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Old 03-13-2013, 08:10 AM
Jorda75078 Jorda75078 is offline
Larry Jordan
Join Date: Apr 2011
Location: Inverness, FL
Posts: 17
Default Use live view to tell you if optical viewfinder autofocus is tuned properly

Before you go and spend money on autofocus fine tuning test kits or software, you can do a test using your camera’s two focus methods to see if you need to do fine tuning with any of your lenses. It takes a few minutes to get setup to do the testing, and to switch lens to do each test, but it is faster than actually doing a fine tuning procedure. And it is free!

To do this test, your camera must have a live view mode and the lens you test with must have a focus distance indicator (distance scale on top of the lens).

The testing method I will describe in this posting is the use of live view focusing to compare with optical viewfinder focusing (phase detect autofocus or PDAF). If you need no autofocus fine tuning for a lens, then the focusing should produce the same results with both methods. If the focus is different for the two methods, then you have more work to do to eliminate the difference.

First, you need to do some setup for the test.

Find a place that has a stable target subject with good contrast such as a brick wall. You can also use a black and white test chart that is made for focus testing (see attached example) or any other high-contrast target as long as it you can make it stable for the test. The target should be one that is flat and vertical -- perpendicular to the line between it and your camera…some people call that planar. Make sure you have good lighting on that subject.

Now, place your camera/lens on a firm tripod or other support that will allow you to press camera buttons with no camera/lens movement. The ideal camera position relative to the target is to be close enough to the target to get a shallow depth of field when testing.

Now, aim at your target and set the lens to the focus length that you would normally use for that lens. For telephoto zooms, most people zoom out to the longest focal length.

Before you do any testing, be sure autofocus fine tuning is turned off in your camera. You do not want any prior testing or calibration to interfere with this new test. Also set the camera so that shutter speed is at least one divided by the lens focal length and ISO is no higher than 800. Turn VR off if the lens has VR and turn autofocus on.

You can use any aperture you want for this quick AF accuracy testing, but I recommend setting it to the lowest f/stop. That way you get minimum depth of field and maximum light intake for the focusing tests. Live view normally stops the lens down to the aperture setting you choose to evaluate focus and depth of field. You will use live view for part of this test.

You do not want the camera to wander around trying to find where to focus, so be sure you are using a single focus point. For Nikon, this is an AF-S setting; for Canon, it is Single Servo mode.

If your camera has settings for Live View, set the mode to Tripod.

The most accurate focus sensor in your camera is normally the one in the center, so move the focal point to the center if it is not already there.

Now turn the lens focus ring so the target subject is out of focus. This will give you a chance to see the lens focus distance indicator move as the camera focuses in live view.

For additional stability, you can lock your camera mirror up and use a cable release to perform shutter activations. However, you will have to touch the camera to switch between live view and optical viewfinder between test shots. If you touch the camera between tests, be sure to pause for five seconds before activating the shutter to allow any movement to dissipate.

The first focus test you will do is in live view mode, so switch the camera to live view. Now gently press the shutter button half way down (or the back focus button if you use that focusing setting) and wait until the camera/lens focuses on the target. The contrast detection method used in live view performs focus testing by trial and error which is slow compared with the phase detection used when you focus using the optical viewfinder. The camera/lens my “hunt” back and forth momentarily until focus is achieved. Watch the lens’ focus distance indicator (distance scale on top of the lens) to see how the lens is changing focus during this action.

When the image is in sharp focus on the viewfinder, trip the shutter to capture the image.

Without touching anything else on the camera, turn off live view.

Now gently press the shutter button half way down (or the back focus button if you use that focusing setting) and watch the lens’ focus distance indicator as it focuses on the target. If the focus distance changes any, then the autofocus fine tuning in the optical viewfinder mode does not match that of the view finder mode. This indicates that autofocus fine tuning for that lens will improve the focus sharpness for images captured with that lens on that specific camera body in optical viewfinder mode.

You can also load the images captured with live view and optical view into your computer and zoom in on the images to 100% to see the differences in sharpness. If there is no difference, then autofocus fine tuning is not required. If the image captured using the optical view is less sharp, then autofocus fine tuning is required to get the best autofocus sharpness.

You can read an excellent article by Nasim Mansurov about this technique of doing fine tuning at:
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Larry Jordan
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