I think the subject of frame rates for underwater video is filled with a level of non-sense second to none. Part of this is GoPro generated, the GoPro being an action cam started proposing higher frame rates as standard and this triggered a chain reaction where every camera manufacturer that is also in the video space has added double frame rate options to the in codec camera.
This post that no doubt will be controversial will try to demistify the settings and eliminate some fundamental misconception that seem to populate underwater videography.
The history of frame rates
The most common frame rates used today include:
- 24p – used in the film industry
- 25p – used in the PAL broadcasting system countries
- 30p – used in the NTCS broadcasting system countries
PAL (Phase Alternating Line) and NTSC (National Televion System Committee) are broadcasting color systems.
NTSC covers US South America and a number of Asian countries while PAL covers pretty much the rest of the world. This post does not want to in the details of which system is better as those systems are legacy of interlaced television and Cathodic Ray Tubes and therefore are for most something we have to put up with.
Today most of the video produced is consumed online and therefore broadcasting standards are only important if you produce something that will go on Tv or if your footage includes artificial lighting that is connected to the power grid – so LED does not matter here.
So if movies are shot in 24p and this is not changing any time tomorrow why do those systems exist? Clearly if 24p was not adequate this would have changed time ago and except some experiments like ‘The Hobbit’ 24p is totally fine for today use even if this is a legacy of the past.
The human eye has a reaction time of around 25 ms and therefore is not actually able to detect a moving object in the frame at frame rates higher than 40 frames per second, it will however detect if the whole room moves around you like in a shoot out video-game. Our brain does a brilliant job of making up what is missing and can’t really tell any difference between 24/25/30p in normal circumstances. So why do those exist?
The issue has to do with the frequency of the power grid and the first Tv based on Cathodic Ray Tube. As the power of the grid runs at alternate current with a frequency of 60 Hz in the US when you try to watch a movie on Tv that has been shot at 24p this has judder. The reason is that the system works at 60 cycles per second and in order to fit your 24 frames per second there is a technique called Telecine. To make it short artificial fields are added each 4 fields so that this comes up to 60 per second however this looks poor and creates judder.
In the PAL system the grid runs at 50 Hz and therefore 24p movies are accelerated to 25p and this the reason the durations are shorter. The increased pitch in the audio is not noticeable.
Clearly whey you shoot in a television studio with a lot of grid powered lights you need to make sure you don’t have any flicker and this is the reason for the existence of 25p and 30p video frame rates. Your brain can’t tell the difference between 24p/25p/30p but can very easily notice judder and this has to be avoided at all costs.
When using a computer display or a modern LCD or LED Tv you can display any frame rates you want without issues therefore unless you are shooting under grid power artificial lights you do not have to stick to any broadcasting system.
180 Degrees Angle Rule
The name is also coming from a legacy however this rule establishes that once you have set the frame rate your shutter speed has to be double of that. As there is no 1/48 shutter 24/25p are shot at 1/50s and 30p is shot at 1/60s this makes sure also everything stays consistent with possible flicker of grid powered lights.
The 180 degrees angle rule gives each frame an amount of motion blur that is similar to those experienced by our eyes.
It is well explained on the Red website here. If you shoot slower than this rule the frames look blurry if you choose a faster shutter speed you eliminate motion blur so in general everybody follows this and it works perfectly fine.
Double Frame Rates
50p for PAL and 60p for NTSC are double frame rates that are not part of any commercial broadcasting and today are only supported officially for online content.
As discussed previously our reaction time is not able to detect more than 40 frames per second anyway so why bother shooting 50 or 60 frames per second?
There is a common misconception that if you have a lot of action in the frame then you should increase the frame rate but then why when you are watching any movies you don’t feel there is any issue there even if you are watching Iron Man or some sci-fi movie?
That is because those features are shot well with use of a lot of equipment that makes the footage rock steady, the professionals that do it follow all the rules and this looks great.
So the key reason to use 50p or 60p has to do with not following those rules and not being that great of shooting things in a somehow unconventional manner.
For example you hold the camera while you are moving for example a dashboard cam, or you hold the camera while running. In this case the amount of changes in the frame is substantial as you are moving not because things around you are moving. So if you were still in a fixed point it will not feel like there is a lot of movement but if you start driving your car around there is a lot of movement in the frame.
This brings the second issue with frame rates which is panning again I will refer to Red for panning speed explanation.
So if you increase the frame rate from 30 to 60 fps you can double your panning speed without feeling sick.
