How To Test If Camera Is Focusing Correctly

Camera Lens Testing - Focus Testing

Part 5 - Focus Accuracy Check (for AF lenses)

A common complaint seen on web photo forums is that cameras (and/or lenses) aren't achieving accurate focus. Sometimes the focus point is in front end of the intended subject and sometimes information technology'southward behind information technology. Some of these observations may be due to technical bug with AF, but some are due to user error or unreasonable user expectations.

Of class no manufacturer of cameras or lenses claims that autofocus is admittedly perfect. For case, Canon land that the spec for focus on "consumer" bodies (such as the Digital Rebels and the EOS 20D/30D/40D) is that information technology should be inside the DOF (Depth of Field). On the "pro" bodies (EOS ane series) focus spec for f2.eight and faster lenses (f4 on some models) is that it should exist within 1/3 of the DOF. With slower lenses the AF accuracy of the "pro" bodies reverts to the same as that of "consumer" bodies which don't accept the loftier precision sensors.

Then the bottom line is that focus should be within the DOF, or to put it another style, the prototype should wait sharp. If it doesn't look sharp, focus is not probable to be in spec.

Note besides that both the photographic camera and lens are involving in focusing. A focusing error can be the result of either a camera or a lens scale problem.

The Lens Testing Focus Nautical chart

I've written this commodity so that users tin can test their DSLR under a set of "standard" atmospheric condition. The lens testing chart which is shown at reduced size below tin be downloaded (as a Zipped JPEG) Hither

You can see two sets of parallel lines which are your focus indicators. The numbers 1, 2 and 3 represent 1cm, 2cm and 3cm distances when the chart is used at 45 degrees equally explained beneath. In the middle is a single vertical line (or horizontal line, depending on how the target or oriented), and that'southward your focus signal.

The Lens Focus Examination

To use the lens testing chart you set information technology up at 45 degrees to the axis of the lens as shown below.The easiest way to exercise this is to have the camera horizontal and tilt the target back by 45 degrees. Y'all can besides lay the target on the flooring and shoot down on it with the camera at 45 degrees. In this configuration (with the target at 45 degrees to the optical axis of the lens) the 1, 2 and 3cm distance marks are correct. They are actually spaced at 1.41, ii.82 and four.23cm from the focus line on the test chart, simply when viewed at 45 degrees these distances are modified by the Cosine of the viewing angle (Cos 45 = 0.707). The lens should be at maximum discontinuity for minimum DOF and maximum sensitivity to focus errors.

Use is easy. Once you have everything setup you select your focus betoken (allow's use the center indicate) and brand sure the focus zone includes only the unmarried focus target line. It's important to notation that the area of the AF sensor may not be exactly the same as the markings on the viewfinder screen, so make certain there's nothing near the AF zone just the focus line. Then you accept your shots. Take several and refocus each time. Attempt several manual focus shots besides, using Live View with magnification if your camera has that ability. Use the maximum aperture of your lens and so as to get minimum DOF.

The Results

Below is an instance cropped from a frame taken with a EOS 10D using 50mm f1.8 lens at f1.8 and focused manually on the single vertical line. Focus altitude was about 0.45m (I have the Mark I version of this lens which has a focus calibration, the newer Mark 2 version does non).

This image displays authentic focus. The "1", "2" and "3" characters are pretty much every bit blurred both in front of and behind the focus line. There's maybe a very slight bias towards the front, but on the side by side manual focus shot y'all might see an equal bias towards the rear. There's zip wrong with the manual focus of this this camera or lens. The lines are spaced at 2mm intervals. 1, 2 and 3 represent 1cm. 2cm and 3cm distances in front of and behind the focus line. Note that at longer focus distances the DOF is non symmetrical about the focus indicate and you lot should see slightly more DOF behind the betoken of focus than in front of it. The important affair is that the lines at the zilch position (at which you focus) are the sharpest.

The side by side shot shows the same view, but taken using autofocus.

Here you can see a slight bias in front of the focus line. The "1 cm" marker is sharper in front of the focus line than behind it, though the focus line itself is still quite abrupt. I'd estimate that focus is maybe 2mm in front if the focus line, simply the line itself is still within the "sharp" zone, perchance close to the rear limit of the DOF. For a 50mm lens at f1.8 focused at a distance of 0.45mm the calculated DOF is 5mm (+/- 2.5mm). So the focus isn't "perfect" but information technology's pretty proficient and within spec for autofocus. I'd doubtable that such pocket-size focus offsets are pretty common but nobody e'er notices them. Below is the same shot at f5.half dozen and the focus offset is undetectable to my optics due to the increased depth of field at f5.half dozen.

The distance between the target and the camera should be whatever you typically shoot at, or about 25x the focal length of the lens if y'all want the best cess of general focusing accuracy. If yous are as well close and get into the macro range, it's possible that focus errors could be different from what y'all might detect at infinity. Just be sure that the focus zone of the camera but covers the single focus line. If y'all are so far away that the focus zone extends across the other marks, print the target larger to compensate. If yous double the size of the target, the 1cm, 2cm, 3cm marks will become 2cm, 4cm and 6cm respectively.

What if the machine focus is off?

If you get great, sharp, properly focused images using manual focus (liveview + magnification), simply your shots using AF are off focus, you have a couple of choices, depending on which lens and which camera is involved. Either the lens or the camera could be out of aligning, simply if only one or two lenses seem to take a problem and the residuum don't, then it's probably a lens issue. If every lens y'all own has a trouble, and then information technology may be a camera adjustment upshot.

The equipment manufacturer can make focus correction adjustments to their cameras and lenses and so you can send either (or preferably both) of them off for the manufacturer to calibrate and adapt. However if you are shooting with a camera that has microfocus adjustment (like most electric current EOS DSLRs except for the Insubordinate series), you can make corrections yourself.

Canon EOS AF adjustment can be washed in +/- 20 steps and the camera remembers the lens and just applies the correction when that particular lens (actually lens type) is mounted. You tin find the right amount of adjustment pretty easily by systematic trial and error. You might starting time out trying shots at 0, -5 and +5 settings. If the focus looks better at +5 than at 0 then you know you need to add together a +ve adjustment. Try +10. If +10 is not as good as +five then you know the right number is less so 10. If +ten is better than +v then you know the right aligning is more than +5. You keep on iterating until you have the all-time focus and at that betoken either reducing or increasing the AF adjustment makes the image less precipitous.

A few Sigma lenses are compatible with their "USB Dock". If you attach a compatible lens to the USB dock you can employ a computer to adjust the lens focusing parameters. Still it only works with Sigma lenses and simply with a few of their newest ones (35/i.4, 17-lxx/2.8-4 and 120-300/2.eight as of 11/13)

Some DSLR models from Nikon, Sony, Olympus and Pentax also have AF microadjustment adequacy. Generally the more recent, higher finish, models have the feature, only y'all'll accept to cheque with the camera manual to be sure.

Here's a listing including cameras released upwardly to around 2013:

Canon: 1Dx, 1DsMk3, 1DMk3, 1DMk4, 5DMk2, 5DMk3, 7D, 6D, 70D, 50D (note non the 60D)
Nikon: D4, D3,D3x, D3s, D300, D300s, D600, D700, D800, D800E, D7000
Sony: A900, A850, SLT-A77, SLT-99
Olympus: E-30, Eastward-620, East-5
Pentax: K20D, K2000, K200D, K-5, K7D, 645D

Source: http://www.bobatkins.com/photography/technical/focus_testing.html

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