You might have come across a feature on your camera called Mirror Lockup. It’s sometimes shortened as MLU (or sometimes written as Mirror Lock Up).
But you’ll only see it on DSLRs (digital single-lens reflex cameras). There’s a simple reason: those are the main types of current cameras that have a mirror. 1 So you won’t find it on mirrorless cameras (obviously) or compact point-and-shoot cameras.
Mirror Lockup is used for two things:
- Preventing mirror slap when shooting long exposures, with long telephoto lenses, or macro photography.
- Cleaning the sensor.
Most cameras will separate these two functions into different settings on the camera, with a shooting mirror lockup and a cleaning mirror lockup. They’re both doing the same core task–moving the mirror out of the way–but you don’t want all the other exposure functions active, nor the sensor activated, when cleaning the sensor, so that version is simplified.
1. Preventing Mirror Slap
The rapid movement of the mirror getting out of the way when you press the shutter makes both a sound as well as ever-so-subtle vibrations. Together, it’s known as mirror slap.
If you need the camera to be as quiet as possible, you can lock up the mirror to prevent the mirror slap. It might not make the exposure totally silent (although newer, higher-end cameras can get very close), but it can drastically reduce the shutter noise).
Similarly, if you need the camera to be absolutely still when shooting with a tripod and a remote trigger for a long exposure, long telephoto lens, or macro shot, where even the smallest vibrations can interfere with an image’s sharpness, you can lock up the mirror. That will prevent the small vibrations that come with the mirror’s movement. It’s rarely necessary when using high shutter speeds, but as you drop down to medium or long shutter times, those small vibrations can become a factor in an image’s sharpness.
2. Mirror Lockup for Cleaning the Sensor
Cleaning a DSLR’s sensor from the inevitable specks of dust and dirt that manage to find their in, especially when you change lenses, is a much larger topic that I’m addressing here. There are multiple ways to do it, and different methods have their own champions (and products).
But something all the different methods have in common is that you can’t clean the sensor if you can’t access it. And that’s where the mirror lockup for cleaning comes in. With the lens removed, you then use mirror lockup so you can apply whichever cleaning method you’ve settled on, whether that involves a swap with sensor cleaning solution, a simple air blower, or an ultra-sonic and anti-static brush.
Things to Watch Out For
There are a few things worth noting when using mirror lockup.
- You usually have to press the shutter twice: once to move the mirror and again to take the shot. Moving the mirror doesn’t by itself take the photo.
- The downside of locking up the mirror when exposing a photo is that it means you can’t get a live view through the optical viewfinder. But newer DSLRs often allow you to use the back screen as an electronic viewfinder, which still works with the mirror up.
- If you’re using mirror lockup to prevent mirror slap when shooting, it usually continues to consume battery power with the camera in an active state. So the battery can drain much more quickly.
- When cleaning the sensor, it’s best practice to put the mirror back down and put the lens back on in order to help prevent, as much as possible, new dust settling on the sensor.
What is a DSLR’s Mirror For, Anyway?
The mirror in a DSLR (and film SLRs before that) directs the light coming in through the lens and diverts it through the optical viewfinder. The light comes in the lens, reflects off the mirror at a 45° angle up into the pentaprism (or pentamirror), which flips the image vertically and laterally so that it displays the right way through the viewfinder.
Here’s a visual representation:
It’s a mechanism that makes it possible to see exactly what the camera sees. And that’s crucial for composing and focusing the image. It’s also very important for an interchangeable lens camera, because it means you can change the type of lens, say from telephoto to wide-angle, and still see a live optical preview of how the photo’s composition and focus. 2
Before the innovation of the SLR camera with its mirror, the optical viewfinder used a standalone lens or viewport of some kind. Various styles of cameras, from rangefinders to twin-lens reflex cameras had different ways of dealing with this. But they all gave a close approximation of what the main exposure lens could see but not an exact view.
The complication with using a mirror to redirect the light coming in through the lens to the optical viewfinder is that the mirror sits between the lens and the image sensor (or film). So the mirror has to get out of the way quickly when you press the shutter, otherwise no light would be hitting the sensor to record the photo. When the shutter button is pressed, the mirror flips up, allowing light to pass through the lens and reach the camera’s image sensor, which captures the image. And it’s that rapid movement of the mirror out of the way that creates mirror slap, which has both a sound and an ever-so-slight vibration.
When the mirror moves up for taking a photo, it does so extremely quickly and then immediately flips back down once the shutter closes. Mirror Lock Up simply stops the process half-way, holding it up without releasing it back down.
Things Worth Knowing
- Some film SLR cameras, especially Canons, had a variation on Mirror Lockup that was known as mirror pre-fire. That was a sequence that went like this: press the shutter, the mirror folds up, the camera waits for a second or two for vibrations to die down, and then the film was exposed.
- Well, OK, that’s not technically true. Film SLRs also have a mirror. But I’m focusing here on digital photography for the most part, and DSLRs are far more commonly used these days than film SLRs. But it’s the same principle at work. The main difference is that SLRs have no need to use mirror lockup to clean the sensor, since, well, they’d don’t have a sensor.[↩]
- An optical viewfinder doesn’t reflect exposure in its image preview (the indicators that might be applied as overlays are different). That innovation is something that became possible with live-view electronic viewfinders.[↩]