Henri Cartier-Bresson is famous for his photographs that capture that "decisive moment" when random actions intersect in a single instant that makes an arresting photograph. His eye-hand coordination was unrivaled, and he was able to get the results he did because he was always ready. There was never any fumbling with controls or lost opportunities. Most digital cameras have an automatic exposure system that frees you from the worry about controls. However, these cameras have other problems that make decisive moments hard to capture. One of the things that has driven many photographers to distraction is the delay between their pressing the shutter button and the camera actually taking the picture. This and other delays built into digital cameras affect your ability to respond to fast action when taking pictures.
When you take a picture with a digital SLR, its mirror must swing up so the light can strike the shutter and image sensor. While up, you can't see through the viewfinder. This viewfinder blackout should be as short as possible.
All of these delays affect how quickly you can get off the first shot or capture a series of photos one after another (often referred to as shot-to-shot time). If the delays are too long, you may miss a picture.
- The startup time is how long it takes for you to take a photos after turning on the camera. Cameras used to take a long time to do this, but now many start up almost instantly.
- The shutter-lag time is the delay you experience between pressing the shutter button and actually capturing the image. This delay occurs because of the time it takes the camera to clear the image sensor, set white balance to correct for color, set the exposure, and focus the image before it fires the flash (if it's needed) and takes the picture. The best cameras have almost no lag.
- The processing delay occurs as the image is processed and stored. This delay has been dramatically reduced by the addition of internal memory, called a "buffer." Images are temporarily stored in the buffer while awaiting processing because they can be stored there much more quickly then they can be stored on the memory card. You can continue shooting until the buffer fills
and resume when some images have been transferred from it to the memory card.
Responsiveness is most important when photographing moving subjects.