You point your DSLR at a portrait subject, the autofocus beeps its confirmation, and the shot looks sharp in the viewfinder — yet the eyes come out soft on your monitor while the ears are tack sharp. If this keeps happening with the same lens, you may be dealing with front focus or back focus: a small, repeatable error in the camera’s phase-detection autofocus. Most enthusiast and professional DSLRs let you correct it through a feature Canon calls AF Microadjustment and Nikon calls AF Fine-Tune. Here’s how it works and how to use it.

Why a DSLR Can Miss Focus Even When It Thinks It’s Sharp

In a DSLR, autofocus during viewfinder shooting relies on a dedicated phase-detection AF sensor that sits at the bottom of the mirror box. A semi-transparent section of the main mirror lets a portion of the incoming light pass through to a secondary sub-mirror, which bounces it down to that AF module. The AF sensor measures focus, and the camera moves the lens to where the AF sensor reports the image is sharp.

But the photograph is recorded on the image sensor at the back of the camera — not on the AF sensor. If the optical path to the AF sensor differs from the path to the image sensor by even a fraction of a millimetre — from manufacturing tolerances, a worn or loose lens mount, or a knock — the lens can be focused perfectly for the AF sensor while still being slightly off for the sensor that actually records the image. Because that misalignment is fixed rather than random, the same lens tends to misfocus the same way every time. That consistency is exactly what makes the error correctable.

Front Focus vs. Back Focus (And What the Correction Does)

Front focus means the plane of sharp focus lands in front of where you aimed; back focus means it lands behind. The correction simply shifts that focus point. On both Canon and Nikon cameras the adjustment range is ±20 steps (values from −20 to +20), with each step representing a tiny fraction of the lens’s depth of field. Negative values shift focus toward the camera and positive values shift it away.

Canon offers two modes — Adjust by lens (a value stored for the specific lens attached) and All by same amount (a global offset). Nikon’s equivalent stores a Saved value per lens (many bodies remember up to 20 lens types) plus a Default value applied to any lens without its own entry. For zoom lenses, Canon automatically interpolates the intermediate focal lengths from the wide and telephoto settings, while Nikon lets you save separate WIDE and TELE values.

How to Test a Lens for Front or Back Focus

You need a repeatable target and a consistent method. Set up a flat focus chart or a ruler laid at a 45-degree angle, mount the camera on a tripod, open the lens to its widest aperture, and use a single AF point aimed at a high-contrast line on the chart. Before each shot, defocus the lens (turn the ring to minimum focus or infinity) so the camera performs a fresh autofocus run rather than reusing a previous lock. Take several frames and check where the sharp band sits.

If the sharp zone consistently falls in front of or behind your target line, you have a front- or back-focus error — and a sense of how large it is. Nikon recommends tuning at a focus distance you actually use, since a value that works at close range can be less effective at longer distances. Dedicated software such as Reikan FoCal and physical charts like LensAlign or SpyderLensCal automate this measurement and take the guesswork out of picking a value.

Applying the Correction on Canon, Nikon, and Third-Party Lenses

On Canon, head to the AF menu and select AF Microadjustment, then choose Adjust by lens or All by same amount, set a value between −20 and +20, and shoot-and-check repeatedly until the target is sharp. On Nikon, open the setup menu, enable AF Fine-Tune, then use Fine-tune and save lens (with separate WIDE and TELE entries for zooms). Some Nikon bodies, such as the D850, add an Auto AF Fine-Tuning routine that uses live-view autofocus as a reference to compute the correction automatically.

Importantly, the correction applies only to viewfinder (phase-detect) autofocus — Live View and video use the image sensor and are not affected. Third-party lenses get the same treatment through manufacturer tools: Sigma lenses can be calibrated with the Sigma USB Dock and Tamron lenses with the TAP-in Console, which store the correction in the lens itself so it carries over to every body you use.

Why Mirrorless Cameras Mostly Don’t Need It

Mirrorless cameras do away with the separate AF module entirely: their phase-detection points are embedded on the image sensor itself. The autofocus measurement and the recorded image share the exact same plane, so there is no separate optical path to fall out of alignment. Native mirrorless lenses therefore generally need no microadjustment. Nikon Z bodies do retain an AF fine-tune option, but it exists mainly for older F-mount lenses adapted through the FTZ adapter; Canon RF and Sony E native lenses have no equivalent per-lens adjustment.

Conclusion

AF microadjustment is a one-time, per-lens calibration that fixes a real but subtle optical misalignment. If a lens consistently front- or back-focuses, a handful of test shots and a small correction can recover sharpness that was always there but slightly misplaced. Just remember to tune only when you see a repeatable error, and only for viewfinder autofocus.

FAQ

Do I need AF microadjustment on a mirrorless camera?
Generally no for native lenses, because autofocus happens on the image sensor. Nikon Z cameras keep the option mainly for F-mount lenses adapted via the FTZ adapter.

Does the correction affect Live View or video?
No. On a DSLR, AF microadjustment applies only to viewfinder (phase-detect) autofocus. Live View and video use on-sensor autofocus and are unaffected.

How do I know whether my lens front- or back-focuses?
Shoot a 45-degree focus chart wide open on a tripod with a single AF point, defocusing between shots. If the sharp band consistently sits in front of or behind your target line, you have front or back focus respectively.