, you need to do some testing, no other way around it. Every camcorder AF system I've ever used has a minimum object size below which it simply will not lock. In low light that gets even worse. Seeing as how you're talking of performers, get some white cardboard and cut out a very rough outline of a human head and upper shoulders, life size. Nail or pin it to a stick and park it about 15 feet in front of a wall (a dark one, preferably, most stage backdrops are dark). Set you camera lens to full wide and stand about 5 feet in front of your cutout. Will AF lock on the cutout at that range, every time? Keep repeating this test, moving 5 feet further away each time. At some point AF will fail and keep haring off into the background. Zoom in till it will lock every time again. Keep testing whilst walking backwards at 5 foot intervals. Sooner or later, you'll run out of zoom and AF will fail again. You've just established the limits of you AF system in good light, and the minmum size object in your viewfinder required to lock consistantly. If you can't keep within these parameters in the venue, AF will fail, simple as that. CS

March 31st, 2010, 05:08 PM	#3
K.C. Kennedy Regular Crew Join Date: Sep 2007 Location: New York Posts: 80	How do I avoid focus ghosting in the above scenario using auto focus? Assuming camera set up to the right side of stage and mic is on the way between the camera and performer's face? Manual focus will make me refocus once I change the subject and move camera onto another performer on stage :(