When I was a still photographer I wouldn't have ever dreamed of shooting with an all-in-one zoom lens. These lenses are notoriously cheap and suffer from a slow, variable aperture, soft focus, and other optical issues like vignetting and chromatic aberration. All of these issues are deal breakers for a still image but are much harder to detect with video and the convenience of having a lens that can "do it all" has outweighed these negative aspects for many of my video projects.
Because of this, I find myself using the Panasonic 14-140mm more than any other lens but I'm always interested to know if there is a better option. Let's put this lens head to head with its closest competitor, the Tamron 14-150mm.
The Panasonic 14-140mm used to cost $700. I would know because I bought two of them at that price. Today, perhaps with pressure from Tamron, they have dropped the price to a much more reasonable $500. The Tamron version is cheaper at just $400 but lacks optical stabilization, a feature that may or may not be important to you.
Panasonic lenses for Micro Four Thirds cameras aren't known for their exceptional build quality. The lenses are small, light, and somewhat flimsy. The Tamron lens feels pretty similar when you first pick it up but after feeling the zoom and focus ring on both lenses, the Panasonic is the clear winner. It's difficult to explain, but the consistent, slow, smooth motion of the Panasonic lens feels more precise and higher quality.
The Panasonic lens is sharper in the center while the Tamron was sharper in the corners. The Tamron struggled with chromatic aberration and vignetting more than the Panasonic lens. Both lenses are far from "professional level" but when it comes to video, many of these flaws are imperceptible. If I had to choose a winner here it would be the Panasonic lens by a very small margin.
Perhaps the best feature of the Panasonic GH5 is its internal stabilization. If you purchase the right lens, the sensor and optical element in the lens can work together to create even steadier shots (Dual IS). I expected the Panasonic lens to perform much better than the Tamron (which doesn't have any optical stabilization) but in our tests, we found that both lenses performed almost identically. This goes to show just how impressive the sensor stabilization is on the GH5.
If you plan to use these lenses on a camera that does not have internal stabilization, like the new GH5S, the lack of optical stabilization in the Tamron lens becomes a deal breaker. The Panasonic lens on the GH5S is still a great lens to use handheld while the Tamron lens becomes completely unusable handheld, especially at 150mm (300mm full frame equivalent).
I personally don't use continuous autofocus while I'm recording with the GH5 because I find it unreliable but I do use autofocus before recording each shot. Autofocus on both lenses appeared to be fast and accurate but I did notice that the focus on the Tamron lens would jump around while I was zooming making it very difficult to see and slower to refocus.
If the Panasonic 14-140mm f/3.5-5.6 was still $700, this would be a harder decision but being that it now costs $500, the choice to me is pretty clear. Spend the extra $100 and get the Panasonic lens. Although it's not a wildly better lens, you'll appreciate having the slightly better build quality, optical stabilization, and autofocus.