After looking at the basics of vintage interchangeable lenses, there is a point when you will have to make some decisions about choosing a lens. There are literally millions of vintage lenses out there in the ether. Some are exceptional, most are good, some are mediocre. But even mediocre lenses can be interesting if they are cheap enough, and you want to experiment (even if that involves taking a lens apart and putting it back together again).
Choosing a lens is often quite a daunting experience, because there are so many possibilities, and it can be hard to narrow things down. Some people only buy lenses from a single manufacturer, others only buy lenses of a single focal length, others only buy lenses from a specific time period, and still others buy lenses that have a cool bokeh effect. The type of vintage lens you buy is a very individualistic thing. You can read the reviews about particular lenses, and formulate your own opinion about buying a lens, but you never really know what you get until you use it. Lenses come in different versions, and sometimes from different factories. Choosing a lens is also more than just looking at quantitative data such as lens sharpness, it is often more about the aesthetic appeal of the image produced, than the exactness of the MFT (Modulation Transfer Function) diagrams.
The best place to start is to decide what the lens will be used for. This aids in determining the focal length and lens characteristics, helping to constrain the search. For example someone who takes architectural photographs may be interested in a shift perspective lens such as the Olympus OM 35mm f/2.8 Shift lens. For astrophotographers, a sharp lens that is well corrected for coma is important. Those who take portraits may opt for a 85mm lens. If you have no specific needs, then start with a 50mm lens – it is by far the most common vintage focal length (and every manufacturer produced various models). Don’t forget that a lens behaviour will depend on the size of the camera’s sensor it is used on. For example a 50mm lens with a standard 46° diagonal angle-of-view, will behave like a “full-frame” 75mm lens on an APS-C sensor (with a diagonal AOV of 32°), i.e. the 50mm lens will be “the equivalent of” a 75mm FF lens.
Once you have an idea of the focal length, then you need to decide what features you want: must-haves, nice-to-haves, and things-not-needed – and of course how much you want to spend. Choosing a focal length is of course the easy part. Now you have to choose a brand, and a configuration (aperture, number of aperture blades, mount). This is harder because there are a lot of choices. Sometimes the best approach is to take the lead from someone who has done some of the hard work for you. For the beginner, there is very little difference between any 50mm lenses from the core camera manufacturers: Pentax, Canon, Nikon, Minolta, Zeiss, etc., they are all pretty good. Of course the “elephant in the room” is often budget. Choosing an upper limit on how much you are willing to spend on a lens will help constrain your search from the get-go, which helps avoid pursuing lenses that are nice, but just ooh too expensive. it you can’t afford it, don’t go there.
Now with a focal length, and a budget, it’s time to explore aperture, or lens speed, i.e. when the aperture of a lens is completely open, how much light comes in. In situations where you are shooting in broad daylight, then is there any need for a 50mm f/1.4 lens, or would a f/2 suffice? For example, Canon 50mm rangefinder lenses were produced in three different types: f/2.8, f/1.8, and f/1.2. The f/2.8 is 128g, the f/1.8 is 270g, and the f/1.2 is 322g. The f/1.2 lens will provide a lot of light, but at 2.5 times the weight. A lot of effort can be put into deciding the speed of the lens. However the faster the lens, i.e. the larger the aperture, the more expensive the lens will be. Don’t spend a lot of money on your first few of lenses. You can experiment with these lenses and decide whether using vintage lenses is for you.
Once you have chosen a lens, it is good to review some of the literature on the lens. How expensive is it? What about availability – is it common or rare? Is the lens generally found in good stead, or is it susceptible to abnormalities such as fogging/hazing, or has a front lens which is easily scratched? For example it is common knowledge that some of the Canon rangefinder LTM mount lenses are very susceptible to fogging. Is it easy to repair if there are issues?
Then it helps to look at some reviews, either on blogs, or videos. Just remember that reviews are a subjective viewpoint from someone with similar or differing circumstances to your own. Try and stick to reviewers that have a long history of reviewing vintage lenses. If their lens review, how did they evaluate the lens? Did they show actual photographs taken using the lens, in a good variety of situations? Did they take the shots using the same sensor size as your camera has? Was it a review performed with the lens on a film camera? It’s best to look at several different views, but always keep in mind that the lens you might buy will be different to the one they are reviewing. It could be a newer model of a lens which uses the same optical formula, but is housed differently, or uses a different aperture mechanism, e.g. number of blades; or perhaps it uses a different type of glass in some of the lens elements, e.g. thoriated versus regular glass. There is never a guarantee that the lens you buy will behave in exactly the same manner.
Don’t choose a lens solely on its specifications. Just because a 50mm f/1.2 lens seems like a good idea because it is rare, and some people proclaim it as the “best” lens, does not mean it will work properly for what you want it to do. Don’t be swayed by the large collections of some photographers. Some people take photographs for a living, and so may have more requirements, and also more ability to sustain large collections. Having dozens of 50mm lenses is fine, if you can afford them, and if you are actually going to use them (and it’s easy to fall down this rabbit hole). You may decide after some investigation that the lens you had coveted it really not something you need – for various reasons. Maybe it’s too expensive? Maybe it’s too hard to find? Maybe it is notorious for needing repairs?