Resolution: 4K vs 2160p vs 8.3MPBy John Honovich, Published Apr 24, 2015, 12:00am EDT
4K, 2160p and 8.3MP all basically refer to the same 'thing', just different dimensions of it.
While total pixel count has historically been the most common technique (e.g., 1MP, 2MP, 3MP, 5MP, etc.), naming by vertical count (e.g., 720p and 1080p) have become more common in the past few years.
Now, naming by horizontal count / lines (e.g., 2K, 4K, etc.) is a growing trend.
In this note, we contrast the pros and cons of each element and how to best use them in designing surveillance systems.
This image visualizes the 3 options for '2K' and '4K':
The number next to the P, as in 720p and 1080p, refer to the total number of rows in the video. A 720p video feed has 720 rows of pixels. A 1080p video feed has 1080 rows of pixels.
The 'P' itself stands for progressive (versus interlaced). Today, essentially all surveillance cameras are progressive scan so the 'P' itself does not make much of a difference.
The K, in 2K and 4K, refer to the total number of columns in a video, in thousands (ergo the K). A 4K video feed has roughly 4,000 columns of pixels. Technically, 4K video has either 3840 rows or 4096 rows, though, in surveillance 3840 is almost always the case (note 3840 is for UHD-1 while 4096 is for DCI).
It is unfortunate that the convention for columns (P) and rows (K) use different metrics (by 'ones' for P, by 'thousands' for K). Regardless, one needs to memorize this in surveillance to navigate between the two.
MP is the total number of pixels, found by multiplying the number of columns (P) by the number of rows (K). For example a 1080p camera is 1920 columns (e.g., 2K) x 1080 rows, equal to 2MP (technically 2.07MP but generally expressed either as 2MP or 2.1MP).
Which is best?
The least useful for surveillance is likely 'P' because most measurements are not taken based on vertical count. For example, pixel density (PPF / PPM) is based on horizontal count. To that end, K is useful because it gives a feel for difference in horizontal pixel count, a key input to pixel density. For example, a 4K camera provides 2x the pixel density of a 2K camera everything else being equal.
The least used metric, though, is 'K', which has only started to be marketed / commonly used for 4K cameras. Because of that, many people will not be familiar with it.
On the marketing side, though, MP still sounds / shows the biggest number. For example, a 2K camera is ~2MP and a 4K camera is ~8MP. Notice that the K 'count' only doubles but the MP count quadruples. This is because MP multiplies the columns and rows. The bigger number often sounds more impressive.
A Note on Frame Rate / Avigilon
Though we have focused until now exclusively on pixels, 4K is a standard that also specifies frame rate, which is typically 24fps or greater.
However, many higher megapixel surveillance cameras deliver much lower frame rates. As an example, Avigilon markets a '4K' camera but it is only specified for 12fps max. Indeed, Avigilon also is demoing a 7K camera (meaning ~7,000 columns / horizontal pixels) but it is only specified at ~5fps and delivered choppy video during their own demo.
On the other hand, most users tend to be more tolerant of lower frame rates than lower resolution. For example, most would reject cameras under 720p today but many would be more than happy with a 10fps frame rate.
Image Quality Considerations
Finally, it is important to always keep in mind, that pixel count / resolution does not equal or guarantee image quality (e.g., Pixels Determine Potential, Not Quality).
Many higher pixel count / MP cameras have problems with wide dynamic range and low light as it is more difficult to implement the image processing that delivers true WDR and super low light performance.
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