Thanks again for the helpful explanation. So that does mean a grey card is pretty useless in bright conditions right? That's my deduction. It's ok for me that the card doesn't have to remain grey. In fact, I can use a red card as long as I have a way to predermine it's equivalent to middle grey. What I'm more concerned is the limitations of the grey card.Originally posted by ckiang
Actually, the grey card reflects 18% (or 13%, depending on which school you belong to) of the light falling on it. It not necessarily has to remain "grey". 18% of a bright sunlight is obviously different from 18% of indoor flourescent.
From my experiment, the camera will always try to expose the card such that it becomes middle grey (which is what its metering system is programmed to do anway). In bright light, the card shimmers or glows (whiter in other words) and loses its "greyness". The camera, in turn, tries to bring it back to middle grey. The overall result though, is that I get underexposure. Cos, even if you don't do this simple test yourself, you will know that the card should be of a paler grey or brighter, not middle grey. In other words, the grey card is no longer accurate and becomes useless in helping one get a correct exposure. Please check my explanation as I hope to make sense and get it right.
The next question I have is: how will you know what is the limit of light shining on it that the grey card can hold before it becomes inaccurate?