One of the biggest reasons that filmmakers are falling in love with the GH4 is because of it’s amazing ability to shoot at high frame rates internally. No other DSLR/DSLM has the ability to over crank the way the GH4 can, which offers variable frame rates at up to 96 frames per second. This is pretty astounding, and certainly makes the GH4 an incredibly powerful to own – but unfortunately not all frame rates are going to work as well as others.
If you want to overcrank on the GH4, there are two main ways that you can do it. The first is the old fashioned way (as you would have done with a GH3 or any camera that shoots 60p), which is to shoot at a true 60p setting and slow down your 60p video file to 40% (24p) in post. The other option is to use the variable frame rate setting in the menu to shoot at anywhere from 2fps all the way up to 96fps. If you’re using this option, the GH4 will actually adjust the clip internally so that your file plays back in 24p (in slow-motion) as soon as you’re done recording, much like a RED camera or other higher end cinema cameras would. It’s tempting to use the VFR mode on the GH4 all the time as there is so much flexibility with regards to what frame rate you can use, and it also saves time in post since you don’t need to slow down your footage. The caveat however is that the data rate is on the lower side.
When shooting on any of the frame rate settings in VFR mode, you are limited to a bitrate of 100mbps. This may sound like a lot, especially since you may be accustomed to shooting with DSLRs that have very low bitrates (17mbps, for instance), but when you get up into the higher frame rates this really isn’t enough. To put things in perspective, when you’re shooting at 100mbps at 24p you are essentially spreading out that 10mbps of data over only 24 frames, whereas when you’re shooting at 96fps you are getting the exact same amount of data per second, but now you are spreading it out over 4 times the amount of frames, which means each frame gets 4 times less information in it. In the end, this translates to much poorer final image quality than you might hope for.
When 96fps Is Useable
In most instances, I would highly advise against using 96fps on the GH4 as it just doesn’t deliver great results. The images can appear blocky, thin, and they tend to fall apart very quickly when you’re color grading. Trying to do any sort of color work on it makes you feel like you’re working with iPhone footage, or an older camcorder where there is little or no room to play with in post. The image can also be quite grainy and will have compression artifacts as a results of the low bitrate. That said however, there are situations where it is useable – namely in very well lit situations.
If you shoot at 96fps in broad daylight for example, you may just be able to get away with it. Since you need to use a 180 degree shutter (meaning your shutter speed is twice that of your frame rate), when shooting at 96fps your shutter speed is effectively 1/192, which is very high considering you are used to shooting at around 1/48 for 24p footage. This means that if you’re attempting to shoot any type of night exterior (or even most night interiors for that matter), you will have a very hard time getting the right exposure unless you have a very big lighting kit, and know exactly how to use it. In the middle of the day however, this isn’t a problem since you have such a bright natural light source (the sun) working for you. Ultimately this allows you to easily get a bright enough exposure to compensate for your frame rate setting, and you’ll be able to get results that are far more useable… Would I personally shoot at this frame rate though? Very rarely, if ever – even in bright sunlight, since the quality still isn’t going to be as strong as it would be at other frame rates. But I would certainly consider it as an option in a pinch if I absolutely needed that extra speed in camera.
60p vs 48fps
So 96fps clearly isn’t the best option for VFR mode, but how do some of the other frame rate settings hold up to the gold standard of 60p that we are all accustomed to? From my experience, they can hold up pretty well – especially if you’re shooting at 48fps or below. Shooting anywhere near 96fps is going to be problematic, and once you drop down to 60fps territory you are naturally better off shooting in 60p mode and slowing down in post (since you can shoot at 200mbps in 60p, as opposed to only 100mbps in 60fps variable frame rate mode). But once you drop below 60fps, things start to get much more useable, specifically at the 48fps mark. When you shoot at 48 frames per second, you are effectively slowing down your footage by exactly 50% meaning you’re cutting that 100mbps data rate in half, which comes out to 50mbps – not bad at all. It wasn’t long ago that many of us were clamoring for GH2 hacks that would let us shoot at 50mbps since the quality was so much better than the stock options on the camera, and in many shooting situations 50mbps is more than enough.
So why would you shoot at 48fps at a lower bitrate when you could shoot at 60p with a higher bitrate? There are many reasons for this, ranging from saving time in post conforming the footage (on quick turnaround jobs), to matching the frame rate with other cameras (maybe your GH4 is acting as a b-cam for an EPIC which is shooting at 24fps). Or in my case this past week, I used it for a music video shoot. As many of you know, often times when you shoot a music video you want to increase the speed of the song during playback so that you can overcrank your shots in camera, and get a slow motion effect in post while the band/singer is still sync’d up to the track. This was exactly what was done on my shoot last week… The music track was doubled in speed and so was the frame rate, which allowed for some nice in camera slow motion without going overboard. If we had attempted to speed up the song even more to match with my 60p setting, it would have been impossible for the band to keep up with the speed. So even though I would have preferred to shoot at 60p as I am always trying to get the absolute best possible image quality, 48fps ended up being a happy medium and gave us some great results. Here’s one of the frame grabs from the shoot, shot at 48fps:
48fps in this case worked very well and I was really happy to see that the images not only looked nice in camera, but graded well too. That said, I still wouldn’t recommend using this setting over 60p unless you are shooting in a situation like this where you absolutely need to. 60p is always going to give you slightly better quality, and in my opinion it is well worth the extra few minutes in post that it takes to slow down the footage. Not to mention that you can get some really nice slow motion with 60p since you’re able to pull the footage down to only 40% speed, which isn’t really that far off from the 25% speed you would get while shooting at 96fps (which is very slow!).
Over the next little while I will update this post (or add a new post) with some screen grabs comparing 48fps, 96fps and 60p, so be sure to check back soon.