Construction of a photographic lens
Each individual glass lens inside a complete photographic lens is referred to as a "Lens Element." These elements are grouped and can move in relation to each other for focusing and zooming. They may also be cemented together to form a Doublet.
The Aperture is controlled by the position of the blades inside the aperture mechanism, which can be managed through mechanical or electronic linkages to the camera. Photographic lenses typically have discrete ("clicky") positions for setting the aperture in F-stops, while cinema lenses usually don't. Some photographic lenses can be modified to remove the "clicks" to achieve smooth movement of the aperture blades, ideal for video work. The aperture blades are usually made of very thin metal.
Manual-focusing lenses often employ a helical focusing mechanism. The helicoid is a geometric object used as a model for the threads of the focusing mechanism. It acts as a screw, allowing the focusing groups to slide forward and backward along the main axis in a smooth and precise motion.
Most Common Issues
Fungi can find their way inside lenses through naturally occurring spores carried by the wind, typically during focusing and zooming actions. Fungi thrive in dark and damp conditions. To prevent this, it's advisable to store your lenses in well-lit and ventilated spaces. If stored in closed spaces, using silica packs and exposing the lens to indirect sunlight can be helpful. Some lens coatings are more resistant to fungal growth than others, depending on the brand, model, and vintage.
Stiff Focusing Ring
A stiff focusing ring can have various causes, such as dry or contaminated lubricant on the threads of the focusing mechanism. Lens damage from dropping can also be a factor, as well as loose internal components, like rusty screws jamming the focusing mechanism.
Sticky Aperture Blades
Sticky aperture blades can lead to incorrect aperture settings and exposure issues. In vintage lenses, lubricant and dirt between the blades are common causes for this problem.
Element separation will manifest as an overall haze over a lens element or sometimes as a concentric rainbow pattern. The effect will be lower contrast images and prone to flare. This is caused by the failure of the cement holding a Doublet together, creating an air gap between lens elements that should act as one. This is common on older lenses that used Canadian Balsam as cement, which is prone to dry and separate over many decades, compared to newer ones that use stronger UV-cured resins.
Tools for lens maintenance
JIS and flathead screwdriver set
I recommend the VESSEL TD56-S as it packs the heads you'll most likely encounter on vintages lenses. Do you really need this? Yes, Philips standard drivers will cam-out and damage the screws. Read further on the differences between JIS and Philips.
Lens spanner wrench
This will be used to loosen retaining rings. The tips of the wrench fit on the notches, then you apply torque. There are different designs of wrenchs, the one I trust is the one pictured - the other designs tend to not be precise enough or not allow enough torque to work on stuck rings.
Lens rubber tool / Filter wrench
These are useful to loosen retaining rings that don't contain notches for a spanner wrench, usually the one on the front of a lens. These come in sets with the most common diameters. The work principle is friction, you push against the ring and apply torque. Because of this, the material it's made will influence the grip - the best ones are silicon. Also useful to remove stuck threaded filters.
Suction cup tool
These are useful for a clean reassembly of lens elements, avoiding smudges and helping position the elements neatly inside the barrel without simply dropping it in place.
Microfiber cloth and Lint-free lens cleaning paper
The cloth is useful for a rough cleaning and resting glass elements, but overall I avoid using it to clean glass since most often than not the cloth isn't 100% clean and any debris may scratch. For that I prefer disposable lens cleaning paper right from the package which I can guarantee are clean.
For cleaning the glass elements. The higher grade (90%) will evaporate faster and avoid smudges while drying, but on the side you'll have less time to work. The proper technique for cleaning with alcohol is to wet a new cleaning paper (not directly the glass), and then work it on the glass softly, in a circular motion and moving outward, but in one single movement. Going back and forth will just spread any dirt and oils across the glass, our objective is to dissolve and capture it on the paper. Throw this paper away. Repeat if necessary until there are no residue.