10 Tips For Better Autofocus

Text And Photography By Rob Sheppard

Phase-Detect AF Vs. Contrast-Detect AF

With phase-detection AF, a portion of the light entering the lens is diverted to the AF module, where it’s split into two parts, which are directed onto a pair of CCD line sensors. The points where the two beams strike the sensor tell the camera’s AF computer whether the image is in focus or not, and if not, in which direction it’s out of focus and by how much. A phase-detection system can thus establish focus with a single reading and adjustment, making it quicker than contrast-based systems and much better for moving subjects (and in dim light).

All of today’s D-SLRs use phase-detection AF systems for normal shooting. Contrast-detection AF measures contrast at the image sensor, the idea being contrast is at its maximum when the image is sharply focused. A contrast-based AF system must take multiple readings to determine and set focus: After the first reading, focus is adjusted and another reading is taken. If the contrast is greater, another adjustment is made in the same direction and another reading is taken, and so on, until contrast starts to decrease. If the second reading shows less contrast than the first, an adjustment in focus is made in the opposite direction, then another reading is taken, etc. The result is that contrast-detection AF requires multiple readings and adjustments, taking longer than phase-detection AF to establish focus. But it can be more precise, and there’s no disruption of the live image during focusing as there is with phase-detection, so it’s excellent for tripod-mounted live-view work, where speed is not of the essence. Some D-SLRs offer both types of AF in live-view operation.

AF In SLRs: A Timeline
1981 Pentax introduced the ME-F, the first interchangeable-lens SLR with TTL autofocusing capability. It took all Pentax SLR lenses, but autofocusing was possible only with one special AF lens: a 35-70mm zoom, which contained both the AF motor and the four AA batteries it needed to operate.
1985 Minolta introduced the Maxxum 7000, which really started the AF SLR revolution. The body contained not only the AF sensor, but a focusing motor as well, so AF worked with all lenses. Caveat: Only the new Maxxum lenses could be used with the camera; the Maxxum cameras couldn’t use previous Minolta system lenses.
1987 Canon introduced the EOS system, with the EOS 620 and EOS 650 models. EOS AF SLRs don’t contain focusing motors; rather, each lens contains its own motor optimized for its requirements. The drawback was that longtime Canon users couldn’t use their earlier lenses with EOS cameras; only EF (and today, EF-S) lenses can be used on EOS bodies. Other manufacturers put the AF motor in the AF SLR body to retain compatibility with previous as well as new AF lenses (only the AF lenses would autofocus, but users could use their existing lenses with manual focusing). Today, most D-SLR manufacturers offer higher-end lenses that contain their own focusing motors. (Nikon has even introduced entry-level D40 and D60 bodies that don’t have focusing motors and thus must be used with the AF-S lenses that do contain AF motors if one wants autofocusing capability.)
2006 Olympus introduced the first D-SLR with a Live-View monitor. The EVOLT E-330 had two Live-View modes, one that used the same phase-detection AF as was employed for normal shooting and a full-time Live-View mode with manual focusing.
2007 Nikon introduced the D3 and D300 with Live-View modes, featuring both types of autofocusing. In Handheld mode, phase-detection AF is used; in Tripod mode, contrast-detect AF is used.

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14 Comments

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Jeff makes this comment

Monday, 17 August 2009

The AF button on my camera has brought autofocus to a completely differant level. It tracks extremely well and is a feature that I will be using frequently. Great Feature......
rebecca makes this comment

Wednesday, 12 August 2009

I agree with auto focus not always working. But I do need to explain how much I love all my film cameras. I have a Canon F-1, and and A-1, they are the split screen focus ring. I love it. I am in control of my picture not the camera. Remember it is you that is behind the camera and pressing the button not camera. I understand that digital can be crystal clear but the right film it also can be clear.
Ola makes this comment

Monday, 27 July 2009

Great article it just met me where i am Thanks
Outofocus makes this comment

Thursday, 23 July 2009

www.katzeyeoptics.com For those who miss split sceen manual focusing.
Jim Horne makes this comment

Thursday, 23 July 2009

Article
image masking services makes this comment

Monday, 22 June 2009

Extremely informative article, well researched and clearly presented.Thanks for the hard work.
Janis makes this comment

Saturday, 16 May 2009

The best advice I've ever seen about focusing is to use the back-button AF exclusively. I'm amazed at what an improvement that makes for me. In the menu I turned off any focusing by the shutter button method, and then put the switch on the front of the camera on C for continuous. As long as I keep the center AF point on a bird or animal and my thumb on the AF button, it tracks its movement very well. Then I take my thumb off the button to freeze the focus when the animal stops. Therefore I can keep the front switch on C at all times. This also works on flowers in the breeze, etc. Try it! You need to take a few dozen shots to get in the habit of always using that button but after that I predict you will never go back.
Mike Lysecki makes this comment

Monday, 13 April 2009

Just found out you had a newsletter. Joined in and within minutes found it to be helpful!!
George makes this comment

Friday, 27 February 2009

I agree with the person who advocates bringing back the split ring screens. It is almost impossible to have tack-sharp focus with the flat matte screens we have today, unless we are able to fully utilize the auto focus function. If we could always depend on auto focus it'd be great, but in low light situations it isn't dependable.
Frank makes this comment

Thursday, 26 February 2009

All good advice, but what I'd really like to see is a return to the old style split ring focusing screens. It is virtually impossible (for me) to manually focus, or manually adjust focus, with these new screens. There's nothing to go by other than whether the light comes on...well, the light comes on in auto-focus and it's not right sometimes. The only way to really see focus is to stop down the lens, and that's just too time consumming in most cases. Also my 2x is virtually useless because it must be manually focused. Please bring back split rings!
Ray makes this comment

Friday, 23 January 2009

I some times get the problem in darker areas with auto focusing. To overcome that, i did exactly what has been explained in this article. On my Olympus Camera E500 i can lock the AEL on the back of the Camera Body and it will stay on until pressed again. That way i keep the exposure metering value for that area and can still move to try to focus on something around the subject to get the camera to auto focus. Go back spot i moved from and press shutter all the way down to take the Photo. When taking night shots and i have the Camera on a Tripod, auto focusing often is a problem. I lock the AEL on the are i want to take the shot. Then i move my camera to something of similar distance or best equal where it will auto focus. I then take my finger off the shutter release, re aim the camera back to where i initialy wanted to take the shot. I then set my Camera to manual focus mode so that my camera does not try to re focus again and hence spoiling my focus. I then take my shot.
Michel makes this comment

Thursday, 22 January 2009

Great article that summarizes the key knowledge to autofocus. I'd like to mention that a lot of Canon camera's have a custom function. This function let's you use the AE button on the back of the camera to focus. When you release the button focus is locked. Pressing the shutter halfway will lock AE and pressing further will let you take the photograph. This is very handy since you can seperate the focus and composition steps without having to set the camera in MF mode. Don't know if other brands offer this option also?
Rob Schley makes this comment

Saturday, 13 December 2008

Great Article!
Auguste makes this comment

Tuesday, 25 November 2008

Intereting and usefull to step back and be reminded of the basics one has started to take for granted, also very useful for new(er) photograhers.

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10 Tips For Better Autofocus

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