Today’s Question: Under what circumstances do you use the camera’s mirror lockup feature?
Tim’s Quick Answer: The general rule of thumb is that you should employ the mirror lockup feature (if your camera is so equipped) when you are working with a shutter speed of around 1/15th of a second.
More Detail: One of the features that makes a digital SLR an SLR (single lens reflex) camera is the mirror that enables you to use the optical viewfinder to observe the view through the lens as you are composing your scene. Needless to say, mirrorless cameras do not include the mirror, which is where the name fo this type of camera comes from.
When capturing a photo with an SLR, the mirror moves out of the way, the shutter opens to create the exposure, the shutter closes again, and the mirror moves back down into place.
The movement of that mirror can impart a degree of vibration to the camera, which in turn can cause a degree of motion blur in the photo. This won’t be the major type of motion blur you might see if you captured a long exposure while moving the camera around, but rather will generally just make the image look a bit out of focus. Needless to say, that is something you want to avoid.
With a fast shutter speed, the short duration of the exposure means the vibration caused by the movement of the mirror will not be significant enough to create any visible blur. And for relatively long exposures, the vibration caused by the mirror movement will be a small portion of the overall exposure, and therefore will not create a visible blur.
But for an exposure duration of about 1/15th of a second, that vibration can have an impact on the overall sharpness of the photo. So, whenever you are close to a 1/15th of a second exposure, I recommend employing mirror lockup if your camera includes that feature.
And of course with a topic like this, photographers who have already transitioned to a mirrorless camera can revel in the knowledge that vibration caused by a moving mirror is not something they need to worry about anymore.