Unerstanding exposure
Posted by reedcat | Posted in Articles, Uncategorized | Posted on 18-03-2007
0
In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph.
There are three controls which affect exposure:
- Aperture – defines the size of the opening in the lens. Aperture is metered in f-numbers. f-number is a ratio between the diameter of aperture and the focal length of lens (e.g. for 50 mm lens f/2 means that diameter of aperture is 25 mm). Traditionally f-numbers are set in a sequence so that each higher number (called f-stops) halves the amount of light falling on the image sensor or film (i.e 1 ev). This sequence is – f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 etc. Modern cameras also allow you to change aperture in 1/2 or 1/3 ev steps.
- Shutter speed – defines the exposure time i.e. the time for which the shutter is held open during the taking of a photograph to allow light to reach the sensor. Shutter speed is metered in seconds or fractions of a second (e.g. 1s, 1/30s, 1/60s, etc.). Many cameras also have a bulb shutter speed, which means that the shutter is opened as long as you keep the shutter release button pressed. Traditionally, shutter speed is set in a sequence so that each number (called f-stops) changes amount of light approximately two times (i.e 1 ev) from the previous number. This sequence is – 1s, 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/500s, 1/1000s, 1/2000s, 1/4000s etc. Modern cameras also allow to change shutter speed in 1/2 or 1/3 ev steps.
- Neutral Density Filter – a third option, particularly in bright sunlight, is to place filter in front of the lens which simply reduces the intensity of the captured light. The advantage of using a filter rather than a faster shutter speed is that the contrast of the image is increased, however midtones are more poorly resolved. For black-and-white photography, a monochrome filter (eg. green for portraits and red for sky) is more suitable, as this will enhance the dynamic range, and eliminate chromatic aberrations, producing a sharper image.
For a given set of illumination conditions and sensor ISO speed, there usually are several combinations of aperture and shutter speed which will make a correctly exposed image (e.g f/4 – 1/500s, f/5.6 – 1/125s, f/8 – 1/60s). Choices are more limited in dark or extremely bright conditions. For example in a night scene you may already have your ISO speed set to your cameras maximum and the lens aperture wide open, so your only choice to allow more light in is to increase the shutter speed.
For artistic/photographic reasons, you may want to use specific settings for some of these options. To ensure the same brightness is maintained, you will then need to inversely change one of the other options. For example, if you are shooting a flower and want the background to blur out, you will want a short depth of field… hence a more open aperture, say F4 or F5.6. This also lets more light in than the automatic settings would have chosen, so you will need to speed up the exposure time to compensate. Or, if you are doing a sports shot and want to show action blur, you will want a slower exposure time. Since this also lets in more light, you will also have to close down the aperture a bit to compensate and still get the correct exposure. This will be covered in more detail as we continue.
Sensitivity of the sensor – setting ISO value
In analogue photography, ISO measures the sensitivity of the film used. What you do need to remember is that a higher ISO will give you a brighter image but that the image will have more noise.
Generally it's best to use the lowest ISO (normally 100) available unless you have a good reason to use a different one. There is no universal standard so ISO 100 on one camera may be as good as ISO 50 on another. The only way you can find out what ISO level is acceptable to you is by experimenting.
If your camera has an automatic ISO selection function don't use it. Digital cameras are notoriously bad at automatically choosing ISO. If you let them do it you'll end up regretting it when the perfect shot ends up noisy.
ISO refers to film speed and a low ISO film is regarded as slow film and a high ISO film is fast film. Slow film offers better picture quality and fast film has a lot of grain referred to as noise in digital photography. Fast film does have its place when shooting in low light environments.
ISO is also known as ASA. ISO stands for International Standards Organisation and ASA stands for American Standards Association. In terms of photography they are both identical in that 100 ISO film and 100 ASA film is one and the same.
ISO settings in consumer-level/ entry-level digital cameras are significantly different from pro-level digital cameras. This is primarily due to the different manufacturing techniques used in high production, middle quality sensors and low product, high quality sensors. When you compare photos across cameras, an entry-level camera will typically have very high noise at around ISO400, whereas a digital SLR will likely not have a speck at ISO400. As such, ISO values can only be compared within camera, not across camera.
Prosumer SLR cameras tends to perform very well up to ISO800, and even at ISO1600, the noise is noticable but tolerable. As can be expected, ISO1600 offers a tremendous advantage to photographers in low light/ night environment.