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Why Do DSLR's Have Mirrors?

by Roger N. Clark

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Why do DSLR's still use mirrors? One answer is simple and due to the requirements of predictive autofocus and tracking of moving subjects. Predictive autofocus systems provide the fastest autofocus and fastest autofocus tracking technology currently available in consumer and professional cameras, both film and digital.

In general, the cameras that fit in your pocket are small low-end cameras that have slow response times. But even if you find one with low shutter lag, it will not be of much help on subjects moving toward or away from your camera. For such a moving subject, the full press shutter lag is only a small part of the story in getting a sharp picture.

Many cameras have what is called "Live View." Live view is an electronic method of reading the "digital" sensor and displaying the result on an LCD screen. The term "Live View" is a misnomer. It takes time to read out the sensor, and the more pixels the sensor has, the more time it takes. Very high speed electronics in high end cameras can read out at rates of around 100 million pixels per second. On a 10-megapixel camera, that means the time to read out is about 1/10th second (100 milliseconds). On lower cost cameras, slower electronics are used, so the readout times can be longer. It also takes time to send that data to the LCD. On some cameras, the shutter speed can also influence the cycle time, adding another delay. All this means that "Live View" is actually delayed view. Often this delay is longer than 100 milliseconds, and that means when you press the shutter the subject is in a slightly different position than what you see on the LCD screen. This is another factor in fast action photography and why those who do such photography usually choose an optical viewfinder. But it is not the main factor. The main factor is predictive autofocus.

P&S cameras, especially small cheaper ones use the sensor for autofocus using a contrast detection method. The lens is moved, the sensor read out, and the contrast checked, the lens moved, the sensor read out, the contrast checked and if it is getting better keep moving in that direction, if not, go back, hunting for best focus.

The full press shutter lag, the time from when you press the shutter to when the camera actually takes the picture, is the best time for a static (not moving) subject in good light. If the subject moves, the camera gets confused because the search for the best focus is continually moving too. That increases the lag time. You can experience such increased lag on static subjects too if you are swaying back and forth: the camera to subject distance is changing, confusing the camera. I have often had P&S cameras take 2, 3 and more seconds to take the picture when their shutter lag is rated about 1/2 second due to these effects.

For static subjects, a DSLR does what is called a phase detection: the phase is measured which tells the camera how much the subject is out of focus. The camera calculates how much to move the focus and does that in one step. No second check is required. For moving subjects, put the camera in what is called predictive autofocus and the camera monitors the phase of the focus as the lens is moved (or not moved). The changing phase tells the camera what direction the subject is moving and how fast, it then moves the focus to the best position but also continually tracks it. Even more impressive is that the camera knows its own shutter lag and predicts where the best focus will be when the shutter actually opens and sets the lens to that point.

When holding down the shutter on a DSLR and taking multiple frames, each time the mirror drops, is allows you to see the subject to help you track the subject, but the phase detection system does another measurement of the focus. With each measurement it continually tracks the focus, the velocity and the changing velocity. The phase detection system in DSLRs is a set of optics and sensors behind and below the reflex mirror. Because of the requirements of a phase detection system design, a mirror is necessary, thus reflex mirrors are not likely to go away. One could do this with beam splitters, but that loses light, compromising light for the phase detection system, light metering system and the picture you want to obtain. Again, the reflex mirror serves a very important part of the phase detection system so is not likely to go away.

Here is a demonstration of autofocus tracking of a DSLR:
Wildlife Action Photography: Autofocus Tracking with Digital Cameras


Think of this example: your baby is about to take his/her first step.

You grab the camera aim, and in a blink of an eye the event is over. So did you get the picture (or several)?

P&S: Unlikely. The baby does two steps and falls; the camera finally focuses and gets a picture of the baby on the floor. That's assuming the camera had "instant on," otherwise you may have simply watched the event as the camera slowly turned on.

DSLR in single shot mode all focus point enabled: No! The camera focused on the wall in the background. ;-)

DSLR in single shot mode, one focus point on the baby: One frame in focus then as the baby moves forward, successive frames are increasingly out of focus.

DSLR in predictive autofocus mode, one focus point on the baby: All frames in focus. Many excellent pictures of the event.


The difference in "focus lock" versus predictive autofocus: In predictive autofocus the camera fires when you press the shutter all the way, regardless of where the focus is. If the camera locked onto the subject and is tracking it (and its speed can be tracked by the lens AF speed) the prediction is usually quite good and stays in excellent focus, or, with some slower AF consumer lenses and bodies, at least better focus than if using single shot mode on moving subjects.

Bottom line: if you want to take photographs of things in action that may be moving toward or away from the camera, choose a camera with predictive autofocus. All DSLRs that I know of have predictive autofocus. I do not know any P&S cameras with predictive autofocus.

One may use "pre-focus" on cameras without predictive autofocus. With this method, the photographer must anticipate where the peak action will occur, set the focus for that positions and wait. This can work well for things like the finish line of a race. Unfortunately if the best action occurred at a slightly different position, you miss the best image. That is where predictive autofocus shines.

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First Published November 6, 2008
Last updated December 9, 2011.