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1. Off Centre Lens Optics & Sensor
Alignment Testing
  for problems mostly encountered when using ‘fast’ lenses wide open.
(Test procedure devised by and article © Adrian Harris)

Test Method for Handholding  Camera:

1. Set lens to wide open = fastest aperture.
   (IE. Lowest f number. Example: f2.8, not f8. As using a small aperture such as f8 or f11 will disguise lens and camera sensor alignment faults).
2. Ensure shutter speed = minimum of 2 x equivalent FF focal length.
   (On M43, 20mm = 40mm FF = 1/80th. The faster the shutter speed used for the test the better, but stay within reasonable ISO limits to prevent a poor image.)
3. Turn OFF all stabilisation.
   (Stabilisation could cause erroneous test results.)
4. Set camera to the smallest focus square or point, and set it central.
   (This will help prevent focus errors).
5. Stand firmly, brace your arms against chest, hold breath, aim carefully at the target point and fire off a short burst of shots.
   (I use a series of 4 to 6 photos* to ensure I get at least one good image in case of camera user error!)
6. Now turn the camera upside down by inverting it 180 degrees. Focus on the exact same point and shoot of another burst - easier said than done!
   (If there are any obvious misalignment faults, inverting the camera should appear to move any visual discrepancies to their diametrically opposite side.)

Method if Camera on Tripod : Only practical if you have a suitable tripod/head that will
                                                    allow 180o inversion…

1. Prepare camera as in 1 - 4 above.
   (Using a 2sec timer delay or a remote shutter control will allow better images from slower shutter speeds if desired.)
2. No need for a long burst if using a tripod, but is always wise to take a couple of shots – just in case.
3. Invert camera as in 6 above and repeat the test, again taking great care to focus on the exact same spot.

Examination:
    Turn the inverted images the correct way around to Check Results.
    Using the best image from each inversion, compare the two images
       for any left-right, and top-bottom sharpness discrepancies.

Differences would indicate a lens or sensor misalignment. To check which may be at fault, repeat the tests using another fast lens on the camera, and if you can borrow one from a friend, try another camera with the lens.

Important Note:

• If testing a suspect zoom lens, repeat above test at different focal lengths.
• To ensure the camera focus is EXACTLY the same when inverted, it may be best to use fully manual focus control – but take care NOT to knock the focus control ring !
• * EXIF data does not indicate whether stabilisation is On or Off, so if you do this test and also are comparing the effects of stabilisation versus no stabilisation, ensure you make note of the number of shots taken in each sequence!   

Real World Test - Example of lens with a Squint Fault:

Click to view a larger photo to examine.

Article © Adrian Harris