Well... A little prologue: every single image that you capture during shooting it's ruled by aperture (the amount of light that is gonna to impress your sensor, generally the values start from f1.4, f2.8 to f16 or f22) and the TIME that the sensor will receive the light. If we were talking about still, I wouldn't had any problem to say: "Hey, It's a still, use a time capture that does't make you blur your picture (faster than 1/125s usually) but it's a movie so you have to understand some basic rules.
Shooting at 1/48s (or at 1/50s if your camera does not have it and you live in PAL area) it's a standard created by cinema industry. Why? Simply, for get that "cinema" look you have to start as the cinema starts, recording at 24p (so check you camera settings and choose 24p if available or 25p).
Every second 24 frames are captured; from the early stage of cinema the shutter was netherless that a wheel:
Image courtesy of: surfacestudio.com
The wheel spin, it takes 1/24s to cover one lap, but the wheel (from now we'll call it with the real name: shutter) as you can see is not a proper one, it's shaped by and angle, that angle does the time of exposure of the frame, so with a shutter angle of 180° and a spin time of a 1/24s the frame is exposed for a 1/48s!
All clear? In modern camera we don't have fisical wheel that spins but the basics is the same, in order to get "that" filmic look you have to expose the images at a shutter of 1/48s.
The reason is to get that natural blur that your eyes see, you're getting an image after another and you have to get a smooth transition between them, with a time faster (1/100s or 1/200s) you will start to notice a some unnatural images:
Image courtesy of cinemashock.org
Let's say that the wheels above are spinning at the same speed, in real life which one you will see? I think surely not the last on the right (or you're Superman!), not the center one but the one on the left, your eyes naturally blur the movement, so the same happen in cinema. Shot at 24p with a shutter speed of 1/48s!
