Similar to another person asking the same question that I've googled, I'm also wondering if anyone has used an Olympus 12-40mm F2.8 on their GH3?

One big advantage I'm seeing is direct manual focus clutch system compared to the Panasonic 12-35. I'm looking to only get "the 1 lens to rule them all" and just set & forget if I ever get the GH4. Not really too concerned about the lack of IS.

Otherwise, sounds like a REALLY good MFT lens, but still am curious on how it fares with a Panasonic.

Not really too concerned about the lack of IS.

That's the main reason why I paid premium to get the 12-35, to have stabilization, if I turn IS on or off on the gh3 the difference is obvious and gets worse the closer I get to 35mm

It's a tough call. IS is definitely useful, but that focus by wire thing is utter crap. I was using the 12-35mm on a gig this week and tried to do a rack focus. I couldn't nail it, so I threw on my Voigtlander and got it right away. I have both the 12-35 and 35-100. Both very useful, but big focus changes are impossible. I used the 35-100 filming a comedy show this week with ETC mode switched on as I was shooting at a distance from the stage, and focusing was easier because the comedian had a small stage on which to maneuver. IS was nice as I was on my monopod roving around the room.

I really wish the Panasonic lenses could incorporate a manual focus ring. I was going to hold out for the new Panny 42.5mm, but am going to get the Voigtlander 42.5mm for half the price. I was also looking at the Olympus 75mm, but am going with the Rokinon 85mm. Manual focus and speed are more important than autofocus and IS, and those two win out. Best of luck!

Much depends for what you plan to use the lenses, I used to own a set of Samyangs and you are right they have much better feedback when trying to do a focus pull, especially their cinelenses are better for manual focussing and setting exposure.

In my case I prefer the Panasonic lenses because I can much quicker and more accurately set focus to a certain object using the autofocus capabilities which snaps on very quickly, I always found it difficult to manually nail the focus with the Samyangs. for that I press the shutter button halfway to make the focus lock on and leave it in manual after that. Also the image stabilization is something I need as I do a lot of handheld work as well. But if my line of work required me to do accurate focus pulls I"d definitely would go for a Samyang type of lens but work more on a tripod, or get myself a Olympus em1 with build in stabilization and a set of Olympus lenses.

Is the Olympus parfocal? Can't seem to find an answer anywhere.

Is the Olympus parfocal? Can't seem to find an answer anywhere.

I would doubt it but the zoom range is small enough that it might function as parfocal within the range. I have found that with non-parfocal zoom lenses you can stay in focus for about a 2x adjustment in either direction. Each lens is different however.

If you have enough lighting contrast for autofocus to work well, rack focus is pretty easy with "touch focus" enabled. The 12-35 should work well with this as well as most Olympus m4/3 lenses.

If you use touch focus the transition between focus points is nowhere near as smooth as with a manual focus pull.