Calibration questions and observations

richard plumb

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Posted this in the c9 owners thread got it’d be useful as a standalone thread to get some discussion going

found this post while looking at options for tying to calibrate brightness without an expensive service or dedicated meter - using a DSLR. Has anyone tried something like this, and how were the results?

You can easily adjust the brightness using your camera as a light meter. Display a white screen on the monitor. An empty Word document or similar works just fine. Set the camera on full manual, ISO 100, f/11, 1/4 sec. Meter the screen using spot metering. Make sure the screen fills the entire metering area. Adjust the monitor's brightness until the camera's meter is on zero. That provides a brightness level of roughly 64cd/m^2. If you want something a bit darker go for 1/8 sec. which yields 128cd/m^2.

summary - set your camera to iso 100, f11, 1/8 shutter speed and adjust brightness until meter reads zero - should give you close to 128cd/m which is about what we should be looking for, right?

how does brightness change depending on room ambient light? If you’re only metering on a patch on the screen then won’t 120cd/m be the same brightness regardless of the room lighting> and therefore potentially look too dim in a bright room/bright in a dark room? Is that 120 target based on an assumption of a dark viewing environment?


edit: found this link referenced else which talks about viewing environment.
Viewing Environment and Calibration

Interesting read I think. 100 nits is only really relevant if you view in the same conditions - 5 nits surrounding lighting (pretty dark room) with some bias lighting around 5-10 nits. If you watch in a brighter normal room you’ll need to counter with higher brightness.

this expectation that most living rooms will be brighter - brighter than 100 nits SDR has a knock on impact to HDR too. HDR assumes standard white is 100 nits so can look dim if you’re using >100 nits for SDR viewing

I’m surprised there isnt a formula for brightness based on ambient lighting? I.e. if we have 100 nits in a 5nit ambient environment, how bright for a 20/40/60 nit environment? Such calculation should be possible as it’d be based on the display overcoming the ambient lighting enough to get back to the original mastering. Also surprised this still seems to not properly address HDR with some way to adjust the tone mapping to allow for brighter ‘normal’ whites to align better with the SDR setting for each individual
 
Whilst the theory is interesting, the actual standards are simple and I'd argue that the the home viewing environment should replicate the grading studio, otherwise, you'll not be viewing the content as it was graded.

If you're interested in accuracy, for SDR you should view in a pitch black room, with the display set to output 100 nits (cd/m2) at peak white, with a D65 bias light set to 10% of that.

For HDR, again you should view in a pitch black room with a bias light set to 5 nits (cd/m2).

The bias lighting will lift ambient luminance, clearly.

You're only really considering 'brightness' too, or peak luminance to be precise. The standards are to ensure the whole image presentation is as it should be and if we just look at peak luminance, we run the risk of having that too high and blowing out near black detail. This is especially an issue with HDR, as Light Illusion's article eludes too.

With regards to HDR, the EOFT is absolute, so there's no room for interpretation until the display is asked to go beyond its capability. This is where tone mapping comes in and is pretty much the wild west at this point, with every manufacturer doing it differently. So there really needs to be an addendum to the standards to define what display manufacturers do with their tone mapping.
That aside, reading you last paragraph, it's important to note that HDR is essentially SDR with the ability to display brighter specular highlights. Or at least it should be. As such, any part of the image that doesn't qualify as 'specular highlights' should be shown at the same levels as the SDR equivalent.

You can see an explanation of this here.

Paul
 

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