How to fix a blurry image at high microscope magnification
If a slide looks sharp at low power but blurry at high power, center the specimen before switching objectives. Check that the new lens clicks into position, then adjust focus gently using the fine control.
Start low and center the feature
Focus with the lowest suitable objective and move the feature of interest to the middle of the field. The field narrows at higher magnification. A feature near the edge can disappear even if the optics are working.
Distinguish focus from low contrast
With the high-power objective in place, make small fine-focus adjustments. Check the illumination, condenser and aperture using the instrument’s instructions. If edges remain soft everywhere, compare a clean prepared slide and inspect the lens and coverslip for contamination.
Check the objective label
Use immersion oil only with an objective marked for oil and clean it using the manufacturer’s procedure. Keep coarse focus away from a short-working-distance objective unless the manual specifically instructs otherwise. More magnification cannot recover detail the optical system does not resolve.