The Three Things:
- Shutter Speed
Back in the old days, ISO referred to camera film. Specifically, it indicated a film's sensitivity to light. The higher the ISO, the faster it would respond to light, giving a brighter image with less exposure time. On modern cameras the ISO refers to the sensor amplification, so that raising the ISO value has the same effect: less light for brighter results.
Put another way, higher ISO lets you work with less light. Indoors, cloudy days or shady places may require higher ISO.
The aperture in your camera's lens is the same as the pupil in your eye: if it gets smaller, it lets in less light. Every lens has a rating for the aperture, the lower the number is, the larger the aperture gets. Lenses with a larger aperture (smaller number) are considered fast (F3 and below). A slow lens has a smaller aperture (F3.5 and up).
Faster lenses cost more, because the wider your aperture is the more readily it reveals flaws in the glass. Lens-makers have to spend more time and money and use higher-quality materials and construction to make faster lenses.
Shutter speed refers to the time your camera is physically open, allowing light to hit the sensor or film. A faster shutter speed means the camera is open for a shorter time. This allows you freeze the action, but with less time to gather light, the image will be darker.
What does this all mean?
This is where the balance comes in. You need to consider what you're shooting, and what qualities are important to you. Shooting a high-speed athlete requires a different balance than flowers in your garden, and both need to be re-balanced if you're shooting in the dark, or using flash, etc.
Every one of these three things has advantages and disadvantages when you adjust them, and you need to decide what's most important first:
- Fast moving objects: Shutter Speed
- Shooting dark places: Aperture
- Image quality: ISO
If your subject is moving fast and you want to freeze the motion for clarity, you have to have a fast shutter speed. When shooting athletes on the move, I try to shoot 1/500th of a second, or faster. In broad daylight this is easy, but in the dark you have to adjust the aperture and ISO to keep the shutter speed high.
Wide-open aperture and higher ISO allows you to keep using a fast shutter speed, but if your ISO is low or aperture too small, you'll get very dark pictures.
If you're shooting in shadow or indoors, raising the ISO is probably the easiest way to get good results, but it comes with a big caveat: higher amplification means more noise. In the same way that maxing the volume on your stereo or ipod results in distorted sound, maxing out your ISO results in blotchy colour and unattractive grain in your images.
As a rule, the cheaper or older your digital camera is, the lower your ISO can be without seriously compromising your image quality. You should always keep your ISO as low as possible for best results, but don't be afraid to raise it when you need to. It's better to get a grainy picture with high ISO than not get the image at all.
Aperture is your blur control. A wide-open aperture makes the background blurry, while the subject is in focus. The higher your F-number, the smaller the aperture, and as a result more of your image will be in focus. While you may think you want everything to be in focus, sometimes this means the subject is competing with the background for the viewer's attention.
Max the aperture when you have to: in dark places, you want a large aperture to let in more light, but this will usually reveal flaws in your lens. Typically your lens performs best when it's a little less than widest: an F2 lens is probably sharper and more reliable at F2.8 or F3.2. This differs for every lens, but you can rely on it as a general rule.
- Keep your ISO as low as possible, but raise it when you need faster shutter speed.
- Max your aperture for extra background blur, but remember that too-wide can reduce the quality.
- Faster shutter speed to freeze the action, but you get less light.
That's all there is to technical photography. Learn to balance these three things, and the rest is just art.