What is back focus, and how can you diagnose and fix it on DSLR or mirrorless cameras?

• A. Back focus occurs when the camera focuses in front of the intended plane; diagnose with a test chart and adjust AF microadjustment if available.
• B. Back focus occurs when the camera focuses behind the intended plane; diagnose with a test chart and adjust AF microadjustment if available.
• C. Back focus is when the lens cannot autofocus at all.
• D. Back focus is when the camera's sensor is physically misaligned.

Answer: (B)

Back focus means the autofocus system is landing the sharpest plane behind the subject you intend to be in focus. In other words, the image you want to be crisp sits in front of the actual focus plane, so the subject appears softer while something behind it seems sharper.

To diagnose, use a flat, high-contrast test chart or a ruler on a level surface and set the camera on a tripod. Focus using a single point on the chart's center and shoot at a consistent distance and lighting. Review the image at 100% magnification in a stable way (live view magnification helps). If the area you aimed to be sharp is blurred and you can see that the plane behind that target looks relatively in focus, you’re seeing back focus. You can repeat this at a couple of distances or with different focal lengths to confirm.

Fixing it starts with calibration if your camera supports AF microadjustment (AFMA). Enable AFMA, then run a controlled test and adjust in small steps (often in small increments like ±5) while shooting test images at the same distance. Recheck until the intended plane becomes sharp. Be aware that AFMA can be lens-specific and sometimes varies with focal length, so you may need to test at multiple focal lengths or for zooms. If AFMA isn’t available or the problem persists across lenses, the issue could be a lens misalignment, a decentered lens, or sensor/mount alignment, which typically requires service or replacement. As a quick check, try manual focus with live view at high magnification to confirm the intended plane can be sharp when you control focus yourself.