Understanding Mirror Lockup on DSLRs: What It Is & When to Use It

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 or Mirror Lock-up). What it does is simple: it holds the reflex mirror up out of the light path instead of letting it flip up and snap back down as part of taking the shot.

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:

1. Preventing mirror slap when shooting long exposures, with long telephoto lenses, or macro photography.
2. 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.

Mirror lockup is also used to maximize image sharpness. 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.

On Nikon DSLRs there’s a related setting worth knowing about: exposure delay mode. It raises the mirror, waits, and then fires the shutter on its own. On the D850 you can choose a delay of 0.2, 0.5, 1, 2, or 3 seconds. You get the same vibration-free frame as mirror lockup without the two-press routine, which helps if you don’t have a remote release with you.

Benefits of Mirror Lockup & when to use it

1. Improved Image Sharpness: MLU minimizes vibrations caused by mirror movement, resulting in sharper images.
2. Enhanced Long Exposure Photography: MLU is particularly beneficial for long exposure shots, where even slight camera shake can cause undesirable motion blurring (as opposed to desirable motion blurring of the subject).
3. Better Macro Photography: When capturing close-up images, MLU can help achieve better focus and image sharpness.
4. Astrophotography: For capturing the night sky or celestial objects, MLU can keep mirror vibration out of the frame at the start of a long exposure. It won’t do anything about star trails, though — those come from the earth’s rotation during the exposure, not from camera shake. This is mostly for when starting and stopping a long exposure; the mirror is already up during the exposure.

When to use Mirror Lockup

1. Long Exposure Photography: When using slow shutter speeds, MLU can help prevent motion blur.
2. Macro Photography: Due to the close focusing distance, even small vibrations can affect image sharpness in macro photography.
3. Telephoto Lenses: The longer the focal length, the more susceptible the lens is to camera shake. MLU can help reduce this issue.
4. Tripod Use: A tripod takes your hands out of the equation, but it doesn’t do anything about the vibration the mirror makes inside the camera. MLU is what deals with that part.

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.

There’s a wrinkle there worth spelling out. Once you switch to live view, the mirror is already raised and stays raised, so mirror slap isn’t part of the exposure to begin with. Shooting off the back screen sidesteps the problem rather than adding a step to it.

How long you get between those two presses depends on the brand, and the two big ones behave in opposite ways. Nikon’s mirror-up release mode takes the picture for you automatically if you don’t press again within about 30 seconds. Canon’s mirror lockup instead drops the mirror back down after about 30 seconds, and you press the shutter fully again to raise it. So on a Nikon, dithering costs you a frame you didn’t mean to take; on a Canon, it costs you the lockup.

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 way in, especially when you change lenses, is a much larger topic than 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 swab with sensor cleaning solution, a simple air blower, or an ultra-sonic and anti-static brush.

If the menu option isn’t there, it’s almost always a power issue rather than a fault. Nikon grays out Lock mirror up for cleaning unless the battery is well charged, because it needs a reliable power source to keep the mirror up and the shutter open while you’re working in there. On the D850 it’s also unavailable while the camera is connected to a smart device over Bluetooth or to a computer over USB. Charge the battery (or plug in an AC adapter), disconnect anything attached, and the option comes back.

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.

Mirror Lockup on film SLRs

• 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.

Frequently asked questions about Mirror Lock-up

Notes & References:

1. 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. [↩︎]
2. 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. [↩︎]