Hints: exposure
This section is about taking control of exposure. If you are happy to leave your camera set on automatic exposure and generally take 'average pictures' (whatever they are!), then you won't need to worry too much about this.
Auto mode is fine for many - or perhaps even most - shots. The problem with "AUTO" mode is that the camera makes its decisions based on fixed choices built into the camera's own operating system. These may or may not be how you want the camera to operate.
If you just want to know a bit more about the operation of your camera and make better pictures when things are not 'average', the following notes might help. Or they may help if you have picture problems. But do remember that these notes can only scratch the surface, and if you want to get into more detail there are many books that deal with the subject. Or come along to the Club and see if we can point you in the right direction (although our meetings do not always include formal tuition, there is usually someone who can help out).
Appropriate exposure for a given amount of light depends on three factors: the aperture, the shutter speed, and the film (or digital processor) 'speed'. Let's look briefly at each.
Aperture
This is the adjustable hole within a camera's lens. The size is indicated by a number. The larger the hole, the smaller the number, eg, f2.8; the smaller the hole, the larger the number, eg, f8.0 or 16.
- A larger hole (lower number) lets in more light. Major aperture numbers let in twice or half of the light of the next number. This is a typical sequence:
(lets in most light) 2.0 - 2.8 - 4.0 - 5.6 - 8 - 11 - 16 - 22 (lets in least light)
- If using a smaller aperture which lets in less light, it will be necessary to use a longer shutter speed (see below)
Related matters
- The smaller the aperture (larger number), the greater 'depth of field', i.e. the more of the picture that is in sharp focus from front to back.
- So when taking landscape shots which includes near foreground objects that you also want in sharp focus you would want to use a smaller aperture (larger number)
- But when taking portraits and you may want the person to be in focus but the background to be blurred so you would use a larger aperture (smaller number).
Using a camera's 'aperture priority setting' allows you to set a chosen aperture and the camera will set the correct shutter speed (or will warn you if you have too much or too little light for the selected aperture setting).
Shutter speed
Shutter speed is how long the shutter is open, ie, the length of time the film [or digital image processor] is exposed to the scene being photographed.
- A faster shutter speed e.g. 1/1000 of a second lets in less light; a slow shutter speed e.g. 1/50 seconds lets in more light.
- Each shutter speed number let in twice or half of the light of the next number. This is a typical sequence:
(lets in less light)1/1000 - 1/500 - 1/250 / 1/125 - 1/60 - 1/30 - 1/15 (lets in more light)
Related matters
- A fast shutter speed will be better to freeze action, and a slow shutter speed may blur anything that's not absolutely still.
- If hand holding a camera, for sharp pictures without 'camera shake' it is generally advised that the shutter speed should not be slower than the reciprocal of the focal length of the camera lens. So for a 200mm lens the shutter speed should not be slower than 1/200 of a second, or for a 50mm not slower than 1/50 of a second, and so on.
- If your shutter speed is slower than the reciprocal then to avoid camera shake you should try and support it: lean against a wall, place on a surface, or fix it on a tripod. Consider using a cable release or the self timer function so that you're not touching the camera during the exposure.
- For digital cameras, you have to know the equivalent 35mm focal length as the above relationship refers to 35mm film cameras. The handbook will quote this. Also camera shake is more likely in a digital compact - particularly when holding the camera at arm's length and composing using the screen - and the effect more noticeable because the image processor is so much smaller than a frame of 35mm film; so it might be advisable to at least double the above speeds e.g. equivalent to 100mm focal length, shoot at 1/200 of a second.
Using a camera's 'shutter speed priority' setting allows you to set whatever shutter speed you want and the camera will set the correct aperture [or will warn you if you have too much or too little light for the selected shutter speed setting.]
Film speed (or the digital processor equivalent)
In simple terms, this is the sensitivity of the film or digital equipment to light - how much light it needs to make a picture. It is usually represented by ISO numbers. Lower numbers are known as 'slow film' and higher numbers relate to 'faster film'. Slow film (lower numbers) need more light to create a picture. Fast films need less light to create a picture.
- A slow film speed e.g. ISO 50 or ISO 100 will need a longer shutter speed for a given aperture. A fast film speed e.g. ISO 400 or ISO 800 will need a shorter shutter speed for a given aperture.
- But the image quality of slow film or the equivalent setting on a digital camera will generally be a better quality than faster film speeds, eg ISO 400 or ISO 800. [The effects in a digital camera in terms of grain and sharpness are broadly the same as film when comparing slower and faster film speed settings in the digital camera].
- So for the best quality, use the slowest film speed possible. In a digital camera, this just means setting the ISO number as low as possible (given the light available).
- The advantage of digital cameras is that you can change the film speed after each frame (if you really need to!) whereas with film cameras you have to shoot the whole roll of film at the same film speed and then put in faster or slower rolls of film (as needed) which could mean carrying around films at different film speeds!
