Next to poor focus, the biggest killer of sharpness in your images is vibration. Not just the kind of vibration that comes from hand-holding your camera at too slow a shutter speed. That kind is more properly referred to as camera shake, and the solution there is a sturdy tripod. But, even if you use the biggest bad-est tripod you can carry, vibration can still affect image sharpness under certain conditions.
Despite the largely electronic and software driven processes that digital SLR cameras employ to create images, there are still elements in the picture making chain that are mechanical in whole or in part. Chief among these are your shutter, and the instant return mirror that allows you to view the image as your lens sees it right up to the moment of exposure. Between the these two, the mirror is the big offender when it comes to generating image sharpness destroying vibration.
If you remove the lens of your SLR camera and peer inside, the instant return mirror is easily visible (DO NOT touch this mirror, or attempt to clean it using conventional lens cleaning methods. The silver coating on this mirror is on the front surface of the glass and very easily damaged). The mirror’s function is to re-direct the image formed by the lens up through the pentaprism, where it is in turn redirected out through the eye-piece. Only problem is, the mirror sits right in the line of fire between the lens and your the digital image sensor, which resides behind the mirror and is normally covered by the shutter curtains up to the moment of exposure. So when you press the shutter button, the mirror has to quickly get out of the way, so that the image forming light from the lens can reach the sensor.
Let’s look at the sequence of events you start when you press the shutter button:
- Press the shutter
- The lens stops down to your chosen aperture.
- The mirror flips up out of the way
- The shutter opens
- The image is formed on the sensor (or film)
- The shutter closes
- The mirror returns to its original position.
Events 1 through 4, if they cause vibration can affect image sharpness. Events 6 and 7 can’t, since the image is formed and the shutter is already closed. Between the two of them the mirror flipping up is the big culprit, the shutter opening, since the mass of the shutter is so low is not really an issue.
When the mirror flips up the vibrations it creates are initially significant, but quickly dissipate. Usually there isn’t enough time to allow these vibrations to settle down before the shutter opens, but this isn’t always as big a problem as it might seem. At faster shutter speeds, the shutter isn’t open long enough for the vibrations to have much effect. At very long shutter speeds (several seconds) the period during which the mirror vibrations last is only a small portion of the overall exposure time, and because of this, they are not a big factor in these situations either. It’s in the range of shutter speeds of roughly 1/30 second to perhaps 1 second that mirror induced vibration is an issue. In these situations the trick is to get some separation in time between “Event 3″ and “Event 4″ above. Separating the mirror flip from the shutter opening allows the vibrations caused by the mirror flipping to settle down before the shutter opens. Once the shutter opens we want things to be nice and calm.
The answer is called “mirror lockup”. When mirror lock-up is set, the first press of the shutter (via a remote cable release, of course) simply causes the mirror to flip up out of the way. After a second, or two or three, a second push of the shutter button causes the shutter to open for whatever time you have set, after which the shutter closes and the mirror returns to its original resting position.
Mirror lockup on modern digital SLR cameras sometimes means accessing your camera’s menu system, and even perhaps accessing the custom functions. On Canon cameras like the 20D, 30D, and 5D mirror lock-up is engaged though Custom Function 12. On Nikon cameras like the D200 this is much more conveniently accessed through the shooting mode selector on the left side of the top deck of the camera.
So whenever you are faced with a static subject and a shutter speed of between 1/30 and 1 second, get out your remote release and set your mirror lock-up. Just don’t forget to reset the mirror lock-up function when you’re finished