When selecting normal range zooms I dislike those with very limited ranges. I'm not a fan of the big, heavy 24 to 70mm f2.8 zooms (on FF) because they are too cumbersome for what they deliver and they can't get me into a portrait range that I like. When I shot with Canons I nearly always had their venerable 24-105mm f4.0 L lens attached to a camera. Now that I'm shooting frequently with Nikon cameras I lean toward the (surprisingly good) 24-120mm f4.0 and am happy to have the extra 15mm of focal length at the long end. It's a nice lens for a portrait photographer! But when I bought the Panasonics I relied on my recent memory of having purchased and used their twin "pro" lenses, the 12-35mm f2.8 and their 35-100mm f2.8. I remember sometimes being frustrated by the limiting 70mm equivalent at the long end of the 12-35mm and miffed at having to always carry two cameras, each with a lens mounted on it, it cover the range at a fast moving event, conference or even theater dress rehearsal.
Those lenses left my inventory in my purge of the Olympus EM-5.2 cameras a while back and so I came to populating the GH5 system inventory with a clean slate. I had the prejudice that lenses with less extreme ranges would be better optimized than those with extreme ranges and that belief made me leery about looking at the Olympus 12-100mm, even though the focal length range is like something from heaven for the kind of work I routinely do. But my (extremely good) experiences with the breathtaking range of focal lengths provided by the Sony RX10 iii went a long way toward at pushing me to at least be open to a trying the longer range on the Olympus professional zoom.
I read many reviews before I decided to try the lens for myself. I borrowed one from a local camera dealer and spent a weekend shooting all kinds of images in all sorts of places. I tried every aperture and every focal length. When I finally sat down in front of my computer and started editing and then post processing my raw files a smile spread across my face. Here was a lens that was clearly as sharp wide open as it was stopped down to f5.6 or f8. It wasn't just "useable" at f4.0 it was superb at f4.0.
It also worked flawlessly with the Panasonic GH5. The system defaults to using the image stabilization in the lens rather than the in-body stabilization but I haven't found that to be a negative. In fact, when I bought the GH5S, which does not feature image stabilization, a lot of my comfort in buying that particular body came from the knowledge that my favorite Olympus lens would do a great job stabilizing images on that body.
I also put the Olympus 12-100mm ahead of the Panasonic twins and the Panasonic/Leica 12-60mm f2.8-4.0 because it is a superior lens for video work. Let me explain why. All the lenses discussed here use "fly-by-wire" to manually focus but only the Olympus (with it's separate setting for MF) allow you to access a marked focus ring which stops at infinity and also allows you to set a repeatable focus point which you can see and return to on the focusing ring. This allows you to easily and accurate preset a distance for focus and come back to it again and again. And, while the focus throw is a bit short you can, with practice, roll focus between two marked points reliably. Not really possible with the other candidates.
I've shot with the 12-100mm for over a year now and last week's big shoot was a good example of why I like using the lens so much. I was covering a conference for three long days, shooting tight shots of speakers on stage, wide shots of branding and stage design, shots with ttl flash and a wide open aperture and even a fair amount of video of the same subjects. I used the lens mostly at f4 and have yet to find a frame (if I didn't screw up on focusing) that wasn't sharp and pretty. I could use the lens on the GH5S and get image stabilization in both video and still photography and there were times when I switched into manual focusing just for the hell of it. After shooting 4,000+ images over the course of the event, everywhere from dimly lit ballrooms, exteriors, tented venues and in crowded team rooms, I found that the vast majority of images were done with this one lens.
A much smaller percentage of shots were executed with the 40-150mm f2.8 (an amazing lens that is sharper than any Canon or Nikon 70-200mm lens I've used) because I used it mostly to get tight headshots of speakers on stage from a discreet position in the back of a hotel ballroom (not a very large ballroom). An even smaller number of images were done with the Sigma 30mm f1.4 contemporary lens and I used it just to see how it handled at its widest aperture. It was great as isolating people in the crowd.
The 12-100mm is not a cheap lens to buy but it is cost effective once you realize that it's usable on nearly every project you'll end up doing with micro four thirds systems. The lens is water and dust resistant, built almost entirely of metal, has very effective image stabilization and, for the range, is not heavy or cumbersome. An added bonus, if you shoot with Olympus EM1.2 and EM5.2 cameras, is that the lens I.S. and camera I.S. will work together to give you pretty spectacular stabilization.
In the course of the last year I've used this lens extensively in video projects and even more extensively in photography assignments and it is one of the gear investments that paid for itself many, many times over.
The Olympus Pro series lenses are in a class by themselves. They are remarkably sharp and well corrected. I'm not sure how much of the correction is being done by processing in the cameras but I can see that I'm not experiencing image degradation in the corners, even with the 10 megapixel camera, and that kind of image damage is usually a clue of too much processing and not enough file information. I'm not seeing those effects with the 12-100mm.
So, bottom line, how good is the Olympus 12-100mm f4.0 lens? Literally, it's my "desert island" choice. I'm using it for a project this afternoon and for a video on Saturday. It's also the first lens to hop in my camera bag and head for Iceland at the end of October. If I dropped, lost or otherwise didn't have one I'd rush to my local dealer and replace it immediately. There are few other lenses whose absence wouldn't at least trigger a "what a good chance to see what else is available...." but the 12-100mm is a non-negotiable part of my system. The other "must have" is the 40-150mm f2.8 but we'll save that for another day.