Clarification - does the vray light meter use lux as it's units?

[joconnell]

Hi folks,

As part of an effort to get my cg more accurate I'm looking in to a few more on set measuring things so I was wondering if the vray light meter was using lux as it's measurement value? So in theory I'd be able to place a light meter on the floor (as an example) of a live action set, get a lux reading for that place and then when I'm recreating things in vray afterwards place a vray light meter helper in the same location with vray lights the same distance and angle away as they were on set and then gradually adjust the output of my lights until the same amount of lux is falling on the light meter helper?

If it's not lux, what unit is it using instead?

Cheers!

John

[svetlozar.draganov]

Yes LightMeter measurement unit is LUX and it should work fine in the described workflow.

[joconnell]

We like this

[LL_Will_J]

Hi,

I know this is an old post - but I was wondering if you have had any luck with the technique of using a real life light meter and the vray one and using them to balance CG lighting.

I am also keen to try this idea to get a ''real world'' intensity value for my HDRIs that I can use alongside the physical camera.

I think this can be a really good workflow - especially if using a macbeth chart to calibrate plates.

Reading that it is a Lux value is good to know - but I am only getting an EV reading out of my current light meter. You can do a conversion on this - but it has a margin of error between camera manufacturers apparently. Still can be a really good foundation.

I would be really interested to hear your experiences of this.

Many thanks!

Will

[joconnell]

Buy yourself a lux meter on ebay or amazon, it'll be about 15 dollars!

Lux does indeed work fine - you'll end up with a slight difference in look though as your stills camera has some built in "look" curves applied to the image as opposed to it being a pure measuring device. You're right on being able to scale hdr's using lux as a target too, the only thing you might need is to get some neutral density gel paper and pop it on to the back of your fisheye lens so you can shoot the sun unclipped. Sebastian deguy did a run through it at siggraph - https://www.youtube.com/watch?v=zs0oYjwjNEo&t=7104s

The next thing I've to do is find a good way to measure the difference between what my camera is shooting versus what it's giving me to make a really good linear profile for it. It's tricky in some ways as our eye's don't view in linear colour, we're kind of log which is what stills cameras emulate - the camera is trying to recreate what we would have seen in front of us through our log colour eyes rather than the pure linear values so proper linear is reeeeeeally flat and boring looking by comparison to what we'd expect

If you haven't bought a chart yet, I reckon the kodak q-13 chart might be even better than the macbeth, there's similar colour patches but also a 20 step luminance chart - this might be more important to you than some of the colour things since your hues might not be accurate on the camera but the luma values are more important. You can white balance your image based on the grey scale values and see does your camera start to shift towards one colour or another as it goes towards the shadows or highlights.

I still have more stuff to do, I haven't produced a hdr accurately yet but it's on the list for the next two months.

Post things here as you find them!

[jasonhuang1115]

In the same boat here John. Just got a lux meter and implemented it in HDR calibration. I am fighting exterior scenes where I didn't use ND filter and see if I can still line everything up. The sun is clipped and it's giving me troubles... painfully enjoying it....

[LL_Will_J]

Thanks for the tips. That is real gold dust.

A LUX meter is definitely going on the shopping list. Do you have a recommended brand?

I have never heard of the kodak q-13 chart. Working with the Macbeth I have found it pretty useful - but that's simply in terms of using a nuke gizmo to balance stuff. I never thought about the luminance being more useful than the actual colours. Definitely something I will look into. Watching the video you posted they mentions using DCRAW to get truly linear images from RAW. Sounds like a good way to tackle the colour pipeline side of things. I have just always let PTGUI get on with it and not really questioned it during the combining and stitching phase.

I enjoyed the video from Sebastian deguy - but I am mainly shooting HDRIs on set between takes. I think if I had to take 10 minutes to get a calibrated HDRI the first AD would kill me. Still it has some nice points and definitely a few things that can improve my workflow. I would maybe do some tests of effort vs reward as I have never noticed fine details like chromatic aberration in an HDRI in a final rendered shot. But things like colour calibration and using the LUX meter sound very cool. I also wonder if you could shoot the sun clipped and then use the LUX values to cheat it in Nuke towards the super brights. I think all it really adds is intensity and no real detail (a super bright circle). Would save you a lot of exposures and messing with ND filters.

I have recently been using the Theta Rico on set. At first I was a bit skeptical because it takes quiet long to fire off shots from an app. But I am actually really impressed with the results. You can really get it right into places where you want to place your CG. So for small objects its really cool. It has quiet a lot of lens distortions and weirdness. But we are currently doing some work with miniatures and its been great. I think getting the camera exactly where the CG is has been a great improvement for me and normally unachievable with a chunky DSLR/nodal ninja/tripod rig.

All the best

Will