August 2026: Hardware Focus Locks
How do microscopes stay in focus automatically over time?
Anyone who has run a long live-cell imaging experiment knows the frustration: a sample begins perfectly focused, but hours later thermal expansion, stage movement, evaporation, or mechanical drift gradually push the focal plane out of alignment. Modern microscope manufacturers have addressed this challenge with hardware autofocus systems such as Nikon’s Perfect Focus System (PFS), Zeiss Definite Focus, and Leica Adaptive Focus Control (AFC). These technologies allow researchers to collect time-lapse data for hours or even days while maintaining consistent focus.
Nikon Perfect Focus System (PFS)
Nikon’s PFS is one of the most widely used focus-maintenance systems for live-cell microscopy. Rather than repeatedly analyzing image sharpness, PFS uses an independent near-infrared (870 nm) optical path to continuously monitor the position of the coverslip relative to the objective. Because infrared light does not interfere with visible fluorescence imaging, focus monitoring can occur continuously during image acquisition.
The system projects infrared light through the objective and detects reflections from a stable reference interface, typically the glass-medium boundary of the coverslip. If thermal drift or mechanical movement changes the distance between the objective and the reference plane, PFS detects the shift within milliseconds and commands the microscope’s Z-drive to compensate. As a result, the focal plane remains locked even during long acquisitions.
A particularly useful feature is Nikon’s optical offset capability. Researchers can focus on cells several micrometers above the coverslip while PFS continues tracking the coverslip reference. The system therefore maintains the desired imaging plane without requiring periodic autofocus scans.
Zeiss Definite Focus
Zeiss Definite Focus projects infrared light (approximately 835 nm) onto the bottom of the culture vessel and analyzes the reflected pattern to continuously measure the distance between the objective and sample carrier. Any detected drift is corrected through the microscope’s motorized focus drive. This approach is specifically designed to stabilize long-term live-cell and time-lapse experiments where temperature changes can otherwise cause focus loss.
Leica Adaptive Focus Control (AFC)
Leica AFC functions much like an optical distance meter. An 850 nm LED sends light through the objective, and the reflected beam is measured on a dedicated sensor. Changes in the reflection position indicate changes in objective-to-sample distance. The system then adjusts focus automatically to restore the original spacing.
Why Hardware Focus Locks Matter
Traditional software autofocus methods periodically acquire Z-stacks and search for the sharpest image. While effective, they consume time, increase photobleaching, and can temporarily interrupt imaging. Hardware focus-lock systems avoid these drawbacks by continuously tracking a physical reference plane and correcting drift in real time. This allows more reliable long-term imaging, reduced light exposure, and improved reproducibility in live-cell experiments. Please note that hardware focus locks rely on refractive index mismatches between the sample and the objective to work properly. These systems are designed for use with air or water immesion objectives and live/aqueous samples. This type of focus maintanence does not work well with oil immersion lenses and fixed samples with higher refractive index mounting media.
References
- Nikon MicroscopyU. The Nikon Perfect Focus System (PFS). https://www.microscopyu.com/tutorials/the-nikon-perfect-focus-system-pfs
- Nikon MicroscopyU. Perfect Focus Offset System Mechanics. https://www.microscopyu.com/tutorials/perfect-focus-offset-system-mechanics
- Nikon Instruments. Perfect Focus System (PFS) Product Information. https://www.nikonusa.com/fileuploads/pdfs/Instruments/TE2000_PFS.pdf
- Zeiss. Definite Focus Information and Documentation.
- Leica Microsystems. Adaptive Focus Control (AFC): Be in Focus – Always. https://www.unige.ch/bioimaging/photonic/application/files/6516/7422/0826/Leica_AFC_Be_in_Focus__Always.pdf