Colour temperature and lightroom

[arad85]

I've invested in some lights to do some product photography. I'm currently playing about with the setup and camera settings before going live. The products I'm shooting are patterned materials, so colour consistency is very important - particularly when a number of images will be used side by side to promote the product.

I've been playing with setting the colour temperature on my 5D2 so I can understand what is correct given the lights I am using. So setting White Balance to be a dialled in Kelvin value I did some test shots at different temperatures. The exif data in the RAW files shows that the photos were taken at

7000
5500
6000

The develop module in Lightroom has the colour temp slider at

6250 tint -3
5100 tint +1
5500 tint -1

respectively. Anyone any idea what is going on?

 

[specialman]

Might help: http://www.lightroomforums.net/archive/index.php/t-8648.html?

Seems your camera's idea of neutral is the same as Lightroom's, it's just they call them different K temperatures.....

The tint function is to change green and magenta apparently.

Do the images all look different?

Were they taken in quick succession? If so, could be a drop in temperture of the flash.....

 

[arad85]

Thanks Pat.

Just to clarify, this was me setting the temperature manually on the 5D2 and dialling in the settings in the first list - there should be no "auto" anything here. The images do look different - but then you'd expect that given I've set them differently in each photo. I don't believe the flash to be a problem - I was using a couple of 300W studio flash units.

Interesting article you linked to there. It says the col. temp is stored as a set or coordinates so can be picked up differently. I'll do some more digging here....

 

[arad85]

An update...

It appears that the white balance is not derived from a single colour temperature, but relative ratios of R:G and G:B. What happens is as the raw file is decoded (developed if you like) the R, G & B values are multiplied by the appropriate scaling factors to generate the RGB data. So, for my 5D2 and a camera set colour temperature of 5400K, my RGGB (there are twice as many G pixels as R or B pixels on the sensor due to the Bayer nature of the sensor) ratios are: 2335 1024 1024 1633.

These are the values that are used to calculate the RGB values for display but more crucially, this is how ACR generates Col Temp and Tint values. For more in detail on this (and other aspects of RAW conversion) see this page on dcraw: http://www.guillermoluijk.com/tutorial/dcraw/index_en.htm and specifically the white balance section. Also, this link: http://photo.stackexchange.com/ques...oom-get-the-colour-temperature-of-a-raw-image demonstrates how these values affect the conversion.

So far, I have found that if I set a specific colour temperature in camera, the values in WB RGGB levels are always the same. LR also translates these numbers into a temp/tint pair entirely consistently too, just that the headline temperature values don't match.

What is frustrating about all this is that if I know the colour temp of my lights (OK, they are never exactly the same flash to flash and will change as power output gets set differently too) I can't translate that to a colour temperature value directly in LR. Well, I can, but it's not 1:1 (i.e. if my lights are 5400K, I have to set LR to 5000K, tint +1).