Had these both existed when I bought my GH3, it's likely the outcome would have been the opposite way. Why?
Olympus is better at still photography.
I bought the GH3 because I needed help with lower light photography indoors. Why didn't I consider the E-M5? Size. It was too small to be useful in sports (just as my OM-1N was, though the heavier, bigger Nikon F2 wasn't all that great), and would require the additional grip just to be functional for me but of course, in true Olympus fashion, the grip was in the way of some controls. (The OM-4Ti grip wasn't bad for the time but sports photography often required using a tripod and flash because there wasn't enough light.)
The GH3 was still a bit small. The Olympus E-1 was practically a perfect size for E-system lenses. The ZD 35-100mm f/2.0 can be used handheld quite easily, although the balance is improved with the E-5. However, I use the GH3 with the ZD 14-35mm f/2.0 and the ZD 50-200mm f/2.8-3.5. Auto focus works faster, especially in poor light, than that of the E-1--at least, my E-1.
|GH3 at front left, with an E-M5 and 2 E-M1 bodies|
The E-M1 auto focus is improved over the earlier micro Four-Thirds bodies made by Olympus. Whether it's better than the GH3 remains to be seen (update: the E-M1 auto focus of Four-Thirds lenses is great in bright light but I didn't get a chance to try it in dimmer situations), but I suspect it will be, except in lower light, where Olympus repeatedly has failed. I'm not saying that it's easy for Nikon or Canon--just that they've done something more than say "that's okay enough" and quit that Olympus have often done. Just last month, Panasonic updated firmware on the GH3 to include auto focus down to -4 EV. If they can do it, Olympus can do it.
Now, for something that drives me bonkers--white balance. The GH3 shifts the white balance from shot to shot just slightly. The best thing to do would be to keep a piece of white paper with me and get a setting and stick with it. I always worry about other things and forget about the piece of paper, and adjust in post processing. Apparently, this is a problem with the majority of bodies from various brands. I'd become so accustomed to the E-1 with the one-touch white balance setting that I hadn't really considered that the rest of the world was so messy. Working in Phase One Capture One Pro 7, I don't really notice it so much, unless there is something dramatically wrong, especially with hundreds of photos at a time.
I believe that the Olympus E-M5 has the edge over the GH3 in lower light photography at high ISO sensitivities, so I expect the E-M1 to work at least that well. Panasonic tends to smush everything at some point above ISO 3200. I get great shots at ISO 3200 but don't bother using anything above that. However, I've seen acceptable shots from the E-M5 at ISO 6400. Apparently, the Panasonic GX7 is much better, but is it better than the E-M1, or even the E-P5?
The GH3 excels at something I've barely used--video. In just a few seconds of video I've taken, I can see that Panasonic really worked at it and you wouldn't need a video camera, although it would be easier to get 30 minutes or more into one file since there is a legal/taxation limitation.
I'm not selling my GH3 to jump to the E-M1. In a bit over 6 months, I'm just barely comfortable with it. In another year, there may be replacements for both. For all we know, Panasonic may make a GH5 with phase detection auto focus points on the sensor and allow smooth operation down to -4 EV. All I can say is that it would be interesting to see how the E-M1 works with my current Four-Thirds lenses but almost good enough isn't good enough.
Update 2013.10.05: Having spent more than an hour or two with the E-M1, and having used it with my Four-Thirds ZD 35-100mm f/2.0 and ZD 14-35mm f/2.0, I can say that the E-M1's auto focus was quick enough in the store to be useful. Whether it's good in a darker gym (where I photograph a lot of sports) or not, I have no clue. People have hinted that E-5 users will not be pleased in low light with the E-M1. Olympus have struggled with low light AF as far back as I can remember--my E-1 is terrible in low light. For casual use with the Four-Thirds mount lenses, I think the E-M1 will be quite good. Perhaps, I'll just wait for the next one or hope that Panasonic adds the same sensor to the GH3 replacement.
Given my encounter with the E-M1, I thought about what annoyed me where the GH3 is better: the angular grip, the sliding, tilting mechanism of the rear display. The GH3 has a comfortable grip (bigger battery), even when holding my SHG lenses and the fully-articulated rear display is helpful, even though it doesn't have the excellent Olympus Super Control Panel or the excellent EVF.
Pros for GH3:
- Fully-articulated display
- Comfortable body
- Auto Focus to -4 EV--starlight
Pros for E-M1:
- Image quality, especially from ISO 6400 and up
- Phase detection used for Four-Thirds lenses and for tracking with micro Four-Thirds lenses
- Electronic Viewfinder
- Super Control Panel
Also, with the newfound processing power to handle 4K video, still photography is getting a boost. Hopefully, that means that everything will be more responsive, including auto focus. I don't care for auto focus, but with the short, short lenses, it's better than trying to manually focus them. Subject tracking on the GH3 needs a huge improvement and if what Panasonic has said about the GH4 is real, there may be some improvement. They have suggested that, because of the improved processing power, there was no need for Phase Detection pixels on the sensor. I am skeptical, but hopeful.
The electronic viewfinder and rear display have been enhanced. The EVF still isn't as large an image as on the E-M1/VF-4 (0.71x), which is smaller than that of the FujiFilm X-T1 (0.77x), at 0.67x but improvement can come in other ways. They made statements about using aspherical elements in the magnification to counter the appearance flaws of the GH3's EVF. They also increased the contrast ratio to 10,000:1, and redesigned the eye cup.
Of course, the GH3 was US$100 cheaper but the GH4 is US$300 more expensive. The E-M1 is only adequate in video and that's where the real difference is.
Update 2014.03.22: I've seen many shots from the E-M1 now and I don't see that they're as good as shots from the E-M5 or E-M10 at ISO 6400 or higher. I'm not sure why carving out 35 phase detection pixels (or is it many more within 35 areas?) would be the cause of the degradation since FujiFilm's X-Trans II sensor has some 86,000 pixels dedicated to phase detection.
It doesn't seem that the E-M1 is quite the low light magician that it might be, unfortunately.
Update 2014.10.17: I traded my GH3 back in June, while the trade-in value was good, for the E-M1. The switch was not awful, but also not great. After four months, I find that the E-M1 is still not as comfortable as the GH3/GH4 body but it is so responsive that it makes up for it somewhat. The tiny body also means that I run out of battery power much earlier than with the GH3. I believe that I never switched to my second GH3 battery, while I often have to switch to a second battery with the E-M1.
The viewfinder of the E-M1 is better but not without the blackouts I also experienced with the GH3. I switched to a different eyecup, hoping that it would not scratch my sunglasses so much, and help with blackouts. There are still blackouts.
It is too easy to change the focus point location, even while viewing photos. The 4 way controller buttons will move the focus point, while the OK button will bring up the Super Control Panel, and the front dial will change the exposure compensation. Then again, with the GH3's Quick Menu, I was never sure that anything had been changed at all, and many times, I had to go to the menus to change things.
I'm still looking at the GH4 for video work, but I'm looking more at lenses and other equipment at the moment.
Update 2014.12.14: Got the GH4 a couple of weeks ago, and it feels good. I've begun to do video and I'm mostly pleased with the results, especially considering my lack of experience.
As with the GH3, it's a good tool. Unlike the GH3, the GH4 handled a darker indoor location much better than the E-M1 with the same lens, and I didn't have to set a certain ISO sensitivity to achieve auto focus or anything else. The E-M1 seems to need to be set precisely for what you want.
The GH4 really fixes the electronic viewfinder issues for me. It may still not be perfect, but it works very well.