I want to use my D800 tomorrow and was getting my gear in order. I spent about 35 minutes and 5 sensor swabs cleaning the sensor. That is something I've never worried about with 4/3s or m4/3s!
In the last 10 years of using 4/3 cameras, I've never once found a need to clean the sensor and that's even after many lens changes in dusty conditions. Lots of DSLR sensors come full of dust from the factory. Also, as the SSWF resides just above the sensor, dust never actually settles on the sensor itself and so is even less likely to affect the image.
I once had a stubborn dust spec that was visible at f8 or even f5.6 if you knew where to look for it. I survived my attempts to remove it with a wet cleaning so I returned the camera to Olympus which will clean the sensor for free. And I have had to blow off a few dust specs from my GX7 that were visible at f11. But in general dust is much less of a problem than with most SLRs. They usually have an effective dust removal system but so do many SLRs. The advantage comes from having a thicker sensor stack, not having to stop down as much to get deep DOF and using wider apertures in P mode. The thicker sensor stack (AA and color filters) keeps the dust further away from the actual sensor which makes it less visible at any given aperture. Dust becomes more visible at smaller apertures. If you think your sensor is dust free take a picture of a blank wall or the sky at f22. Since diffraction becomes a problem sooner on m43 most people never shoot above f8 or f11. Even when I want deep DOF I try to avoid anything above f11 and prefer f8 if possible. A lot of time when I see SLR owners complaining about dust they are using f16 or higher. Many are probably in aperture or manual mode but it could be SLRs are more likely to pick higher f stops in program mode too.
Except that dirt can get under the SSWF. That's exactly what happened with my E-M1. Some dirt (they suspect shedding from the shutter box) got underneath the SSWF and had to be "extracted". Luckily it was under warranty.
I think that would be extremely rare and with the m/43 bodies, that is probably less likely. But at the end of the day, anything is possible.
If dust was directly lying on the sensor under the SSWF, wouldn't it look incredibly solid and sharp-edged?
Yes... but only at certain focal lengths - there's still a very thick glass between the SSWF and the sensor surface. I had no idea anything was there until I got the 75-300. At any aperture (even wide-open) beyond 150mm you could see two sharp squiggly lines - one in the top right (bottom left of sensor) and one closer to the PDAF area. It wasn't blurry like the Canon example above. The SSWF is still in front of the very thick coverglass-pack. So in most cases the dust was invisible. Since I only use a 17mm and a 75-300 right now it was invisible with the 17mm at any aperture. The key was my inability to clean it. Looking at the sensor using my Zeiss loupe, they actually looked like two scratches. Since then, the issue hasn't returned. Yes, I have seen dust on the sensor, but yes, a flick of the power to re-run the SSWF has cleared it every time. Olympus claims they re-sealed the sensor pack to be sure. Perhaps it came from the factory with a bad seal.
On the bright side, you can clean it yourself. All the current Olympus models (E-PL7, E-P5, E-M5, E-M10, E-M1) require sending it in to be cleaned, thanks to the free-floating sensor assembly. Granted, they don't often get dust, but when they do, it's not a user-serviceable thing any more. Older Olympus models and Panasonic models don't have this issue.
I have cleaned my EM5 twice. First time for curiosity because I was already doing another camera and had the stuff out..Second time for a dust particle that wouldn't come off with a blower. With about 50K pictures taken since 2010 on four different M43 bodies, I believe I've only needed to do this kind of cleaning three times for an actual dust issue for M43. When I shot a Pentax DSLR, it needed cleaning every other month, so I had plenty of practice to learn how to wipe a swab. . .
At some point I got a really problematic smudge on my EM5 sensor. I have no idea what happened or how, but it was large enough to create issues. Luckily I needed to send the camera in for the screen bezel crack issue, and Olympus gave it a thorough cleaning inside and out. Have made sure to keep things clear ever since.
