Let us start by talking about what a lens is. Often when we talk about lenses, we mean an objective we put on our camera, an array of optical elements which focuses the light rays on our sensor or film. Purely technical speaking, a lens is also a single element of an objective, one piece of glass, whereas an objective consists of many elements in a row to focus the incoming light in specific ways. To lessen confusion, an entire optical assembly will henceforth be referred to as a lens, and a single lens inside a lens will be referred to as an element.
How a lens is designed influences three very important things, of which two everyone is well aware of; The focal length, which combined with the size of your sensor or film determines the angle of view, and the aperture, which determines the amount of light which lands on said sensor or film. For both technical specifications are almost always printed on the front rim of the lens, in old times even written with a pencil on the side of the actual glass inside the lens. We know them as a length specified in mm or cm, whereas the aperture is noted as f/ or t/.
The third is rather logical, but bears no technical representation; The way a lens renders the image. The problem with this third aspect of a lens, is that it is not an objective one. Whereas aperture and focal length can be measured, resulting in an absolute number, the way an image looks or feels cannot be represented in mere numbers. How a lens renders the image determines many aspects of how the final result will look, from the rendering of fine detail (Sharpness), the quality of the out of focus rendering (Bokeh), any possible aberrations caused or fixed by the lens, contrast and many others. The whole package is the character of a lens.
Over time people have found ways to measure specifics of a lens' rendering by measuring the amount of fine detail a lens can pick up, shooting endless test charts at every possible aperture, often noted as lp/mm (Lines per mm). Spherical distortion can be measured, vignetting, and a few others. This leads to, what I believe, is a serious problem of our current age. It leads to the believe that sharper is better, that less distortion is better and that less vignetting is better and so forth. While some esthetics of a lens, such as vignetting, might be done in post-production, many cannot, simply because in post-production you do not have the actual light present to work with.
With this article I hope to persuade you into looking further then numbers, and create your own subjective view on lenses.
Anatomy of a lens
To understand the subjective nature of lenses, it is good to understand the way a lens is constructed. Most, if not all lenses will share these aspects. Besides the helicoid of a lens, these all have a direct influence on the way the image is rendered. How these parts of the lens influence the image we will discuss later on.
The optical formula
First and foremost is the actual glass in the lens. Many many types of glass exist, each with their specific nature and diffraction. Going that deep into this is futile, as we cannot get any useful information from the type of glass used. However, what we can in fact read and understand, is the optical formula of the lens. The optical formula is the way the glass inside a lens is arranged, which elements are chosen, where they go, where the aperture is located and so forth.
The optical formula of a lens consists of three things. First is the lens element, a single piece of spherical glass, which alters the path of incoming rays of light. The second is that of what is called a lens group, a combination of multiple lenses glued together to form one piece of glass. By glueing lenses together we can create a more complex lens, whilst using different types of glass. Most often you will simply find two elements glued together to form a group, but up to four or five lenses is certainly possible. The third is the aperture, often noted by a line with an opening in the middle.
Optical formulas might not be easily found for every lens, however, most specification sheets or old folders will be able to tell you a 'number-fied' version of this formula, noted with two numbers. For example, a lens might be noted as being a 6/2 lens, which refers to the amount of lens elements and the amount of lens groups. In this case, that means there are 6 lens elements in this lens, divided over two groups. From this information we can also understand that if we have 6 elements over 2 groups, the lens must consist of two groups of three lenses, three lenses glued together.
The amount of lens groups is, in these cases, a bit different from our previous definition of that. In these cases the amount of groups refers to any lens or group of lenses, so a single lens element will also count as a group. For example, a 4/4 lens refers to a lens consisting of 4 elements in 4 groups, thus each group is a single lens. From this we can get that this lens contains no glued lenses.
The lesser the amount of elements inside a lens, the easier it will be to understand or read the lens' character to a certain degree.
The aperture of a lens is any apparatus which lessens the amount of light passing through the lens. In our current age, the dominant method is that of a mechanism which uses multiple metal leaves, which can move all together to create a smaller hole in the middle of the lens, thereby limiting the light passing through. In the past this could have been achieved in many ways. One is the system of waterhouse stops, named after it's inventor John Waterhouse. These are metal plates with a specific hole in the middle, with a handle to insert them straight into the lens. The downside is that it requires a new plate for every aperture you would want to use.
Lens coatings are a rather late invention, but a very useful one. Normally, when you look at any optical glass, it will be pure white. Purely transparent. However, what if you now take a look at the lens on your phone or camera? On any modern lens nowadays you will see a purple, blue or orange reflection. This is the coating of the lens. It is an extremely thin layer of anti-reflective coating. On cheap lenses this wi ll be done on the front side of the lens and possibly on the rear. In more expensive lenses every single element inside the lens will be coated as well. A very surprising thing is that lens coating can actually happen naturally over hundreds of years, depending on the type of glass used. Slowly a lens might begin to build up a natural layer of anti-reflective material from the air. This process is called blooming. The discovery of this process is in fact the reason lens coating was invented, as it lead to purposely reproducing this effect in artificial ways.
Coating can be done in a single layer or multiple layers, the former known as single-coated, often visible as a yellow-ish or blue / purple reflection of the lens, the latter known as multicoating, often purple but with tints and hints of many other colors, green, orange, blue and what not. When no color tint is visible at all, the lens is most likely not coated.
The helicoid / barrel
The helicoid is not truly a part of the lens itself, but is important to talk about nonetheless, especially with our eyes on DIY work when considering lenses. The helicoid is the part which makes it possible for a lens to focus, it is an object that changes the distance between your sensor or film and the actual glass itself. This can be done in a multitude of ways, one of the more basic methods nowadays is to use an actual helicoid, a construction of two tubes with screw threads, which by turning pushes the other forward or backwards. Other methods are for example those used by view cameras, where the entire front standard of the camera moves to make focussing a possibility. This is also the reason why lenses pre-dating celluloid film often do not have any form of focusing, as they rely on the camera doing this. Other lenses from this era can be found with a so called rack and pinion system, where a row of flat tooth is layer along the lens and driven by a gear on an axis.
The barrel takes care of all other functions, such as an accessible way of controlling the aperture, a method of mounting the lens on the camera and what not. Often when adapting an ancient or unique lens to whatever camera you will be using, there is a lack of helicoid and mount. Either they are missing, unusable on your preferred platform or broken. Thus it becomes the goal to find a method of making your own focus apparatus and aperture control. Mounts might be one of the easier things to fix, due to the availability of broken lenses and spare parts or adapters.
-The influence of lens design on the image
[Abberations and their uses]
[Contrast (air to glass spaces)]
-The history / mistery of lenses
[Rapid Rectiliniears, Aplanats, Petzval]
[Triplets & Tessars]
[Post-computer lens design]
-What can I do with this?
[Where to look for old lenses]
[Ways to use old lenses]
[Making or altering things yourself]
[Get rid of sharp == good mentality]