| First of all, what is the P mode? Your camera is | | | | smaller the aperture (e.g. f4 instead of f 11), the |
| blind to the world and has to make a lot of | | | | more light is let in. |
| assumptions about it including how much light is | | | | In summary, a DSLR controls how much light |
| out there, where to focus, how fast your subject | | | | registers on the camera's sensor by varying the |
| is moving and the ideal depth of field (or how | | | | size of the aperture and by varying the time the |
| much in front and behind your subject should be | | | | shutter retracts. It stands to reason that different |
| in focus e.g. 2 metres in front and behind the | | | | combinations of shutter speed and aperture will |
| subject or. as much as possible). Switching your | | | | result in the same amount of light reaching the |
| camera to P mode means the camera has to | | | | sensor. |
| make the best 'guestimate' it can for all these | | | | For example, if your sensor needs a shutter |
| unknowns. | | | | speed of 2 seconds and an aperture of f8, if you |
| Why does the amount of light matter to your | | | | double the shutter speed to 4 seconds (which will |
| camera? Think of your own eyelids and how each | | | | let twice as much light in), then you need to |
| of your eyes has an iris. When it's a very sunny | | | | change the aperture from f8 to f11 in order to |
| day you tend to squint and your irises expand to | | | | halve the amount of light let in by the shutter. |
| protect your retinas. Conversely, in a gloomy | | | | Before you worry about what combination of |
| room, you open your eyelids as wide as possible | | | | shutter and aperture to use, you need to work |
| and your irises contract. | | | | out how much light is required by your DSLR's |
| Your eyes irises and eyelids vary the amount of | | | | sensor. Cameras have built in light meters to |
| light because your retinas like a certain amount of | | | | measure the prevailing light condition but they |
| light to register information - too little light and | | | | have to make an assumption about how much |
| your eyes only see limited information, too much | | | | light is being reflected from the objects you want |
| light and your retinas are overloaded and you see | | | | to photograph. Why? The camera's light meter |
| nothing. Your camera's sensor is the monocular | | | | measures light that is being reflected from your |
| equivalent of your retinas - the place where | | | | choice of subject and the general environment |
| information about light is registered, and just like | | | | around your subject. However, the camera's light |
| your retinas, the DSLR's sensor likes an ideal | | | | meter cannot know the reflectivity of your |
| quantity of light. Unlike your eyes, your camera | | | | subject matter so it has to assume that overall, |
| has no eyelid or iris to regulate the quantity of | | | | your subject reflects as much light as a light grey |
| light. Instead the camera controls light by varying | | | | (known as "18% gray") piece of paper and this |
| the shutter speed and aperture. | | | | assumption works for the majority of subjects. |
| Two new terms, shutter speed and aperture, | | | | To understand why this assumption isn't always |
| deserve an explanation. Think of a shutter as a | | | | appropriate, imagine three tennis balls, one is |
| primitive eyelid for your camera, primitive in that | | | | white, one is grey and one is black. |
| the shutter is either open or closed and cannot be | | | | Let's assume the white ball is three times as |
| anything in-between. When you take a photo, the | | | | bright as the black ball. In other words, the white |
| shutter temporarily retracts so light can come | | | | tennis ball reflects three times as much light as |
| through the lens and register on your camera's | | | | the black ball. Now let's imagine what your DSLR |
| sensor. The longer the shutter retracts, the more | | | | assumes when you aim your camera at the white |
| light hits the sensor. In a bright room, you only | | | | tennis ball. Does your camera assume it is a black |
| want the shutter to retract for a short time or | | | | tennis ball in a very bright room, a white tennis ball |
| the sensor will be overloaded. In a dark room, | | | | in a very dark room or a light grey tennis ball in |
| you want the shutter to retract for much longer | | | | average lighting.? Faced with these three options, |
| to give your camera's sensor enough time to | | | | your camera assumes the tennis ball is light grey, |
| register the far dimmer light. The typical shutter | | | | whether it is in fact white, black or grey. |
| speed is 1/125th sec which means the shutter | | | | In order to make a white tennis ball a grey tennis |
| temporarily retracts for a tiny fraction of a | | | | ball photo, your camera will underexpose the |
| second but most DSLRs' shutter speed ranges | | | | photo i.e. it will speed up the shutter speed from |
| from 30 seconds to 1/4000th of a second. A | | | | say 2 seconds, to 1 second so that the sensor is |
| later article in this series will later explain why you | | | | only exposed for half the time it should be. |
| would want to vary shutter speed. | | | | In order to make a black tennis ball a grey tennis |
| Now you know what a shutter does, where does | | | | ball photo, your camera will overexpose the photo |
| aperture fit in? The aperture is the iris of your | | | | i.e. it will slow down the shutter speed from say 2 |
| camera. It is physically part of your camera's lens, | | | | seconds, to 4 seconds so that the sensor is |
| it is usually octagonal and it restricts light by | | | | exposed for twice the time it should be. |
| expanding and contracting. | | | | If this is all a bit too theoretical, think about |
| Unlike an iris, an aperture has a ratings system | | | | photos you have taken that were incorrectly |
| e.g. f5.6, f8, f11. What does this mean? The 'f | | | | exposed. A classic example is snow - instead of |
| stop' rating simply explains how much light is let in | | | | pristine white landscapes, your pictures have dull |
| by the aperture using a scale where f2 lets twice | | | | 'sooty' snow because your camera didn't know |
| as much light as f4 as f5.6 lets twice as much | | | | that the scene's overall reflectivity wasn't light |
| light as f8 which lets in twice as much light as f11.... | | | | grey but much brighter so it exposed the focus |
| This is a confusing scale - just remember, the | | | | to make your snow light grey! |