First of all, what is the P mode? Your camera is blind to the world and has to make a lot of assumptions about it including how much light is out there, where to focus, how fast your subject is moving and the ideal depth of field (or how much in front and behind your subject should be in focus e.g. 2 meters in front and behind the subject or. as much as possible). Switching your camera to P mode means the camera has to make the best ‘guestimate’ it can for all these unknowns.
Why does the amount of light matter to your camera? Think of your own eyelids and how each of your eyes has an iris. When it’s a very sunny day you tend to squint and your irises expand to protect your retinas. Conversely, in a gloomy room, you open your eyelids as wide as possible and your irises contract.
Your eyes irises and eyelids vary the amount of light because your retinas like a certain amount of light to register information – too little light and your eyes only see limited information, too much light and your retinas are overloaded and you see nothing. Your camera’s sensor is the monocular equivalent of your retinas – the place where information about light is registered, and just like your retinas, the DSLR’s sensor likes an ideal quantity of light. Unlike your eyes, your camera has no eyelid or iris to regulate the quantity of light. Instead the camera controls light by varying the shutter speed and aperture.
Two new terms, shutter speed and aperture, deserve an explanation. Think of a shutter as a primitive eyelid for your camera, primitive in that the shutter is either open or closed and cannot be anything in-between. When you take a photo, the shutter temporarily retracts so light can come through the lens and register on your camera’s sensor. The longer the shutter retracts, the more light hits the sensor. In a bright room, you only want the shutter to retract for a short time or the sensor will be overloaded. In a dark room, you want the shutter to retract for much longer to give your camera’s sensor enough time to register the far dimmer light. The typical shutter speed is 1/125th sec which means the shutter temporarily retracts for a tiny fraction of a second but most DSLRs’ shutter speed ranges from 30 seconds to 1/4000th of a second. A later article in this series will later explain why you would want to vary shutter speed.
Now you know what a shutter does, where does aperture fit in? The aperture is the iris of your camera. It is physically part of your camera’s lens, it is usually octagonal and it restricts light by expanding and contracting.
Unlike an iris, an aperture has a ratings system e.g. f5.6, f8, f11. What does this mean? The ‘f stop’ rating simply explains how much light is let in by the aperture using a scale where f2 lets twice as much light as f4 as f5.6 lets twice as much light as f8 which lets in twice as much light as f11…. This is a confusing scale – just remember, the smaller the aperture (e.g. f4 instead of f 11), the more light is let in.
In summary, a DSLR controls how much light registers on the camera’s sensor by varying the size of the aperture and by varying the time the shutter retracts. It stands to reason that different combinations of shutter speed and aperture will result in the same amount of light reaching the sensor.
For example, if your sensor needs a shutter speed of 2 seconds and an aperture of f8, if you double the shutter speed to 4 seconds (which will let twice as much light in), then you need to change the aperture from f8 to f11 in order to halve the amount of light let in by the shutter.
Before you worry about what combination of shutter and aperture to use, you need to work out how much light is required by your DSLR’s sensor. Cameras have built in light meters to measure the prevailing light condition but they have to make an assumption about how much light is being reflected from the objects you want to photograph. Why? The camera’s light meter measures light that is being reflected from your choice of subject and the general environment around your subject. However, the camera’s light meter cannot know the reflectivity of your subject matter so it has to assume that overall, your subject reflects as much light as a light grey (known as “18% gray”) piece of paper and this assumption works for the majority of subjects.
To understand why this assumption isn’t always appropriate, imagine three tennis balls, one is white, one is grey and one is black.
Let’s assume the white ball is three times as bright as the black ball. In other words, the white tennis ball reflects three times as much light as the black ball. Now let’s imagine what your DSLR assumes when you aim your camera at the white tennis ball. Does your camera assume it is a black tennis ball in a very bright room, a white tennis ball in a very dark room or a light grey tennis ball in average lighting.? Faced with these three options, your camera assumes the tennis ball is light grey, whether it is in fact white, black or grey.
In order to make a white tennis ball a grey tennis ball photo, your camera will underexpose the photo i.e. it will speed up the shutter speed from say 2 seconds, to 1 second so that the sensor is only exposed for half the time it should be.
In order to make a black tennis ball a grey tennis ball photo, your camera will overexpose the photo i.e. it will slow down the shutter speed from say 2 seconds, to 4 seconds so that the sensor is exposed for twice the time it should be.
If this is all a bit too theoretical, think about photos you have taken that were incorrectly exposed. A classic example is snow – instead of pristine white landscapes, your pictures have dull ‘sooty’ snow because your camera didn’t know that the scene’s overall reflectivity wasn’t light grey but much brighter so it exposed the focus to make your snow light grey!
About the Author:
This article was written by John Slaytor. “I find it difficult to narrow my photographic interest. This inattention to detail gives me plenty of subject matter. My range of work includes Macedonian Weddings and Christenings, Nigerian 21st Birthday parties, Presbyterian and Catholic funerals, Indian and Greek family portraits, Chinese and Ghanaian football supporters, Auschwitz-Birkenau concentration camps and Sydney Opera House.
I live and work in Sydney but can and do travel anywhere for my vocation. I would like to think I have been positively influenced by Werner Bischof for his quiet humanistic vision, Jane Bown for her minimalist approach to technology, Eve Arnold for her compassion and Peter Dombrovskis for his pristine imagery. After visiting Auschwitz I came across Michael Kenna whose work has helped me understand how buildings can have mood. (I avoid formality and artifical lighting believing these things draw far too much attention to the process of photographing people. I have no qualms about making buildings endure long exposures with a tripod.) I use Nikon cameras and process my images (RAW only) using DXO. I print with an Epson 4800. My computer is a Mac and my screen is an Eizo.”
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