There are two generally accepted, but different, ways of defining scanning standards ...
If the letter comes first the following number is always the frame rate : i25, p25, p50 (And NOT the image rate when it comes to interlaced systems)
If the number comes first then the following number is the image rate (field rate for interlaced, frame rate for progressive) : 50i, 25p, 50p.
i25=50i (Though lots of people will assume the former is defining a standard with half the image rate of the latter...)
Both are in widespread use in the industry - and some people get them wrong when describing interlaced content (and commonly describe video as i50 or 25i - when they mean i25 or 50i...)
The EBU would define the UK HD system as 1080/i25. Many manufacturers would define it as 1080/50i (and use that nomenclature in their camera settings)...
Personally I think the image rate is a better number as it instantly tells you the temporal resolution.
Lots of people will think i25 and p25 will have the same number of images per second (i.e. the same temporal resolution) whereas it's much clearer that 50i has twice the temporal resolution of 25p. (Even though 50i and i25 are two ways of describing the same, identical, system)
Last edited by noggin on 7 February 2019 1:38am - 2 times in total