Shutter speed
This is the amount of time the film or sensor (i'm going to use the term 'image plane' to refer to these two generically) is exposed to the light. Pretty obviously, longer time open equals more light hitting the image plane. Shutter speeds traditionally run on a doubling pattern, but generally the number displayed needs to be considered as a fraction. For example a speed of '1000' displayed means one-thousandth of a second, a relatively fast speed. '1' means one whole second, and inbetween we generally find 2 (half a second), 4 (quarter), 8 etc up to 64, which is then 'almost' doubled to 125, 250 etc up to whatever your camera maker deems necessary. Mine goes to 4000. Some go much faster than this, and many go to much slower speeds than 1 second. A shutter speed of 'B' signifies 'Bulb' exposure - a throwback term which basically means the shutter is held open for as long as you hold the shutter release. sometimes accompanied by a 'T' setting ('Time') which opens the shutter on one press of the release and closes it on a second, these settings can be used for very long exposures lasting several minutes or even hours when shooting star trails etc. Fast shutter speeds freeze action or movement, slow ones blur it. Different subjects will look good at different speeds. For example you may assume that a racing car should be shot at the fastest possible speed, but if that causes the wheels to be frozen too, the car will appear not to be moving. If you want to experiment with the effects of different speeds, find a fountain or fast-moving stream and shoot it at all different speeds. From swirling milky streaks to sparkling droplets, the effects are very different. Shutter speed also has an impact on flash photography. Your camera will have a maximum sync speed, above which the flash cannot be used. On most digital SLRs this is a respectably fast speed above 1/125 sec. On many (specifically Canon) budget film SLRs it was/is much lower, around 1/90 sec, though some flash units can get around this limitation.
Aperture
The Aperture setting controls the amount of light entering the lens in a different way to shutter speed, though the two are related. The aperture is the name for the hole or gap through which light passes on its way to the image plane. The diameter of this hole can be varied using either a ring around the lens barrel, or a setting on the camera body. For all Canon EOS SLRs and lenses and many of the modern budget nixon lenses, this is a camera body setting, if you're on an older camera, especially anything manual focus, the aperture tends to be set from the lens. Settings range on normal lenses from about 1.4 to 32, in numbers which increase by multiplying by approximately root2, thus the range goes 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32. These numbers are often referred to as 'f-numbers' or 'f-stops' and written as 'f/5.6'. This is because they are expressed as a fraction of the focal length of the lens, and thus the same number can mean the same exposure setting regardless of the lens length. F/32 is thus much smaller than F/2.0, one sixteenth the diameter in fact. The gap between each number in the range is known as a 'stop' and setting your lens to a smaller aperture is known as 'stopping down'. One stop of aperture corresponds to one doubling or halving of shutter speed, an arrangement known as the reciprocity law. If your camera suggests, for example 1/250sec at an aperture of f/11, you could equally shoot with settings of 1/60 @ f/22 or 1/1000 @ f/5.6, and obtain the same exposure, albeit with different effects. We'll come on to aperture and shutter speed related effects in a future entry. For now I'm just concentrating on exposure and as such we can leave aperture there.
ISO (or 'film') speed
This is controlled on film cameras by physically changing the film, or sometimes changing how we choose to treat film. As such it is a 'whole roll' decision that must be lived with for up to 40 frames if we don't wish to waste film. On digital cameras it tends to be much easier, and can be set differently for each and every shot if we so wish. It is a measure of the sensitivity of the recording medium, be that film or sensor. Usually available in doubling hundreds from 100 to typically 800 or 1600, it can be treated as simply another exposure variable, allowing you greater flexibility with your shots and the conditions in which you shoot them, it does have tradeoffs however, which are perhaps not so obvious as those related to shutter speed. Suppose you are set at 1/100sec @ f/4, your lens aperture as wide open as it goes, and your ISO on 400. The light levels drop and you need just one extra stop to expose correctly. Aperture is at maximum so that leaves just shutter speed or ISO. If you reduce the shutter speed (increase the time for which the image plane is exposed) then you risk not being able to hold the camera still. If you take the other option and push the ISO up to 800, your image quality may well be compromised by increased image 'grain' or 'noise' (extra patches or blotches of colour or light where you don't want them) which is a common side effect of higher ISOs. Going one stop higher to 1600 will seriously affect your image usability on all but the highest quality SLRs. It's there to be used, but bear in mind that speeds typically above about 400 ISO will adversely affect your images in some way. One quick addendum for any pedants in the building - the technically correct definition of ISO includes both the European DIN specification and the more familiar numeric American standards. The 200 or 400 or whatever is technically called the ASA rating, but in most publications etc, ISO and ASA are used interchangeably,
This has become something of a monster post, so I'll leave it there for now. Once you've digested the settings, I'll try my best to explain how and why and most importantly when to use them.
No comments:
Post a Comment