HD Video: Frame Rate or Field Rate
Sometimes we miss the good old days where choosing a frame rate was an either/or decision. When it comes to HD, one area of confusion that we get asked about a lot is the difference between frame rate and field rate. It’s common to see numbers like 720p60 or 1080i60 as part of a description.
- The first part describes the vertical resolution.
- The P or I describes whether the footage is progressive or interlaced.
- The last part describes its frame or field rate.
In HD video, frame rate and field rate are often used interchangeably and it can be hard to tell what someone is talking about. Here is an easy way to think of it.
- When an HD format is interlaced, the number generally describes field rate (1080i60 would describe 60 fields per second).
• When an HD format is progressive, the number generally describes frame rate (720p60 would be 60 frames per second).
The interesting thing is that field can describe frame too! This is because field rates are double that of frame rates. So 1080i60 footage has a frame rate of 30fps. Got it? Well, it’s not that simple. Most of the time numbers like 60 or 30 really represent fractional field rates (59.94) or frame rates (29.97) for backward compatibility with NTSC television.
Math—oh, how we hate you.