"Faster lenses are always better"
You often hear that to get the best image quality, get the fastest possible prime lens out there, and then use it stopped down. A faster lens (smaller f-number) can be stopped down more for normal shooting, e.g. at f/5.6, which will give you better sharpness than a less fast lens also stopped down to f/5.6, the argument goes.
This view comes from the first generation SLR cameras, which typically came with a fast normal lens, e.g., 50mm f/1.7. On an SLR camera, with a long register distance, a fast 50mm lens can be made quite cheaply, which is why this was the "kit lens" of the time. These lenses were typically not intended to be used at the maximum aperture. They had this fast aperture to help the user get the focus right, though, which can be harder with a smaller aperture lens. Also, the faster aperture gives you a more bright viewfinder image.
What this argument fails to take into consideration, is that a less fast lens can be made with a simpler lens design, with less optical compromises. A very fast lens, on the other hand, needs more lens elements, and a more complicated optical path. This can reduce the contrast, and create other image quality issues. For this reason, to make an equally good very fast lens is a very difficult task, as it requires more optical compromises in the design.
Still, there is often some merit to this advice that faster lenses are better, but for a different reason: When producing lenses, the manufacturers will put more prestige into the fastest ones, as they are sold at higher margins to the enthusiasts who are more critical towards their equipment. That leads to larger aperture lenses utilizing better materials, having better optical designs, and better quality control.
Today, fast lenses are intended to be used wide open, even if they are still not optimal at the largest aperture. Here is a simple comparison which illustrates this. I have compared the Sigma 30mm f/2.8 DN Art (my review) and the Lumix Leica 25mm f/1.4 (my review)
These lenses are quite similar in focal length: The Sigma lens is slightly longer, but you could view both as typical normal lenses. In terms of aperture, though, they are very different. The Leica lens is two stops faster, and three times as expensive.
To compare them, I took these images with both lenses. The pictures were taken with a bright sky as background, which is typically more challenging to render with a good quality:
|Sigma 30mm f/2.8 DN Art||Lumix Leica 25mm f/1.4|
As the lenses have slightly different field of view, I put the tripod closer when using the 25mm lens.
So, which lens is better? Here are some 100% crops to illustrate this (click for larger images):
And from the right top corner:
So, we see that the Lumix Leica 25mm f/1.4 is better when stopping down a couple of stops, but it is also usable wide open at f/1.4. Further, I would say that at f/4 and f/5.6, both lenses are comparable. The much cheaper Sigma 30mm f/2.8 DN Art is slightly worse at f/2.8 than at f/4, but other than that, I think the image quality is completely adequate, compared with the Leica 25mm f/1.4 lens.
Here is another example where an f/2.8 lens is better than a much more expensive f/1.8 lens, albeit comparing across camera systems.
Leica is a company that makes a range of high grade lenses with different maximum apertures. For example, they have a 50mm f/2.4, 50mm f/2, 50mm f/1.4, and finally 50mm f/0.95. Generally, it is well understood that you don't get better image quality with the f/0.95 lens, you just get the opportunity to use a very large aperture. For the best image quality for your use, go for the lens with a maximum aperture that matches what you need. Most of the time, f/2.4 can be just fine.
"Metal is always better"
When reading online discussions and reviews, you will often find people judging the build quality of camera equipment on the basis of how much exposed metal materials they have. The more metal you can see and feel, the better.
A lens which has a plastic exterior, and, especially, a plastic bayonet mount, is sure to get poor reviews. This leads to a trend we see now: Manufacturers putting a thin layer of metal around all lenses.
One example is the Lumix G 12-32mm f/3.5-5.6 (my review) pancake zoom lens, seen below (to the left) compared with the older Lumix X PZ 14-42mm f/3.5-5.6 (my review):
The older black lens has a plastic body, while the newer silver lens has a bare metal surface.
However, is the left lens better, with the metal surface? Here is what has happened with my lens, and many others, as I hear: Both the front and rear cover have come off:
Sure, it is easy to glue them back on, but I think the lens would have been much better, and less expensive to produce, had it been plastic all around. However, that would have been a market risk for Panasonic, as people would have complained about the "poor plastic build quality".
Here is another example. Two cheap kit zoom lenses, Lumix G 14-42mm f/3.5-5.6 (left), and Nikon 1 11-27.5mm f/3.5-5.6 (right):
The Lumix lens has a plastic bayonet, while the Nikon lens has a chromed plastic bayonet. (Read more about it here.) The Lumix lenses are routinely getting poor reviews for the plastic bayonets, while most seem to think that the Nikon lens is genuine metal.
In reality, the plastic material used is completely adequate for the bayonet. I have accidentally dropped the Lumix kit zoom lens on a hardwood floor a couple of times, and, after careful checking, I have confirmed that it performs just as good as before. Even if it has a body made from 100% plastic, it is very solid. Plastic is simply a very good material for moderately sized lenses.
"Pro zoom lenses must have a constant aperture"
Traditionally, professional grade zoom lenses have always had a constant maximum aperture across the focal length range. A typical example is the Lumix X 12-35mm f/2.8 (my review). This very good lens is modeled after the traditional high end standard zoom lens, with the 24-70mm equivalent focal length range.
Olympus made their take on the lens a bit less traditional, with a longer 24-80mm equivalent range.
However, there are no rational reason why the maximum aperture range should be constant. Perhaps it makes the lens slightly easier to use if you set the exposure parameters manually, but this hardly a big issue.
The relationship between the focal length (f), aperture f-number (N), and entrance pupil diameter (D), is: N=f/D. This shows us that as the focal length becomes longer, the entrance pupil diameter must be much larger, given the same aperture.
Or, put the other way around, given the same entrance pupil diameter, the aperture can be larger in the shorter end of the zoom range.
Of course, zoom lens design is much more than just solving the equation N=f/D. But it goes that many pro zoom lenses are often deliberately limited in the shorter end to get the constant aperture property.
Samsung have gone against this tradition, with their Samsung NX 16-50mm f/2-2.8 pro grade standard zoom lens. Samsung's customers are probably much less conservative — if they were conservative, they would have chosen Canon or Nikon — hence they can afford to make a more rational, but less traditional lens design.
Oddly enough, the most traditional of all, Leica, have also made a non-constant aperture zoom lens, for their new SL system:
Hopefully, more manufacturers will deviate from the traditional norm here, to give us more rational lens designs.