I still get significant questions about sensor cleaning with mirrorless cameras, plus a lot of need-for-cleaning denial from Olympus mirrorless shooters. So let’s once again try to come to grips with what’s happening and what you should do about it.
Sensors on mirrorless cameras are more exposed than on DSLRs during lens changes. Well, not exactly the sensor, but the filter that sits on top of the sensor, which is there to filter UVIR light, and on some models provide anti-aliasing.
Dust and other things can get lodged on the top of that filter, and that’s what we need to clean.
In particular, the one thing that I find you have to be very careful with on mirrorless cameras is “splashes.” Obviously, don’t change lenses next to breaking waves or a waterfall, or in light rain. But I’ve seen some other interesting cases in seven years of using mirrorless. For example, I had someone sneeze on my sensor while the camera was open for a lens change. To which I’d add that you probably shouldn’t change lenses in front of a camel ;~).
So, yes, right up front it should be obvious that there will be instances where things get on the filter in front of your mirrorless camera sensor that should be dealt with (cleaned).
Aside: Why is the shutter left open on mirrorless cameras? Good question. First, the shutter needs to be open for the LCD or EVF to let you compose. There’s no reason why you can’t design a mirrorless camera that closes the shutter when you remove a lens, but there are implications of doing so: you can’t use a lens or adapter that doesn’t have electrical contacts. Moreover, shutter blades, when closed, are vulnerable to touch, so it would be easier to injure the shutter if you accidentally poked something that hit the blades.
Olympus users are in denial, though. I’ve heard quite a few of them claim that their sensor never needs cleaning. Actually, they just don’t see anything on their sensor, but that doesn’t mean it isn’t there.
There are two primary reasons why Olympus users in particular never notice things on their sensor: (1) they rarely are shooting at even f/8, let alone smaller, because of diffraction impacts; and (2) Olympus uses a very thick filter stack above the sensor, which makes it much more difficult to see how dirty the sensor is in the pixel data. The further the front of the filter is from the actual photon detection area, the more pixels those tiny dust spots spread their visual impact over; the visual impact is reduced from being a distinct black shadow to just being a contrast inhibiter.
That’s the problem, though. Anything on the filter will essentially reduce contrast, just as significant amounts of dust on the front of the lens will. Just because you can’t see a dust bunny at the pixel level on your computer screen doesn’t mean that the sensor is clean on an Olympus camera. If you don’t clean your sensor at least once in a while, you’re simply going to be slowly building some very low level of contrast reduction into your camera. Yes, that may be so low a level you don’t notice, but why would you want any contrast reduction in your image data?
So, periodic cleaning is probably a reasonable thing to do.
Unfortunately, the cameras with image stabilization at the sensor are a bit of of a problem when it comes to cleaning. Olympus seems to still recommend that you don’t do it yourself. Sony had real issues with the original IS system they used: people were breaking one of the supports for the stabilization swivel (not necessarily from cleaning, but it was a contributor). That’s since been fixed with design changes, but just be aware that if you have IS-on-sensor on your mirrorless camera, you really want to avoid using any substantive force when cleaning.
Note that I wrote “when cleaning.”
I firmly believe that you should be periodically cleaning your sensors. Not as much as folk think they have to (mirrorless or DSLR users). Even with my Nikon DSLRs I tend to do proactive sensor cleanings only once a year or so; that's on top of whenever I can see something clearly creating a problem in my images. I always carry cleaning tools with me around the world, but I rarely use them in the field.
Let’s see, at this point I’ve been shooting m4/3 for eight years and E-mount for more than five. In that time, I’ve probably cleaned my sensors about once a year per camera, typically provoked by something I noticed on close inspection.
So you don’t have to get anal about this. You just have to get yourself out of denial. At some point, your sensor could probably benefit from a cleaning. At some point you’re going to encounter something getting on the sensor filter that requires cleaning.
Thus the answer to the questions I get about mirrorless sensor cleanings is this: no, mirrorless cameras are not immune from sensor cleaning.
My advice on sensor cleaning: Thom’s DSLR Sensor Cleaning Article. But if you don't want to do it yourself, have someone that knows what they're doing (good camera dealer, service center, etc.) do it for you once a year. Or at least inspect the sensor filter carefully to make sure there’s nothing on it.