The FPS business is confus


The FPS business is confusing at best partly because FPS is used in two different ways. 24 FPS generally means 24 frames per second which is a film standard. 60 FPS generally means 60 fields per second which is the interlaced TV standard. Since a TV frame is made up of two fields, 60 fields per second equals 30 TV frames per second. Actually, the TV standard is 29.97 frames per second, but that’s not important in this discussion.

It would seem that there is not much difference between 24 and 30 frames per second, but there is because the TV frames are painted on the screen twice – once for the odd numbered scan lines and once for the even numbered scan lines. This is where the 60 fields per second comes in, one field contains the odd numbered lines and the other field contains the even numbered. This is called interlaced scanning and is abbreviated in HDTV as 1080i or 1080 lines interlaced. In TV, the frame rate is always 30 frames per second.

In the case of 24 frames per second, the frame is usually scanned progressively which accounts for the abbreviation 1080p. This means that the scan lines are painted on the screen one after the other so there is no need to interlace two fields.

Interlaced scanning was originally implemented to reduce flicker on CRT displays, but it has the added advantage of smoothing out motion. The cost of the smoothing is a reduction in sharpness on fast moving subjects. That sharpness reduction is due to the fact that two adjacent scan lines are actually created 1/60 of a second apart and the subject has moved during that time interval. The smoothing comes from the fact you are getting 60 “looks” at the picture every second as opposed to either 30 or 24 in the progressive scan modes.

Progressive scan provides a sharper picture of moving subjects because, presumably, all scan line were exposed at the same time. This is true if the camera has a shutter system that allows that simultaneous exposure, most do not. Most progressive scan cameras scan every line from top to bottom meaning that the bottom of the picture is captured slightly later than the top. The result is a slight horizontal stretching of rapidly moving objects.

To my eye on the videos we shoot, neither the slight blurring due to interlacing nor the stretching due to progressive scanning is important because the eye doesn’t focus clearly on fast subjects. But I also think the interlaced scanning produces smoother motion because the 60 fields per second provide more views of the subject (higher sampling rate).

If you watch closely, fast motion on film movies is not really smooth nor sharply focused. Pans are also jerkier than on TV, especially fast pans. That is neither good nor bad, it is a characteristic of the art form.

In the end, I shoot everything in 1080i 30 frames per second because nobody has every told me they wanted jerkier pans etc.


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