Hi folks. In this Quick Tip, I want to touch on the subject of Mirror Lockup. What it is, when it matters, and when it may not make a difference, even though you've heard that it will.
What Is Mirror Lockup?
If your camera uses a mirror box to take the light from the lens and reflect it into an optical viewfinder, there is a mirror involved. In order to make an exposure, the mirror has to get out of the way. This is a mechanical or electro-mechanical process that involves moving the mirror. Of course, if you have a mirrorless camera, this does not apply to you and neither does mirror lockup because there is nothing to lock up at all. Lockup means to choose to move the mirror out of the way, long before the shutter goes off with the goal of removing any micro shake caused by the movement of the mirror. Such shake occurs, if at all, when the mirror starts moving or stops moving, which involves a change in inertia and a transfer of energy.
By locking up the mirror, we reduce the probability of such micro shake impacting our images because the shutter is opened and closed after the mirror has moved out of the way and the mirror only resets after the image has been made.
Why Can This Be Useful
Some cameras have very energetic mirror systems. A larger mirror has greater inertia and so there is greater probability of the movement or stopping of the mirror transferring that energy into micro shake. I have a couple of old medium format 6x7 cameras where you can hear the mirror slap across a busy street, however in most cases the delay between mirror movement and shutter actuation is sufficient to avoid any issues.
I also have a Canon 5Ds which has tons of mirror slap and is known to create micro shake as a result of mirror slap in certain conditions.
If we could eliminate that slap from causing micro shake that would be a good thing,
The Most Common Use
When we go for longer exposures, say longer than 1/15 of a second as a guide, we may be more susceptible to micro shake than at higher shutter speeds because the exposure is made so quickly. As shutter speeds increase in duration, the possibility of vibration caused by mirror slap goes up. Thus it is often recommended that mirror lockup be used when making longer exposures. That's quite reasonable even if not all cameras allow for mirror lockup.
However, the vibration from mirror slap as the mirror rises out the way is very well damped in all modern cameras, and this damping causes the vibration whatever there is of it to disappear in short order. As the shutter speed duration continues to increase the percentage of the overall exposure that is coincident with mirror slap falls off very fast indeed and so the portion of the exposure made during the vibratory period is very small and makes very little contribution to the overall exposure. Because of what we know of mirror rise time and vibration dampening, mirror lockup benefits are gone once the shutter open duration hits one second.
Real Long Exposures and Mirror Lockup
It's a very common treatise to use mirror lockup for really long exposures such as night sky photography, or light painting or when using a very dense neutral density filter to create creamy movement of clouds and water.
The fact is, mirror lockup makes no difference in these situations. There is no harm in using it, but no gain either. So if you planned on doing it, and realize at the end of your 15 minute star trail exposure that you forgot, don't fret because it will not have made any difference at all.
In fact, what is often attributed as mirror slap causing micro shake is typically something else. A poor quality tripod with insufficient capacity in the legs and or the head will do much more micro movement than mirror slap every will. Even with a good tripod, walking around on the ground near the tripod is going to create vibrations that even with the best carbon fibre legs may transit into the camera and be seen as micro shake. I have seen it happen during a night sky shoot that a freight train a few kilometres away generated enough ground vibration to show shake in the shot.
These possibilities are often the root cause of blur incorrectly blamed on mirror slap.
If your camera has mirror lockup and you want to use it, go for it. However if it does not, or it seems like a pain in the butt to have to activate the mode, think about the overall shutter open duration and whether mirror slap is going to make a difference or not, Not touching the camera and using a remote release for longer exposures typically creates a lot more value than Mirror Lockup. If you know that your camera is susceptible to mirror slap choose shutter speeds accordingly, use a short duration high intensity light source, such as flash, fire the camera remotely, and when making exposures between ⅛ of a second and 1 second inclusive, use mirror lockup.
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