Camera bodies come in different prices and sizes, with different features, the bottom line being that there are many things to take into consideration. Much of this is also applicable to lenses, which means that there is no such thing as the best lens.
What is a mount?
A mount, or, more specifically, a lens mount is the interface between the lens and the camera. If you take a look at a camera body allows interchangeable lenses, it is the metallic, shiny ring on the front of the camera body. There are different types of mounts, of which the bayonet type is the most common these days. The idea behind its design is simple. You take a compatible lens and insert it to the camera body. The camera body and the lens typically have a mark, such as dot, to indicate how they should be aligned with one another at that point. Once inserted, you then turn the lens in the right direction or the body in the opposite direction, as guided by the lens mount, as you can only go one direction, until you hear a click. That means that the lens is aligned correctly and locked in place. There is a small button somewhere next to the lens mount, on the camera body, that releases the pin holding the lens in place.
While different cameras may share the same type of mount, such as the bayonet type, the specific mount limits what lens can be used with that camera. To keep things simple, you are only supposed to use the lenses designed for that lens mount. You can, of course, use any lenses that are compatible or made compatible with the mount.
In most cases this is a relatively simple matter. You check the lens mount of each camera body and each lens. If they are the same, they are compatible. If they are not the same, they are not compatible. In some cases, it is, however, possible to make them compatible. The simple case involves the use of adapters that operate in between the two. The less simple case is having the lens mount changed by someone who knows what they are doing.
Adapters, a quick fix?
There are two key things to keep in mind when opting to use an adapter. Firstly, not all lenses can be simple adapted to be used with certain camera bodies. Each lens mount has a flange focus distance or FFD, which indicates the distance between the mount itself, i.e., the ring on the camera body and the corresponding in on the lens, and the sensor inside the camera body. As a rule of thumb, you cannot use lenses that have a shorter FFD on camera bodies that have a longer FFD (without resorting to optical adapters), whereas you can use lenses that have a longer FFD on camera bodies that have a short FFD (with a simple adapter that adds extra distance to match the FFD). Secondly, lenses made for certain lens mounts correspond to different sensor (or film) sizes. This means that while you can adapt just about any lens to work with just about any camera body, lenses are typically designed to cover only a certain (sensor or film) frame size. Therefore, if a lens is designed to cover a larger frame size than the camera body frame size, you will have full coverage of the frame with that lens. However, if the opposite is the case, if a lens is designed to cover a smaller frame wise than the camera body frame size, you will not have full coverage of the frame with that lens. In practice, this means that you can adapt a large- or medium-format lens to function with full-frame and APS-C camera bodies, a full-frame lens to function with APS-C camera bodies, while having full frame coverage, but not vice versa (you will have to crop the resulting image or use a crop mode on the camera, as you will get little to no light on that area of the frame).
There are also other considerations. Lenses can be adapted from one system to another, even autofocus lenses, but the results may vary. In some cases, you can even adapt manual lenses to operate as autofocus lenses.
If you have mirrorless interchangeable-lens camera (MILC), you can adapt almost any lens made for single-lens reflex cameras (SLRs), regardless of whether are from the film era or from the digital era, because its FFD is bound to be shorter than the FFD of the lenses designed for those other systems. Of course, you need an adapter for that and the results may vary as you are, indeed, adapting something designed for one system to be used with another system. For some people this is the charm of it. They want to try glass that might not be the best in its class, but has a certain look to it, as flawed as that might be according to others.
In terms of convenience, a lens designed for the system is much more convenient to use than a lens adapted from one system to another. I have little experience in lens adapters, having only used an M43 to Pentax K-mount adapter (screw-threated type lens mount to a bayonet type lens mount), and while I can confirm that it works, as intended, it is far from convenient.
Bigger is better?
While you can use lenses designed for larger systems (larger frame sizes) on smaller systems (smaller frame sizes), it does not mean that results are simply better than with lenses designed for those smaller systems. Firstly, and perhaps most obviously, an optically inferior, yet larger lens will not produce better results than a smaller but optically superior lens. Secondly, lenses designed for a certain system are optimized for that system. Simply put, the magic of larger systems (such as medium-format) is in that film or sensor size, not in the lenses.
In a sense, bigger tends to be better in terms of quality, as I explained in the previous post, but using lenses designed for larger systems does not mean the results are better. In addition, these lenses tend to be fairly expensive, heavy and most likely require an adapter if you wish to use them on smaller system.
The upside of using a lens designed for a larger system is that the smaller frame covers only portion of the frame the lens was intended to cover. Lenses tend to perform weaker in the corners on the rectangular frame than in the center as the lens itself is round. This means that you will, most likely, have poorer resolution in the corners than in the center of the frame. You will possibly also see some vignetting, so that the corners appear darker than the center of the fame. You mitigate these issues by operating at the center of a lens designer for a larger system. This does not, however, necessarily mean that the overall results are better as the quality of lens and the sensor output are what matters.
Smaller is better?
To be clear, simply because bigger is not necessarily better, it does not mean that smaller is better either. What really matters is how the lens performs on a camera body, how a certain lens designed for a certain frame size performs in comparison to other similar lenses designed for the same frame size or to other similar lenses designed for other frame sizes.
In practice, one should be comparing the results that one gets from a lens with the same focal length and the same aperture on systems that share the same frame size (film or sensor size). The bigger lenses will typically outperform the smaller lenses in the same class. It is also likely that the results have less distortion and other optical quirks.
The problem with bigger lenses is that are often more expensive than their smaller counterparts. If you use these lenses in good lighting, stopping down, utilizing the sweet spot of the lenses (in my experience often at around f/8 on APS-C, where their corner-to-corner resolution is maximized), prior to diffraction affecting the quality negatively, you will get comparable results, minimizing the difference between lenses.
The bigger and the more expensive lenses are not, however, designed to be used only in this way, in optimal lighting conditions. They are expected to perform also in difficult lighting conditions, when one cannot rely on the lens sweet spot. This also means that they tend to have a smaller achievable f-number than the smaller and cheaper options. The lens simply needs to let in more light in order to perform in low lighting conditions. Letting in more light also allows the lenses to be used more creatively, to achieve a shallow depth of field (in effect, the subject is in sharp, in focus, while the background is blurry, which gives it three dimension and, sometimes, a dreamy look). This is often also the reason why the lenses are as big and heavy as they are.
Not unlike in the previous post on camera bodies, it is difficult to give clear recommendations on lenses. Like camera bodies that have many features that may or may not be relevant to you, lenses also have many features that may or may not be relevant to you. In this post, I only covered what you should be taking into account when buying lenses.
If we combine the discussion with the previous post, you first need to consider the camera body, to account for whether it is suitable for what you will be using it, and for the lens mount that restricts you to it, at least in terms of convenience. Then you need to think what kind of lenses you need for your purposes and whether they are available for that camera body. If they are not available for that camera body, then you may have to consider another camera body.
If you are photographing landscapes, you will most likely be looking for wide-angle lenses. A good one will likely cost you some hundreds of euro, whereas an excellent one will likely cost you anything between a thousand and three thousand euro, depending on the sensor size and the camera manufacturer. If you opt to shoot on a tripod, in order to maximize the quality, the difference between the good and the excellent, not to mention the okay, the good and the excellent, is bound to be marginal. This means that you can get very good results without spending a month’s salary on a lens. Of course, you still need to account for other factors, for example weather sealing, but I believe that it still holds that you can get very good results, even on a tight budget.
These larger and more expensive lenses also tend to be designed to be in adverse weather conditions. The materials used to construct them, and to weather seal them, mean that the lenses will be bigger, heavier and more expensive than the smaller and cheaper designs.