Well, I made such a long reply in DPReview, I figured I should make this its own thread here: I have much less experience than it many of you have with weddings, as I'm young and have only shot about a dozen or so smaller ones, so take my advice with a grain of salt. Still, my experience has been that the couple of times I've used a larger-sensor kit I've found myself missing my M4/3 kit more often than I find myself wanting an FF sensor when using my M4/3 gear. In my opinion, and weighted towards my specific shooting style, there are only two things inherently and significantly better about an FF kit for weddings over shooting with an OM-D: battery life and DoF control (particularly in environmental portraits). That's it. That being said, I believe wedding photography is more affected by the specific combination of body, lenses, and gear than by the system format you're using. This is true for any type of photography, of course, but seems particularly pertinent for weddings: there are many subtleties involved with these shoots affecting the balance between practicality and image quality. Sensor IQ, I believe, should be the least of your concerns for shooting a wedding with modern gear. The fact of the matter is pretty much any ILC system with a sensor size of 4/3 and larger will give you IQ more than good enough for weddings. But let's break it down a bit: The E-M5 matches the original FF Canon 5D on DxO, and has less measurable IQ advantages like more malleable RAW files with better shadow noise patterns. Heck, the E-M5 has better dynamic range than even Canon's newest crop of FF sensors (according to DxO, plus my anecdotal experience with a 5D MKII/MKIII). Resolution should hardly be a concern. People did splendid with 4 megapixels, 8 megapixels in more than enough, and 16 megapixels definitely is. In fact, I generally choose downsize all my photos to 10 megapixels on export to allow for better apparent pixel-level sharpness, while still printing up to 16x20 with ease. I even made a 16x20 print at ISO 6400 that came out quite nice after a little bit of careful processing--it was the couple's favorite print. When you consider that you rarely do more than a couple of prints at that size per wedding, all you really need is to do is get a few very sharp images at low ISO. Most of your prints will be much smaller, for which resolution will not at all be an issue. It's also worth noting that most M4/3 lenses have are sharper wide open than FF kits become until stopped down significantly. Color accuracy, particularly at lower ISOs, should hardly ever be an issue. High ISO noise seems to always be the knock-out punch for full frame, and indeed this is one area where M4/3 sensors are about two stops behind... but there are practical considerations that make things more complicated than it seems on paper. First of all, let's consider whether you actually even need to use high ISOs. Certainly people who shot film for weddings would not push their ISOs too high, not to mention old digital sensors. You can just use flash--problem solved. But say you're like me, prefer being discreet, and you absolutely hate using flash at night/in dark venues. Surely FF is better? Well... sort of. Again, it really depends on your specific combination of gear and your familiarity with it. A very typical FF wedding set up is two cameras, one with a 24-70 f2.8, and another with a 70-200 f2.8. On the other hand, a typical M4/3 kit will almost always involve faster primes from f1.4 to f2, or if you like MF, f0.95. I shot a recent event primarily with the 25mm f1.4 and the Voigtlander 42.5mm f0.95. With this set-up, I actually have more light gathering ability than I do with the FF kit, and the 2-3 stop advantage negated the noise differences or even tipped them in favor of my M4/3 kit. Add to this that in my experience the E-M5's stabilization system is better than any I've tried in an FF zoom, and things are tipped even more in favor of M4/3. Of course, not every FF wedding photographer uses zooms only. With Canon you have plenty of amazing F1.2 and F1.4 primes... but not a single one of them is stabilized. The closest you've got is the new stabilized 35mm f2 prime. The E-M5's stabilization consistently gets me 3-4 stops in typical use, 5 if I'm really careful and enable anti-shock, so again, any noise difference will disappear. Naturally, you have to monitor your shutter speeds for subject movement, but so far this has seldom been a significant issue for me; people stand still for long enough. You might get some blur if the person is laughing or talking a lot, but it shouldn't be too problematic. In situations where they are moving a lot, like night-time dancing, chances are you'll need a flash regardless of what you're shooting with. With Sony A-Mount shooters the difference is less apparent, but from my observations Sony's current IBIS technology is about 1 or 2 stops behind Olympus'. Additionally, often times when I'm shooting dinners wedding candids at night, I don't want to have my DoF toooo shallow. If you want more than one person in a picture, F1.2 or F1.4 on FF will make it tricky to have decent focus (not to mention PDAF's difficulties with accuracy in low light), and if you stop down, your light gathering advantages over M4/3 are nil. It's doable, but a hindrance worth noting. Also worth noting that this advantage only really applies to