Exposure is the heart and the art and the act of taking a photograph.
In film cameras, the film must be exposed to light, the light causes a chemical reaction to occur which captures and records the picture.
In a digital camera, the action is the same except, instead of the light falling on a chemical soaked film, it lands on a sensor. Think of the sensor as the camera’s eye, it “sees” the light and records the picture.
I don’t really want to use an eye as an analogy as your eye has a very clever brain behind it helping out. To simplify things your eye needs just the right amount of light to see. Too much light and you are blinded, time to put on your polarised sunglasses (more of this in another article I think), not enough light and it is too dark to see.
The “right” amount of light, is what’s known as the correct exposure. You are letting in just enought light to record a good photo.
If your photo is over exposed, there is too much light, it will be too bright, too much white, you will loose detail in the bright areas. This is also known as “blown highlights”.
If your photo is under exposed, there is not enough light, it will be too dark, you will loose detail in the dark shadow areas.
You can use software to manipulate an image, generally you can salvage some detail from underexposed images but it is very difficult to recover over exposed images. This means you should err on the side of underexposure when you take a photo.
How do we control the exposure that we take?
There are two mechanisms that we can control that will affect the exposure:
The aperture is the “hole” in the front of your camera that lets in the light. Strictly speaking the hole is in the lens. This hole can change in size. The larger the hole, the more light gets into the camera. The smaller the hole, the less light. The aperture is measured in FStops, the larger the FStop the smaller the aperture.
The shutter is a “curtain” that blocks the light once it has passed through the aperture and controls when the light gets to the sensor. You can control how long the shutter stays open for, long shutter speed, more light gets in, short shutter speed less light.
Instead of taking the perfect photo, I want the perfect glass of water. A 1/2 pint is just enough to quench my thirst. I can turn the tap on fast (large aperture, lots of light/water) but I need to be careful and only hold the glass under the tap for a short period of time. Alternatively, I can turn the tap on slowly (small aperture, a little light/water), to fill my glass I now need to hold it under the tap for longer to get my 1/2 pint. If I try and put 3/4 of a pint of water into my 1/2 pint glass, I will have too much water (over exposed), it will spill make a mess and be wasted.
Now that we know that exposure is a function of aperture and shutter speed, how do we control this? We do this through the various modes on our cameras.
The camera will “work out” the correct exposure by looking at the picture, it will automatically set the aperture and the shutter speed. As this is automatic, there is nothing for you to do here other than press the button and then bask in the results. Safe, but boring.
If you use shutter priority mode, YOU control how long the shutter is open for and the camera will decide how large an aperture to set to get the correct exposure. You can get some creative effects by changing the shutter speed. Long exposure, see movement, blur things, short exposure, freeze the action.
If you use aperture priority mode, YOU control the size of the aperture and the camera decides how long to open the shutter for. You can get some creative effects by changing the aperture, small aperture, sharp photos, large aperture, you can blur “parts” of the photo. More on “Depth Of Field” in another article.
In program mode, you can cycle through the permissible combinations of shutter/aperture settings.
In manual more, YOU set the shutter and YOU set the aperture. YOU are responsible for ensuring that the values you select will result in a correctly exposed photo.
Up until now we have assumed that the camera will work out what the “correct” exposure is. The camera may get it wrong. You may want to override this for a particular effect. This is done via exposure compensation. Here you tell the camera that you want to deliberately over or under expose the photo and it will change the settings accordingly.
I have kept this article deliberately simple.
I will discuss FStops for example in another article.
I will add some images to highlight modes etc on my camera (Nikon D50) at a later date.
If you have any preferences on the next article, leave a comment.