Underwater Video Considerations
Now that we have covered all basics we need to take into account the reality of underwater videography. Our key facts are:
- No panning. Usually except some cases the operator is moving with the aid of fins. Panning would require you to be in a fixed point something you can only do for example in a shark dive in the Bahamas
- No grid powered lights – at least for underwater scenes. So unless you include shots with mains powered lights you do not have to stick to a set frame rate
- Lack of light and colour – you need all available light you can use
- Natural stabilisation – as you are in a water medium your rig if of reasonable size is floating in a fluid and is more stable
The last variable is the amount of action in the scene and the need of slow motions – if required. The majority of underwater scenes are pretty smooth only in some cases, sardine runs, sea lions in a bait ball there really is a lot of motion and in most cases you can increase the shutter speed without the need to double the frame rate.
When I see video shot at 50/60p and played back at half speed for the entire clip is really terrible and you loose the feeling of being in the water so this is something to be avoided at all costs and it looks plain ugly.
Furthermore you are effectively halving the bit rate of your video and to add more usually the higher frame rate of your camera is not better than the normal frame rate of your camera and you can add more frames in post if you wanted to have a more fluid look or perform a slow motion.
I have a Panasonic GH5 and have the luxury of normal frame rates, double frame rates and even a VFR option specifically for slow motions.
I analysed the clips produced by the camera using ffprobe to see how the frames are done and how big they are and discovered a few things:
- The 50/60p recording options at 150 Mbps have a very long GOP essentially a full frame is recorded every 24 frames while the 100 Mbps 25/30p records a full frame every 12 frames. So the double frame rate has more frames but is NOT better at managing fast moving scenes and changes in the frame.
- The VFR option allows you to set a higher frame rate and then slows down recording to the frame rate of choice. For some reason the 24p format has more options than all the others and the 25p does not even have a 50% option. As the footage is recorded at 100 Mbps the VFR footage at half speed conformed to 30p is higher quality than 60p slowed down to 30p (100 Mbps vs 150/2=75 Mbps) in terms of key frames and ability to predict motion this is better as it has double the amount of key frames per second see this explanation with details of each frame look for the I frames.
- The AVCI all intra option has actually only I frames and it will have 24/25/30 of them per second and therefore it is the best option to detect fast movement and changes in the frame. If you need to slow this down this still has 12 key frames per second so other frames can easily be interpolated.
- Slow motion – as the image will be on the screen for longer and it is slowed down you need to increase the shutter speed or it will look blurry. So if you intend to take a slow mo you need to make that decision at time of your shot and go for a 90 or 45 degree angle. This remains through if you use VFR or if you slow down AVCI clips in post
- If you decided AVCI is not for your the ProRes choice is pretty much identical and again you do not need to shoot 50/60p unless you have specific situations. In general AVCI is equal or better than ProRes so the whole point of getting a recorder is highly questionable but that is another story.
For academic purposes I have compared the 3 different ways Final Cut Pro X does slow down. To my surprise the best method is the ‘Normal Quality’ which also makes sense as there are many full frames.
Now it is interesting to compare my slow motion that is not ideal as I did not increase the shutter speed as the quality of AVCI is high the footage looks totally fine slowed down
Looking at other people example you get exactly the wrong impression you take a shot without increasing the shutter speed and then slow it down. The reason why 60p looks better is for the shutter speed not for the image quality itself it is also completely unneeded to slow down a whale shark as it glides through the water.
So taking this kind of guidance blindfolded is not a good idea.
Key Take Aways
- Unless you shoot using main grid powered lights you can choose any frame rate you want 24/25/30 fps.
- Shutter speed is important because it can give a motion blur or freeze motion in case of a slow motion clip
- You need to choose what scenes are suitable for slow motion at time of capture
- Slowing down systematically your footage is unnatural and looks fake
- Using formats like AVCI or ProRes gives you better option for slow down than 50/60 fps implementation with very long GOP
- VFR options can be very useful for creating purposes although they have limitations (fixed focus)
How do I shoot?
I live in a PAL system country however I find always limitations with the 25 fps options in camera. The GH5 VFR example is not the only one. All my clips are shot 24 fps 1/50s, I do not use slow motion enough and if I did I would probably keep using AVCI and increase the shutter speed depending on the effect I want to give to the scene, this is also the most natural and easier way to shoot underwater as you do not have to continuously change format. Having all intra frames gives me all the creativity I need also for speed ramps that are much more exciting than plain slow motion see this example.