What is true 4K?

I was recently asked by one of our clients to clarify the reason for the 17:9 aspect ratio on the Sony PMW-F5 & PMW-F55 CineAlta cameras, as well as they were wondering if either of these models had a sensor “crop factor”.


by Casey Bramall – Service Technician at Fusion Cine

I was recently asked by one of our clients to clarify the reason for the 17:9 aspect ratio on the Sony PMW-F5 & PMW-F55 CineAlta cameras, as well as they were wondering if either of these models had a sensor “crop factor”.

The F5/F55 does not crop the sensor as some other 4K/5K cameras do at lower resolutions. The F5/F55 maintains the ‘standard super 35mm’ focal length at all resolutions, which is a very important factor in the fantastic image quality the camera produces. This means that Super35mm cinema lenses used in all resolution modes (HD, 2K, 4K), on the F5/F55 maintain the correct marked focal lengths. Some camera manufactures induce a crop factor (sometimes referred to as telephoto conversion) that you get going from a 4K resolution down to various lower resolutions, because it only uses a portion of the sensor. Sony achieves this by using the entire sensor at all resolutions and then converts the resolution in camera, post sensor. This is one more reason why the Sony PMW-F55 & PMW-F5 is the premiere choice for 4K/2K/HD acquisition, compared to the other available options currently out there, in the approximate same price range.

As far as aspect ratio is concerned, it is a broad subject, but I’ll try to explain the reason for the 17:9 sensors on the F5 and F55 as simply and clearly as possible.

The true 4K resolution is actually 4096×2160 (1.89:1 or 17:9) which is based on the 2K (2048×1080) model. 4K based on the 16:9 HD model resolves to a little less than 4K at 3840×2160 (1.78:1 or 16:9), although they are quite close, just like 1920×1080 or 1.78:1 (16:9) is ‘almost’ 2K, but the ‘true’ 2K resolution is 2048×1080 or 1.89:1 (17:9). For the foreseeable future the 4K at 3840×2160 (1.78:1 or 16:9) will be used for all consumer acquisition and display products, leaving the true 4K 4096×2160 (1.89:1 or 17:9) resolution for film and television applications, where the additional resolution would be helpful when the image is to be cropped, the integration of VFX, punched in later in post, or for theatrical release.

It seems that 16:9 4K will be shot at or cropped in post to the 3840×2160 resolution for display on a 16:9 monitor without top and bottom bars. If the full 4K image is used, there will be small horizontal top and bottom bars on a 16:9 screen.

To understand where the 16:9 or 1.78.1 aspect ratio came from, it was a simple solution to a big problem and was introduced as a simple compromise. The HDTV standard of 16:9 is more or less the average between 1.33:1 (4:3 television) and 2.35:1 (anamorphic cinema). 16:9 was chosen so that when black bars had to be introduced for either of those formats on a ‘widescreen’ 16:9 monitor, they would be minimized equally for each format.

New in the v3.0 Firmware released in December, there is now the ability to select a 2K cropped center extraction. The Center Scan mode for Super 16mm lenses lets you can use Super 16mm lenses and record XAVC HD, XAVC 2K, SStP and 2K RAW.

I hope this clears up some of the questions regarding HD/2K/4K resolutions, crop factor and aspect ratio and thanks for reading.