Another area to check is whether the lens have oil marks. When oil gets onto the blades, it will usually have an over-exposed picture due to the aperture size during a shot.
How does this happen?
The focusing threads mechanism will have grease and over time, the heavy grease oil will flow onto the aperture blades. This process is generally hasten when the lens is kept in a warm place as the oil will be thinner and will flow onto the blades. Old lenses usually suffer from this problem if not kept in good conditions.
What is the effect?
When the oil gets onto the blades, it will cause them to stick (contrary to popular believe). Generally, a the aperture will be wide open during focusing and only when the shutter is pressed, the aperture will resize to the value set. This has to happen very fast before the shutter is open for the shot. Any delay in the movement of the aperture blades will generally cause over-exposure.
What can I do?
Firstly, do not store lenses in a hot environment. Other than that, this generally occur when the lens is old. There may be a need to service the lens after a few years BUT there 20+ year old lens that do not have this problem.
In a hot tropical climate where it is also humid, fungus and oil on blades is something to watch for.