MaxFALL Maximum Frame-Average Light Level

What MaxFALL is

MaxFALL stands for Maximum Frame-Average Light Level. It is one of two content-light-level values that ship with HDR10 video, next to MaxCLL. Microsoft's HDR10 metadata structure defines it as "the nit value corresponding to the average luminance of the frame which has the brightest average luminance anywhere in the content." The unit is nits, the same as candelas per square meter (cd/m2). A colorist's grading notes put it more plainly: the frame in your project with the highest average brightness.

The value is static. One number describes the entire title, and it does not change from scene to scene. That makes it part of the same static bundle as the SMPTE ST 2086 mastering display data, which records the primaries, white point, and the mastering display's maximum and minimum luminance. The formal definition and the normative calculation sit in CTA-861.3, published in 2015, whose Annex A is titled "Calculation of MaxCLL and MaxFALL." The standard also defines the InfoFrame and EDID data block that carry static HDR metadata over HDMI. See static vs dynamic HDR metadata for the contrast with scene-by-scene formats.

How MaxFALL is stored

In practice MaxFALL travels as an unsigned 16-bit integer, so it can express whole nits from 0 to 65,535. The x265 encoder documents the pair as a string of two unsigned 16-bit values, MaxCLL first and MaxFALL second, in cd/m2, and a value of 0 means no maximum is indicated. The documented example is --max-cll "1000,400", which sets MaxCLL to 1,000 and MaxFALL to 400. Windows exposes the same pair as MaxContentLightLevel and MaxFrameAverageLightLevel inside DXGI_HDR_METADATA_HDR10. The mastering luminance fields beside it use finer units of 0.0001 cd/m2, so in x265's master-display string a 1,000 nit maximum is written as 10,000,000 and a 0.0001 cd/m2 minimum as 1.

HDR10 itself can address up to 10,000 nits at 10 bits per channel, but common content is mastered at 1,000 to 4,000 nits.

MaxFALL vs MaxCLL

MaxCLL is the brightest single pixel component in the whole title. MaxFALL is the brightest average over a full frame. Because an average cannot exceed the peak it averages, MaxFALL is generally smaller. A worked example with a 3840 x 2160 frame of 8,294,400 pixels: if 10% of the pixels sit at 1,000 nits and the other 90% at 50 nits, the frame average is 0.10 x 1,000 + 0.90 x 50 = 145 nits. MaxCLL for that frame is 1,000 nits and its contribution to MaxFALL is 145 nits. A frame that was full white at 1,000 nits would give both numbers as 1,000, and that is the only way they can match.

Real clips show the same gap. Chris Dobey's ST 2086 notes list a file mastered on a display with a 1,000 nit maximum and a 0.0001 cd/m2 minimum, with MaxCLL of 999 cd/m2 and MaxFALL of 215 cd/m2. A DaVinci Resolve walkthrough measured a MaxFALL of only 22 cd/m2 on one graded clip, and the author confirmed that most of the picture sat well below that. Microsoft's own sample structure pairs a 2,000-nit MaxCLL with a 500-nit MaxFALL. Dark, high-contrast films have low MaxFALL, while a snowfield, a desert, or an explosion that fills the frame raises it, because every pixel in that frame contributes to the average instead of just a few highlights.

What a TV does with MaxFALL

A display reads the values to decide how much compression the source needs. The standard example: a film mastered with peaks at 4,000 nits on a TV that reaches 800 nits must roll off the top 3,200 nits instead of clipping to white. If the metadata is missing, the tone mapper has no information about the brightest pixel or the brightest frame and falls back to safe assumptions, which usually means clipped highlights. Dobey adds that MaxFALL helps a display avoid damage when a frame exceeds what it can safely sustain.

Whole-frame brightness is exactly where displays differ most. In ecoustics' lab test, the TCL QM8L measured 1,992 nits on a 10% window in Filmmaker Mode but 708 nits on a full-screen white pattern. A title with a MaxFALL of 215 nits sits comfortably under that second number, while a title that averaged 900 nits across the frame would not. How each brand weights MaxFALL inside its tone mapper is not published, so treat claims about specific brand behavior as unverified.

Common confusions

MaxFALL is not a peak brightness spec for your TV, and it is not the same as the average picture level you see on a light meter. It is a property of the content. It also does not apply to HDR10+ or Dolby Vision scene data, which carry dynamic metadata that can change every scene. Encoders and grading tools compute and write the numbers: Resolve and x265 can write them, and ffprobe can verify them with -show_frames. Check them after an encode, because a title with no values leaves the TV guessing, while correct values let it map a 4,000-nit grade onto an 800-nit panel deliberately. Pair this entry with nits and peak brightness for the display-side numbers.