Dust, dirt, finger prints and other deposits degrade the performance of a lens and lead to degradation of the images it produces. It is therefore important to maintain a high standard of general cleanliness, and to keep the rear and front elements of a lens spotlessly clean.

The most important part of a lens to keep clean is arguably the rear element. Any contamination on this surface is likely to produce significant flare in images. The most common problem resulting from contamination of a rear element is a bluish circle in the centre of an image. The front element is also important but can tolerate a certain amount of contamination without producing significant and immediately obvious problems. However, as contamination accumulates, so the images become softer and lacking in contrast. The easiest way to avoid front element contamination and damage is to use a protective ultraviolet or skylight filter.

The correct approach to cleaning lenses is described in the relevant manufacturer's handbook, and should be followed to avoid damage. However, in general, such processes involve two-stages;

• Use an appropriate blower or lens brush to remove dust and loose particles. This should be done whilst holding the lens upside down so that the dust falls away from the element. Compressed air should be used very carefully.
• Use a micro-fibre cleaning cloth to gently wipe from the edge of the lens in towards the centre. Use a lifting motion rather than rubbing which may press particles against the delicate surface of the lens. Alternatively, put a drop of approved lens cleaning fluid on to a piece of lens-cleaning tissue and proceed as with the micro-fibre cloth. Note that micro-fibre cloths do not need lens cleaning fluid. When no lens cleaning fluid is available, blow gently on the lens to deposit condensation on the element before cleaning in a similar fashion.

Most lens cleaning fluids consist of alcohol. silicone or glycerine-based products. However new products, generally known as residual oil removers (ROR), are based upon natural ingredients which remove the difficult microscopic oil residues from the glass and plastic surfaces.