A common question people have about lenses is whether extending lens barrels are worse than fully sealed internally zooming/focusing lenses.
There are two lens actions that can cause the lens barrel to move in and out. Those actions are focusing and zooming. Some prime lenses will have a moving front element and that will cause the lens barrel to move in and out during focusing. On the other hand, many zoom lenses will have lens barrels that move in and out during zooming. A few, mostly older designs, will have front elements that move in and out during zooming and focusing.
Because people see that it is mostly older and cheaper lenses that have lens barrels that move, they often assume that this means the lens is flawed in some way. And in some ways that can be seen as true however there are also several advantages to designs with extending barrels.
Advantage #1, the lens can be collapsed and fit into a smaller space for transportation. With moving lens barrels the lens changes size so that the lens can be fully zoomed in or out to make the lens as small as possible. This makes fitting the lens in a camera bag just that much easier.
Advantage #2, simpler optical design. For the most part we will see that designing zoom lenses with a moving front element makes it easier to achieve a good optical result. In other words, it is one of the best compromises a lens designer can make when designing a zoom. The lens will be cheaper, better performing, and probably lighter and easier to handle. A lot of other factors will play into a lens design but as a rule these things will be true.
Advantage #3, lighter overall weight and better balanced. The thing I hated most about fully internally zooming 70-200’s was how front heavy they were. After hours of shooting at a wedding it really starts to feel heavy. The new RF 70-200mm f/2.8 which has an extending barrel zoom design is much easier to handle by comparison.
I wouldn’t be writing this post if it was all advantageous to have an extending lens barrel so here are the disadvantages of these setups.
Disadvantage #1, changing lens dimensions. In some cases, such as balancing a camera on a gimbal, zooming the lens in and out can cause the camera to go out of balance if the lens extends too far. When the camera goes out of balance this can cause the gimbal to malfunction or even cause the gimbal motors to overheat. If the lens is small enough and light enough you might not have an issue with the camera being just a little out of balance, but with a larger lens like Canon’s new RF 70-200 and 100-500mm telephoto zooms it could be a potential issue.
Disadvantage #2, potential for dust to enter the lens. Modern lenses are usually well sealed against dust but, I still hear about people who are getting dust in their lenses. The reality is you will want to avoid dusty and damp environments most of the time regardless of the lens design as any lens or camera can get fine dust particles in it. But it still does seem like people have more issues with dust when the lens has a large amount of travel on the front element. This could be due to the pressurization effect. Essentially, the lens must suck air inside of it when the front element extends because extending the lens increases the internal volume of the lens. This can cause dust and moisture to be pulled into the lens when zooming in and out.
Disadvantage #3, internal FOCUSING lenses typically focus faster. It is important to note here the difference between internal FOCUSING and internal ZOOMING, most modern zoom lenses are internal focusing even when they are external zooming. Many, many lenses fall into this category such as the RF L zooms like the RF 70-200mm f/2.8. While that lens is externally ZOOMING it is internally FOCUSING which is good because internally focusing lenses typically focus much faster and the RF 70-200mm f/2.8 focuses very fast with its dual AF motors. In most modern designs external focusing elements are limited to prime lenses. Examples of lenses that are external focusing are both RF 50mm lenses, the 50mm f/1.2 L and the 50mm f/1.8 STM, as well as the RF 35mm f/1.8. A prominent issue with external focusing lenses is lens hood usage. If you put a big lens hood on there the focus motors now must move that extra mass around. Canon solved for this with the RF 50mm f/1.2 L which has a slightly different design where the front element moves back and forth inside of the barrel where the lens hood is mounted. This means no extra weight is placed on the focus motors with the hood attached.
Disadvantage #4, it doesn’t look as “professional”. When Canon released their EF 70-200mm that was fully internal zooming and focusing a lot of pros decided that a professional zoom had to be internally focusing and zooming. The reality is Canon never made many zoom lenses that were fully internally zooming and focusing. For instance, the EF 24-70’s all have extending lens barrels when zooming. For the most part if you want a well-sealed lens that is in the wide to normal range it will have to be a prime lens design such as the new RF 85mm f/1.2 L which has an internally focusing design.
There are pluses and minuses to both approaches, but I think that for 99% of shooting scenarios the extra weight and complexity of the internal zooming designs is more off putting than it is beneficial.
On the other hand, internal focusing is clearly superior to external focusing. Even in the case of the RF 50mm f/1.2 L where the front element moves inside of the lens barrel, that is still the largest and heaviest element group in the whole lens, and you feel that mass moving when it is acquiring focus. Ultimately mass is the enemy of responsiveness when it comes to focusing speed and tracking abilities of a lens. Additionally, any suction effect caused by moving the front elements around will affect the AF speed as well.
In conclusion you will pretty much always want an internally focusing lens unless it is a small, cheap, and simple design. But you can rest assured that an externally zooming design is probably not going to affect your lens performance much if at all. For the most part external zooming designs are going to make the lens lighter and better performing for the cost.