dimanche 19 décembre 2004

Snow White? But exposure meters see 18% grey! One of the hazards of being over enthusiastic about something is that you can let your heart take over when your mind should be firmly in control. Photography is sometimes like that... you think there will never be another opportunity to capture a magical moment so you quickly take as many pictures as you can before the subject disappears. In reality it's all happened and been seen before... and will be again. I'm reminded of this by the number of home-made photo-type Christmas cards received at this time of year. Snow to most people is an inconvenience if they have to travel to and from work whilst it's on the ground. But to photographers it presents an opportunity for picture taking that may only happen once a year... if that. On these opportune occasions rational thinking goes out of the window just when we should have a carefully scripted plan to hand on exactly how to tackle this natural phenomenon when it takes place. So what do we do? We grab a camera and dash out into the vivid whiteness rattling off shots in case a thaw is about to set in before we can put another film in the camera! But what appears to be wrong with most of these seasonal, picturesque cards? The answer can usually be summed up in just two words... grey snow! There are no prizes for guessing what has gone wrong... probably a combination of extreme cold, early morning blues and too much concentration on how the composition in the viewfinder will turn out makes many photographers forget the first and most important rule when taking snow pictures which is... ignore what the exposure meter tells you! It's a common misconception that the camera's exposure is always correct. Note: This is where digicam users have a good laugh at the traditionalists because they have the push-button facility to set "white-balance" for different levels of white and different colours of light. What many photographers don't realise in this age of almost total camera automation is that exposure meters are not infallible. Whilst a camera's built-in exposure meter does read correctly the light reflected from a subject - a "reflected" reading... and a hand-held exposure meter correctly reads the light falling onto a subject - an "incident" reading... with the first method the built-in meter can't know what tone the subject is reflecting the light falling on it. Because all meters are calibrated to measure an 18% Standard Grey Card as a mid-tone... that is what they are assumed to be pointed at all the time. Much of the everyday world (grass and tree's leaves for example) averages out to an 18% mid-tone which means that in general exposure meters are indicating a correct exposure much of the time. But that doesn't mean anything when you go out on a winter morning and everything is brilliant white. Your camera's built-in meter tells you that the light coming off the "mid-tone grey" snow is such and such an EV (Exposure Value)... but the problem is that lightly textured snow is very near to white rather than a mid-tone grey. An exposure meter has an eye... but it is blind so we must tell it that textured snow is around two stops lighter than an 18% mid-tone grey. But as a meter has no ears we must lie to it. There are two we can do this. We can either tell the meter that our film is two stops slower than it really is... for example that the Ilford FP4 I used here would be rated at an (Exposure Index) EI 32 instead of its nominal ISO 125... or if it is available as an option we can programme "exposure compensation" of plus two stops into the camera's meter. If using a faster ISO 400 film then rate it at EI 100 instead. Either method will give the same result... the camera's exposure will be fooled into giving two extra stops of exposure to the film and by so over-exposing it will render it nearer to the correct white in the photo print that we see with our own eyes in the real scene. It sounds much more complicated to describe that to do... over 700 words to describe something which, once instilled in your mind, you do automatically... or takes a second to do when adjusting a setting on your camera. But it's important to get it right because your photo Christmas cards will look so much more attractive with white, white snow than the unattractive sludgy-grey tone you see when you drive-and-slide home through it. Note : The trodden snow in the image of Market Lavington Church looks a bit too grey... but it was an evening shot and was just like that. Maybe I should have chosen a better example... although this image does perhaps illustrate the right and wrong of the problem! 8:25:46 PM    comment []