I had the honor of shooting a friend's wedding last weekend in British Columbia, so I decided it would be a good opportunity to try doing a professional gig using the Panasonic GH4 and my brand new Lumix 35-100mm f/2.8 lens (a full review of that lens will come later, hopefully). I've shot dozens of weddings in the past, but I've used a Canon camera for every single one (occasionally supplemented with a Fuji X100). Could a Micro 4/3rds camera measure up to full frame?
Here are some of the pros and cons of shooting a wedding with a GH4, compared to, say, a Canon 5D Mark III.
Weight - WOW this setup is light! You can pack a single small camera bag with the Panasonic GH4, a 12-35mm lens and a 35-100mm lens (the rough equivalent of a 24-70mm and a 70-200mm on a Canon 5d Mark III) and switch between them all day without feeling any impact on your back and shoulders. Those who have shot weddings using Canon gear will know the pain of which I speak. (One of my mentors actually got a pinched nerve from all of her gear!) By far this is the hugest advantage of using the GH4 compared to any full frame camera.
It also has other implications. Since you're traveling lighter, you can carry more stuff and be more nimble and experimental when it comes to finding the right shot. For instance, the photo at the top of this page was taken using a bare LumoPro flash, remote triggered from behind the couple. But it was done in haste between group photos. If I'd had a heavier setup, I might not have been able to move as quickly to try and snap this shot.
Cost - Buying a Canon 5D Mark III and a 24-70mm and 70-200mm lens will cost you roughly $7000, give or take $500 or so depending on whether you buy all the stuff new. Through some deal-hunting and eBay-ing, I was able to purchase a GH4 with lenses of equivalent focal lengths to the above for about $3500 total. That is a huge difference in price, especially if you're just starting out.
Drive mode - The GH4 can shoot up to 12 frames per second. It is SUPER fast. And at a wedding, this can be particularly useful when you're trying to capture specific moments during a ceremony, or with interactions between the couple and their guests. Whenever I saw something interesting happening, I'd just let the drive mode rip and then have a ton of options to choose from in post.
File sizes - RAW files on the GH4 are significantly smaller than on the Canon 5D Mark III. This means fewer cards and more photos (P.S. This is also a negative, as I will discuss below).
Shallow depth of field - I've already discussed this in previous posts, but obviously the shallow depth of field on the GH4 will never measure up to what you can get on a full frame camera. That's particularly unfortunate for a wedding, because a lot of clients are looking for that creamy bokeh in their wedding shots. This camera, even at a 200mm focal length, really struggles to deliver on that front. You'll have greatest success when there's a lot of physical distance between yourself and the object.
Aesthetics - As of this writing, the GH4 retails for $1700. But because of its size and weight, it certainly doesn't LOOK like a professional grade camera. People expect to see a Canon/Nikon-size full frame camera at weddings. It's hard to look "legit" when you are toting the GH4 around. I realize this isn't really a con from a photography perspective, but it's worth noting for people considering this as a tool to build a career on.
Low light performance - The GH4 does just okay up to ISO 3200, but a Canon 5D Mark III blows this camera out of the water when it comes to low light performance. I took tons of shots in low light situations that were just completely unusable. Honestly, I wouldn't go above ISO 1600 on the GH4, which is practically impossible at weddings (nearly all of which involve at least some low light situations).
On that note...
Megapixels - The GH4 has about 16 megapixels. The Canon 5D Mark III has around 22. I'm aware that megapixels don't necessarily determine picture quality and that there were other factors involved, but there were definitely instances where I took photos with the GH4 that I needed to crop, and on a smaller sensor with fewer megapixels, you definitely "feel" that crop a lot more in terms of loss of quality. The resulting image can be muddier or noisier than the same image would have been on the 5D Mark III.
So overall, is this a setup I would recommend for weddings? Yes and no. This gig was for a friend, so it wasn't a conventional client situation. I was pretty happy with the photos that I got and so was the couple. But I also was able to enjoy the evening - the camera was so light that the process of taking the photos wasn't onerous at all from a physical perspective. This can't be understated.
If you are already a videographer/photographer who has decided to go with the GH4 for the advantages that it provides, you should definitely feel good about shooting a wedding with it (but only if you have the 35-100mm lens). The drive mode in particular can be an amazing benefit, and the photos obviously have great focus and sharpness.
However, if you're still deciding between a GH4 and a full frame or APS-C camera for wedding photography, there's nothing that's going to beat a larger sensor for getting great low light photos and shallow depth of field. If I had to choose only one type of camera to shoot weddings with for the rest of my life, it'd be a full frame camera. I'm fortunate to not have to choose, so I will probably use the two for different scenarios as time goes on, based on which advantages from each are important to me at the time.