Eh what ?
Ha – ok some will know what this is about and so you can skip this but thought I’d post for those that don’t.
What I’m talking about is traditionally known as a Grey (or is it Gray…?) card, althought they now come in several different formats. Yes – this is a piece of grey card – but it’s a very specific luminescence/lightness of grey – 18% grey. (PS I’m using “Luminescence” or “lightness” rather than “shade” as that suggests colour and grey has none – it’s just part way between white or black – which are not colours…. penadtry over for now) Allegedly this is what all cameras from early-ish light meters were built to consider a middle and average tone. [I say allegedly as Thom Hogan a well know photographer/blogger says otherwise]
The story goes that someone (possibly Kodak ?) went through thousands of correctly exposed pictures and took an average, overal light value and found this to be 18% grey – and so built cameras/meters to try to achieve this. This is why sometimes taking picture of light things in snow will under expose (pushing it darker towards grey) and dark things on a dark background will over expose…… although modern cameras have got a lot better … anyway I digress..
So the main/original purpose of grey cards was to assist in obtaining the correct meter/exposure value in tricky scenarios. You can hold the card infront/next to your subject and take a meter reading from it which should be correct.
BUT another use for them (and the point which I have been getting to in a very round-about way) is that they can be helpful when trying to get the correct White Balance. (If you don’t know what White Balance is – err – it’s the measure of how warm or cool the light is – our eyes automatically adjust for it but cameras sometimes need to be told – especially with different kind of light bulbs)
Usually modern cameras can do a fairly good job of getting the right white balance – but sometimes they need some help. This weekend I was helping a friend out by taking pictures of her bags which she’s producing and will be selling. As part of this shoot I was taking product shots – the bags on a plain white background – over exposed so that it would be white, using studio lights at a friends studio. I realised that I’d forgotten my grey card – so although I have a rough idea what temperature/tone the studio lights are I didn’t have a proper value to go by. What I’ve done instead is to find a patch of shade that isn’t influenced by other areas and taken a value from that. (You can’t take values from pure white – most of my background – as it has no colour) But one of the bags is proving tricky – the purple one. It just won’t look as it should. Not sure if this is due to my White Balance measurement (in Camera Raw) not being correct or the camera having an issue with that colour. Either way – a grey card would have been handy – I think I’ll cut off part of the card I have and keep a bit in each of my camera bags.
As well as standard cards there are some newer formats as I mentioned.
small and portable:
BUT grey cards are not the final word on full/correct colour management. Although they are a quick and easy method – they only correct the picture based on one sample shading. Theoretically it’s possible that either the light source or your camera’s colour characteristics will vary according to shade i.e. the light might be a little orange in the darker shades, but it is less so when it gets lighter. Other colours might come out more there – in essence – the colour variation of the light or the sensitivity of your sensor may not be linear. So for a comprehensive calibration you’ll need something with more sample points. Something like the X-Rite ColorChecker passport:
As well as a grey card it has a section with many different colours to sample from. There are various others on the market but this seems to be the main market leader at the moment as it comes with software that you can install that will help you create a calibrated colour profile for your camera/lens/light setup and then you can apply it to all images in a batch.
Here’s a video from X-rite on its use:
I do not have one of these – I would like to have one of these 😉