Localised softness, haze or flare usually means contamination on the lens or the housing window.

• Correct cleaning order: blow off dust and grit with an air blower first, then wipe one way with lens tissue or a lint-free cloth lightly moistened with lens solution. Never rub a dry lens hard, as grit will scratch the coating.
• What not to use: no alcohol, thinner, washing-up liquid or paper tissue. These attack the anti-reflective coating and the housing finish.
• Water spots and hard-water marks: mineral deposits left by evaporating rain degrade image quality persistently. Fit a sunshield after cleaning so rain does not strike the glass directly.
• Domes and windows: when the inside of a dome fogs or attracts dust, dismantle and clean following the vendor's instructions, and make sure the seal is seated correctly on reassembly.
• Rule out defocus: if the picture is still soft after cleaning, re-adjust focus or back focus rather than scrubbing harder.

Add lens cleaning to the quarterly inspection list, particularly at coastal sites, construction sites and locations near kitchens where dust and grease accumulate.