ARTICLE MENU

3. Front Focus & Back Focus - Testing
  Identifying mis-focus errors for ‘very long zoom’ lenses.
(Test procedure devised by and article © Adrian Harris)

Test Method:

1. Set lens to max zoom and wide open = fastest aperture.
   (IE. Lowest f number. Example: f2.8, not f8. Using a small aperture such as f8 would keep to much in focus and make test results difficult to analyze).
2. For sharp results ensure shutter speed is high = at least 2 x equivalent FF focal length. (on M43, 300mm lens = 600mm FF lens = 1/1200th.  Faster shutter speeds are better, but keep within high image quality ISO limits)
3. If HANDHOLDING you may wish to try test with Stabilisation ON and OFF* (to see if there is a difference). But if using a TRIPOD, switch stabilisation OFF (and use the 2 second shutter delay or a remote trigger).
4. Set camera to the smallest focus square or point, to take accurate aim at your chosen target letter.
5. Stand firmly, brace your arms against chest, hold breath, aim carefully at the target point and fire off a short burst of shots. (A series of 4 to 6 photos will enable you to select the best in case of any erroneous images).
6. If using a zoom lens, move closer and repeat test using a shorter focal length.
   (This is to test for zoom focus anomalies).

Examine and compare all images from each test sequence: 

Was your chosen target letter generally the sharpest ?

• If letter behind target was sharpest, camera/lens combination suffers Back Focus.
• If the letter in front was sharper your camera/lens combination suffers Front Focus.

       Many new cameras now allow user adjustment to correct Front & Back Focus problems,
          and some with each specific lens. If you do not feel confident to do this yourself, contact
          your local camera shop or service centre.

If results show a focus error:

1. Repeat tests from the opposite angle.
   If focus error from opposite angles using one lens was offset to the same side (ie. Always to the left), this could indicate a misaligned lens element.
2. Repeat all tests above with a second fast lens.
   Identical mis-focus results (ie. both Back Focus, or both Front Focus) would indicate camera is setup incorrect.

Important Notes:

• When handheld shooting, park so that you can use the car to steady your camera.
• * EXIF data does not indicate whether stabilisation is On or Off, so if you do this test and are also comparing how stabilisation versus no stabilisation affects sharpness, ensure you make note of the number of shots taken for each sequence!     

Article © Adrian Harris