Exposure is controlled by three functions on a camera Ė the shutter speed, the aperture and the ISO. Overexposed or underexposed images are almost always due to one of these camera settings not being suitable to the ambient light. Overexposed images are dark and underexposed images are very light but detail is lost in both. A well-exposed photograph has proper detail across the highlights, shadows and midtones.
The ISO, shutter speed and aperture can all be used on automatic or manual modes but to get the best exposure in any type of light and in any kind of environment, itís best to use all three on manual mode if you can.
Using different combinations for the settings will give different results. The exposure triangle explains the effect each setting has on the others.
High Shutter speed = Darker Image and freezing motion
Low Shutter speed = Lighter image and blurring motion
Wide aperture = Lighter image and shallow depth of field
Narrow aperture = Darker image and wide depth of field
Low ISO = Darker image and less grain/digital noise High ISO = Lighter image and more grain/digital noise
The type of subject you are photographing, is often a good indication as to what settings you should use, however ISO should always be the final setting you choose because you want to keep the ISO as low as possible, to avoid having grainy pictures (unless thatís the effect you want).
For example if you are wanting to capture motion in sport, you will want a high shutter speed so you donít get motion blur. However this will make the image underexposed so you should choose a wide aperture to compensate for this. However choosing an aperture that lets enough light in and also allows a suitable depth of field is important, otherwise the focus on the subject might be slightly off.
Alternatively for a portrait, you would want a shallow depth of field, so you would choose a wide aperture, but this would initially overexpose the image by letting a lot of light in. You could choose a higher shutter speed to balance out the aperture (and definitely keep it above 1/125 if your subject is moving) without cancelling out any nice catchlights in the personís eyes.
Once the shutter speed and aperture have been selected, you should choose the lowest ISO you can without underexposing your photo. In most outdoor settings with natural light, you wonít need to go above an ISO of 800. However in indoor settings with artificial light or at night, you may have to bump the ISO up to as high as it goes. While grain isnít ideal, itís still better than having an underexposed photograph.
Sometimes only certain areas of your photo will be overexposed or underexposed, such as the lighter areas (highlights) being ďblown outĒ, such as the sky or a subjectís skin, or the darker areas (shadows) such as trees or a personís hair being too dark and dull. To prevent this you can:
- Turn on your camera's "Blink Function". This will appear on the LCD screen after a photo has been taken to show where the blown highlights are.
- Turn on your histogram. This will show where the light is skewed in a photo so you can see the darkest and brightest areas.
- Look at your light meter. This can be seen through the viewfinder of your camera. 0 is at the centre and when you hold down the shutter to take a shot, the needle will flash somewhere on this line. If the needle is on either side of 0 it usually means the photo will be over or underexposed. However the light metre can be incorrect in certain situations so use one of the other techniques to double check the exposure.