ISO measures the camera’s sensitivity to light. Doubling the ISO number, doubles the camera’s sensitivity to light. However, increasing the ISO also decreases the image quality (or increases the ‘noise’).
When would you increase ISO then? Increasing the ISO is invaluable when shooting hand-held in low light as it allows you to use a faster shutter speed & therefore, avoid camera shake. However, if you’re using a tripod this isn’t necessary & keeping the ISO setting at ISO 100 is usually the best option (the longer shutter speed will make up for the lack of light). Similarly, if you are using a flash, high ISO settings are not required (although increasing the ISO will increase the effective range of your flash).
Is it weird that since taking a keener interest in this hobby of mine, when browsing through the Christmas catalogues, instead of looking at the products being advertised, I find myself trying to work out the lighting & other techniques used to obtain the final images?
Maybe I’ve crossed a line that can never be uncrossed?!
The exposure meter measures the brightness of a scene. This is sometimes tricky as the brightness can vary a lot within the picture (eg. the sky can be much lighter than the foreground). So there are 4 different metering modes within the camera to try to help you obtain the best results. However, there isn’t necessarily a single choice for which one to use in a given scenario. It all depends upon personal choice.
Evaluative metering. This is the default option used in all automatic exposure modes (ie. not M, Tv, Av & P) & is the most sophisticated of the four. It breaks the picture down into zones, analyses them & tries to work out what type of picture you’re taking. It also biases the reading to the autofocus point.
Centre-weighted metering. This light metering mode measures light across the whole picture but strongly biases the reading to the centre. It doesn’t take the focus into account.
Partial metering. This measures the intensity of light in the middle of the frame (about 8-13% of the picture), ignoring everything else.
Spot metering. Measures light in a small circular area (about 2-4% of the frame) in the centre of the viewfinder.
Both spot & partial metering are useful when photographing a subject where the background is significantly darker or, lighter than the subject eg. a spot lit singer on stage. Spot metering is the hardest to use as it is the most precise & therefore, you need to know the spot to pick for the reading in order to get the best results.
Because of the way these metering systems work (measuring the light being reflected from a subject rather than the light falling on the subject) all will give the wrong results in certain situations. The reflected reading is skewed by colour for instance (light objects reflecting more light than dark ones) & so due to the averaging system used, assuming the average from the zones to be a midtone, exposure compensation is needed when photographing very light or very dark-toned subjects.
When I think of shutter speed, the first thing that comes to mind is its use in controlling the amount of blur (or lack of it) in a picture containing moving objects. If you want a moving object to appear sharp in a shot then you require a faster shutter speed than if you were taking a stationery object. When deciding what the speed should be you will need to take into account the speed of the object, direction of movement & how big it appears in the frame (eg. objects moving across your line of sight will need higher shutter speeds than those coming towards you; ones that fill your viewfinder require faster shutters speeds than those which are smaller). You may of course, not want everything moving to be perfectly sharp, instead opting for a more artistic blur to emphasize their motion. This latter idea really comes into it’s own when photographing moving water (eg. rivers, waterfalls etc.) to create a more dream like picture.
Another use of shutter speed, even when taking pictures of stationery objects, is to counter the effects of ‘camera shake’. If the shutter speed is fast enough it will compensate for the slightest movement of your body producing blurred images. How fast the shutter speed needs to be depends upon a number of factors such as, if you’re using an image-stabilised lens, how windy it is but the most important factor is the focal length of the lens. The more you zoom in, the harder it is to keep your body sufficiently still to obtain a sharp image.
The minimum shutter speed should be ‘one over’ the focal length eg. with a 50mm lens use 1/50sec or faster.
Naturally, all this ‘camera shake’ compensation is mute if you use a tripod.