Anyone with a grey card who shoots in Raw? I need help...

[twinquartz]

I have a Canon EOS 600D, a few grey cards, I shoot in Raw
and process the photos in Camera Raw (ACR) in CS6.
I also have Magic Lantern installed in the camera.

Background: I set every parameter in ACR to 0 and use Camera
profile = Neutral and point curve = linear.

I shoot a Lastolite Ezybalance 12% (= 12% reflectance) grey card
and expose so that Magic Lantern's spot meter indicates that the Lastolite
surface is 50%.

Problem: when I import the shot into ACR and measure the grey
area, it does not show 128/128/128 as I expected and hoped for.
Instead, it shows as 119/119/119 when I use Adobe RGB colour space,
and 100/100/100 when I use ProPhoto RGB colour space.

Where is my line of thinking wrong?

Edit 130731:
Values modified. Now I know why
and I will publish full info as soon as I have verified the data...

[RSalles]

Hi,

Well, raw + gray card I use often. and I didn't payed attention to the RGB values difference in different color spaces, but I will.
Notes about practical use: as soon as you get the 3 RGB values equal you're going well, even if the luminosity shows a big difference in luma values. This is good for color balancing, or WB, you'll not have color casts. If you have three cards, grey, white and black, and white and black are well positioned in the curve - step and threshold, but not grey, you came to discover something new, at least for me: the RAW data file is embedding a tone curve even with linear camera profile. I use a 18% grey card, falling straight in Zone 5, if you're old as I am, and Zone System is not new for you.
I'm guessing if with a UniWB WB the diference would be or not the same...

Cheers,

Renato

[visualopsins]

Increase the brightness so the image grey measures 128,128,128. I suspect camera is underexposing...

[Orio]

My personal experience with Canon cameras is that the built-in spot meter is not accurate enough for professional use.
For instance, the spot meter of my 5D Mark II misses the exact value nearly every time, with errors that range from
2/3rds of a stop (minimum) to 1 stop and 2/3rds (maximum).

Remember (this is important) that the built-in meters in Canon cameras do take actual readings only wide open.
When stopped down, they perform a computer calculation instead, based on the ISO, speed and aperture selected
(this is the source of many inaccuracies when using manual lenses on chipped adapters that were burned
at a different max. aperture value than the lens that you are using).

If instead I spot-meter using my Sekonic L-758DR and take the picture in Manual mode, I get perfect results every time.

[twinquartz]

@Renato: "I didn't pay attention to the RGB values difference in different color spaces, but I will"
Yes, please do, since that is the big dilemma in this matter

@visualopsins: "I suspect camera is underexposing..."
and @Orio: "the built-in spot meter is not accurate enough"
Sorry, that would not explain the different RGB values in the two colour spaces.

I have experimented quite a lot the last few days and I believe I have a nice
solution to the problem -- I will verify the data before I publish them, though.

[poilu]

[Orio]

[twinquartz]

@poilu: Exactly, that was my problem!

What makes me mad/irritated/angry is that the manufacturers of 12/18%
grey cards (= 12/18% reflectance, which is a fact that most people seem
to misunderstand) forget to tell you the proper colour space when they
recommend you to set your mid grey to 128. If your Camera Raw is set
to ProPhoto RGB, Adobe RGB or sRGB you will use false values.

Since I have not understood the importance of this until now: you can not
imagine how many bad grey card matchings I have attempted during the years
using their recommendations...

I have just tabulated the correct values needed for a SpyderCube (one of my
favourite tools) in Camera Raw and in different color spaces -- if you want them,
just send me a PM.

[ilguercio]

Another good point is that exposure as it used to work on film is quite not the way to go with digital, where you want to saturate the histogram right to the point where you have clipping and stop there.
ML has also got ETTR, expose to the right, feature so that you can just use that and the LV image will scan the frame and set the exposure according to the acceptable clipping values you have set and so on.
Yes, modern light meters in cameras suck, especially when you consider that they tend to underexpose the image, which is the kind of thing you don't want to do with a DSLR.

[twinquartz]

[ilguercio]

[ilguercio]

[twinquartz]

Grazie!

[RSalles]

Well, I think I know what's happen. Adobe Lightroom uses ProPhotoRGB 16bits as its default color space for editing. The default gamma value for ProPhotoRGB is 1.8, but LR uses gamma 2.2, as sRGB, screwing everything.

Renato