I have read statements like this a number of times (simplified):

Camera X has a bit rate of 50 MBit and if you record in 60 FPS that is really equivalent to 25 MBit at 30 FPS and that means the quality is bad.

This sounds logical because the argument is that having fewer bytes for each picture results in lower quality and thus the video at 60 FPS is at the same quality as a 25 MBit at 30 FPS. However, this is only true for intra-only codecs such as Prores, DNxHD, AVCI or IMX. Let me explain why.

The key factor that makes compression so much more efficient for inter-frame codecs (also commonly called “long GOP”, although that’s a bit of a misleading name) is that they do something called motion estimation. That means they analyze the content of the input signal and try to represent the motion of certain blocks of pixels between frames by something called vectors and those take up very little space. Then the difference between a picture approximated by moving around those blocks from the source signal along those vectors and the actual image is computed and then compressed as an image. The trick here is, that the better the approximation is, this difference image will not contain a high density of information and will compress really well.

The slower things are moving in the picture, the easier it is to estimate the motion and thus compress well. So by filming the same subject in 60 FPS instead of 30 FPS you are effectively cutting the speed of motion in half. The higher the frame rate is, the better the motion estimation will work for a given motive and then the formula used above no longer works.

So what is the right number? The answer is, you cannot say, because this whole effect depends on a number of pretty complex factors. One is the actual codec implementation, e.g. one h.264 encoder is not the same at all as another h.264 encoder as far as quality at the same bitrate is concerned. Making encoders is almost an art form.  Another thing is that it highly depends on the motive (for each individual codec implementation!). All that means that you can not solve this using a formula. The sad truth is, you have to test.

The bottom line is, don’t trust anyone making this statement to dismiss a camera until you’ve seen actual results.

Hope this helps

Just some little practical and maybe for beginners non-obvious things that can make your life easier when dealing with too much or too little light.

Breaking the shutter angle rule

To have filmic look, which many people strive for, you often read that having exposure at 1/frame rate * 2, e.g. at 1/50th of a second for 25 (or 24) FPS is the thing to do. The reason for that is to have the amount of motion blur that is typical for most big-screen films and is sometimes referred to as having a shutter angle of 180 degrees.

Tip: Sometimes it’s completely OK to expose shorter than the rule says

People sometimes despair because when shooting in daylight and shooting with a wide open lense to get shallow depth of field, there’s simply too much light and if you don’t have an ND filter and ISO is already at the lowest setting, you cannot do the shot.

In those situations, the key here is the word motion. If you have very little motion in the shot (e.g. a close-up of a person sitting/standing somewhere), you won’t really see a big difference if you expose at 1/100 or even 1/200. Just try it and develop a feeling for how far you can go in certain situations.

Tip: Sometimes it’s completely OK to expose longer than the rule says

This is quite the opposite but the principle is the same: If there is not much motion in the shot you can play around with longer exposure (e.g. 1/20th) and that way get an extra stop of light when you need it, if your camera allows that. Imagine a close-up in a mexican stand-off at a gloomy bar. That stop can really help and won’t make your shot less cinematic.

Tip: Use a denoiser to be able to use higher ISO settings

After getting to know your camera you develop a feeling for how high you can set your ISO for a still acceptable image (and don’t just trust reviews by anyone containing numbers because this is highly subjective). If you use a denoiser, e.g. like the one integrated in Kyno (disclaimer: as one of the makers of Kyno I am biased), you can drive that up a bit, because up to a certain level denoisers can remove certain types of ISO noise quite well. Many cameras allow in-camera denoising which is sometimes crappy, so a software denoiser typically gives better results but that depends on a lot of factors. It’s just a good thing to have in mind when planning locations and light, to sometimes have a way to squeeze one or two extra stops of light out of your low-budget equipment in post without much hassle.