First, here's an overview of the existing alternative framerates. I have outlined the choices for full HD 1920x1080 resolution only. For lower resolutions, there may be more framerate selections available:
|24 fps||GH3, GM1, GX7, G6||None|
|25 fps||All PAL area cameras||None|
|30 fps||All NTSC area cameras||All|
|50 fps||PAL area cameras: GX7, GM1 (only interlaced), GF6 (only interlaced), G6||None|
|60 fps||NTSC area cameras: GX7, GM1 (only interlaced), GF6 (only interlaced), G6||None|
As you can see from the table above, Panasonic gives you much more choice in terms of which framerate to use. All current Olympus cameras only have one single choice: 30 fps.
24 frames per second, or more precisely, 23.976 fps, is the standard for motion pictures. If your ambition is to record footage for motion pictures, you pretty much need to use 24 fps. For all other uses, you don't need 24 fps.
For video and television, the framerate has been 25 fps in countries with the PAL standard (Europe, China, Africa, Australia and New Zealand), and 30 fps in countries with the NTSC standard (Americas and Japan).
Panasonic cameras are generally hard coded to give you the correct framerate, based on which country it was bought in. So if you buy a camera in the US and bring it to Europe, you can only use 30 fps (or 60), and you cannot record video in 25 fps PAL standard.
Olympus only makes 30 fps video capable cameras, even in PAL regions.
Some Panasonic cameras allow doubling the frame rate, so that you can use 50 fps in PAL regions and 60 fps in NTSC regions. This is useful when you have a lot of movement in your video, as it gives a more fluid footage. However, some cameras, the GM1 and GF6, only allow the frame doubling using when combined with interlaced footage. I would discourage using interlaced video, as it can give you unwanted striping artefacts. So stick to progressive video, is you can.
Olympus only at 30 fps, problems?
From the table above, you'll note that Olympus only has one framerate choice: 30 fps. Is this a problem?
If you are in an NTSC area, e.g., the Americas or Japan, this is no problem, as the local standard is 30 fps.
In PAL areas, TVs and videos operate at 25 fps. However, as most TV sets are multi standard and can handle both PAL and NTSC, viewing the footage is not really an issue. Besides, a lot of people now use computers or tablets for viewing videos. So playing back your 30 fps video footage in a PAL area should not be a problem at all these days.
However, there is another reason why you would want to use 25 or 50 fps in PAL regions. When recording video lit by artificial light, chances are big that the light will flicker at 100hz.
The power grid operates at 50hz in PAL countries, however, as each cycle contains two peaks, you'll have 100 light peaks per second. When recording video at 30 or 60 fps, this can create strange interference effects, leading to distracting flickering.
Here is an example. I have recorded video at both 30 fps (left part of the frame) and 25 fps (right). The recording was done in a PAL country with a 50 hz power grid. As you can see, the 30 fps footage has a lot of flickering:
Both video streams were recorded using the Lumix 14mm f/2.5 at f/2.5, and 1/60s shutter speed. The right video was recorded in the multi aspect ratio capable GH2, hence, the right part has a slightly wider field of view.
If you are only recording video footage outdoors on a sunny day, then this is of course no problem. The sun doesn't flicker.
180 degree shutter
If you are using a framerate of between 24 and 30, you should try to use a 180 degree shutter. This means having a shutter speed around twice that of the framerate, i.e., around 1/50s. The purpose is to achieve some motion blur, to give a more fluid motion. Read more about 180 degree shutter here.
However, if you are able to use 50 fps or 60 fps, then 180 degree shutter is not so crucial. The higher framerate gives you more fluid motion anyway. It will usually be better to not have too fast shutter speed, though. So try to use around 1/100s if possible.
Using 50/60 fps also helps the camera to autofocus quicker during video recording. See a demonstration here, using the GH2. The reason is that with 50/60 fps, the camera has twice as many video frames to analyze for contrast detection autofocus (CDAF).
Olympus Micro Four Thirds cameras are still a bit primitive when it comes to video, offering only 30 fps, regardless of PAL or NTSC region. They have announced, though, that future cameras will be more video optimized, and offer more framerate choices.
Being restricted to 30 fps can be a problem when video recording subjects lit artificially in PAL regions. This can cause flickering in the video stream, which is very distracting.
Generally, try to use the fastest framerate available in progressive mode. Don't use interlace mode, unless you absolutely know what you're doing.