Olympus cameras. Only the Panasonic GX7 has IBIS, and its implementation lags way behind Olympus'. So obviously, I don't think FF has a very noticeable IQ advantage over M4/3 in most scenarios. The only noticeable advantage is in high ISOs, but my experience has been I'm often shooting at ISO 800 and 1600 where I'd have to shoot at ISO 3200 and 6400 on FF. However, there are other things to consider. Battery life is better on FF DSLRs. There's no getting around this. Using an OVF as the primary means of composition means that you're not wasting precious electricity before you click the shutter, and I'd assume the larger body means larger batteries. It's not too big of an issue for me, as I always carry spares and a charger and shoot conservatively, but the fact that I need to worry about shooting and switching batteries at all is noticeable, when I've never had to switch batteries on an FF kit within a day of shooting. The E-M5 will just barely get you through a long active day of shooting with a single battery, and the last thing you want is to have your battery die at an important moment. DoF control. The "full frame look". Of course, this depends largely on your shooting style. Not everyone is looking to annihilate the background and blur people's noses into oblivion, but the fact is with even the cheapest FF kits, you can, and in no type of photography is this look more common than in weddings. Although a lot of people will default to shooting wide open for wedding portraits, with FF you have the choice. On M4/3 your choices are more limited, and you're much more likely to just shoot everything wide open, especially at wide angles. You cannot get this look conveniently on M4/3 (35mm f1.4 wide open): Untitled by David Guimarães, on Flickr You can approximate it with the Voitglanders, but shooting at f0.95 in sunlight might mean you'll need an ND Filter: http://www.flickr.com/photos/mjones41/8379378597/ Likewise, you just can't get the look of an 85mm f1.2 on M4/3: Buckman Flair Promo by conorwithonen, on Flickr But the 42.5mm f0.95 doesn't do too bad: TVC (159) by napilopez, on Flickr And for headshots I doubt you'll need anything more: prospect by napilopez, on Flickr Those options are MF only, and are substantially heavier and larger than their equivalent DoF (f1.8ish) counterparts on FF. Still, if you're considering the switch in the first place, I'd imagine you're already familiar with these DoF consequences. It's a matter of whether you can live with them. But you can always keep an FF kit as a back-up, or rent one on occasion. One other semi-advantage, depending on your shooting style, is that you might have to switch lenses less with FF. As mentioned before, a 24-70 and 70-200 two body kit will cover most of your wedding needs while still providing the option for very shallow DoF. On M4/3 it's a bit trickier. I prefer to shoot with one body because I'm very active and run around every, so I keep my backup in my vehicle and I'm always switching lenses. I don't mind it, and it's never been a problem so far. But if I disliked switching lenses and wanted to absolutely make sure I didn't miss any moment, FF definitely has more versatile potential set up options (at hugely increased weight). Of course, if you're shooting in good light and don't care about super thin DoF, you can get an equivalent, way lighter f2.8 set-up on M4/3. Ultimately, it's pretty clear full frame provides more options for crafting images from your camera kit. You can get super shallow DoF, or you can stop down. You have overall better sensors with greater potential image quality, particularly at higher ISOs. But, for me at least, these slight image-crafting advantages are not worth it over the more understated image-capturing advantages you might already know about shooting with M4/3. The most commonly referred to one is that your kit will be much lighter, which will definitely save you some back and foot pain, but it will also allow you to be more nimble in your shooting. I tend to move run around a lot when shooting events, and I just don't have the same mobility with FF kits. It's hard to chase after a couple when you have 5 pound weights jiggling along your body. You also have other features like articulated screens so you don't mess up your suit by lying on the ground and can properly frame overhead shots (null point if you use a modern Sony), and I find a good, fast EVF to be better than an OVF for composing shots in low light. Likewise, I've found that when shooting with my one camera M4/3 set-up, guests tend to see me more as another person enjoying the wedding who just happens to be professionally documenting the event. They seem to feel more comfortable having me around than the more intimidating presence of a photographer with a huge camera and lens combo. This isn't just some completely subjectively constructed observation: I've had clients and/or guests specifically tell me something along those lines at at least 3 weddings so far. I'm much less conspicuous. M4/3 allows me to feel closer to the people enjoying the event. Take all that as you may; deciding which system's advantages are more important is completely up to you. There are most certainly more differences and advantages for these two formats than those I mentioned, and qualifications will vary from camera to camera and lens to lens. Still, I just hope I was able to help!