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Often when long exposures are discussed, there is a recommendation to use mirror lockup to avoid loss of image sharpness.

I fail to understand why the brief vibration, maybe half a second, could possibly affect a 30 second exposure? At the end of the exposure the mirror probably start to move after exposure end so this is not a problem.

Camera vibration caused by mirror slap should affect short exposures more than long exposures as I understand it.

What is the error in my reasoning?

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  • \$\begingroup\$ welcome to pse, please use the search box first before you ask, your question would probably end up closed as duplicate. see here for answers photo.stackexchange.com/questions/1593/… \$\endgroup\$
    – K''
    Commented May 14, 2015 at 17:59
  • \$\begingroup\$ Yes, that answer also suggests that mirror lockup is more important to long than short exposures. That is exactly what I don't understand. \$\endgroup\$
    – Wirewrap
    Commented May 14, 2015 at 18:03

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The reason people recommend mirror lockup for exposures lasting several seconds is usually because they don't know any better.

Mirror lockup is most effective when the shutter speed is in the range of about 1/100 second down to around one second. Any shorter and the second curtain is closed before the vibration from the mirror reaches the parts that count: the lens and the sensor. Any longer and the duration of the vibration significant to create blur more than one pixel wide is such a small percentage of the total exposure time as to be trivial. If a person can spend three or four seconds to walk across the field of view of a 30 second exposure and not show up in the resulting image then the result of mirror vibration on a 30 second image is likely to not even be detectable.

There is one exception where mirror lockup can be helpful with exposures longer than about one second. If you are shooting in a very dark environment and there are very bright light sources included in the frame, that first second of vibration can cause noticeable light trails, even when the total exposure is 30 seconds or longer.

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  • \$\begingroup\$ true what you said about long exposure however if you are shooting building in the dark and there's a blur in the lights even if you exposure is long the blur in the lights will register on the sensor or the film \$\endgroup\$
    – K''
    Commented May 15, 2015 at 14:09
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Mirror lockup is advised in long exposure to avoid the vibration at the beginning of the exposure. This is what happens when you press the shutter:

1- Mirror goes up 2- Shutter opens 3- Sensor/Film is exposed 4- Shutter closes 5- Mirror goes down

As you can see from the sequence 4 & 5 the mirror lockup won't have any effect after the shutter closes and ends the exposure

But from sequence 1, 2, and 3 if the mirror goes up action cause vibration in the camera body and the shutter is getting open, the vibration will translate and captured in the exposure.

Now back to main question is why it's crucial in long exposure? Because if your shutter speed is 1/1000 of a second, it's faster than the vibration, the shutter will open and close before any vibration or camera shake could change the exposure. But if your exposure is 10 seconds, and the vibration cause by mirror lock up lasts for a second, then this shake will go to the first second of the exposure.

Update:

if short shutter speed is not faster than the vibration wave, it may be only one cycle of it which is okay and will look like there's no vibration happened.

A Wikipedia page that may help

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    \$\begingroup\$ if short shutter speed is not faster than the vibration wave, it may be only one cycle of it which is okay and will look like there's no vibration happened. ------------------ This is confusing to me. Suppose there is one cyle of camera shake (at the mechanical resonance of the camera) during the exposure, can it be any worse? The camera is moving during the whole exposure? If you have a 30 second exposure, the camera will be steady maybe 29 out of 30 seconds. \$\endgroup\$
    – Wirewrap
    Commented May 14, 2015 at 19:32
  • \$\begingroup\$ the sequence for long exposure with mirror lock up to be effective is to lock the mirror up and wait couple of seconds till all the vibration is gone and then open the shutter. That's why mirror lock up involves 2 button press \$\endgroup\$
    – K''
    Commented May 14, 2015 at 21:05
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    \$\begingroup\$ The wiki mentions that the vibration will be most affected with exposure time between 1/2 to 1/60 because the vibration will dominate the exposure. Like the OP, I still don't understand the mirror lock for long exposure because that one second of vibration will produce almost nothing for the camera to capture. \$\endgroup\$
    – Nelson
    Commented May 14, 2015 at 21:34
  • \$\begingroup\$ Keep in mind that with focal plane shutters, exposure times ("speeds") shorter than X-sync are accomplished by narrowing the size of the slit as the second curtain chases the first curtain across the sensor/film plane. For a particular camera, it still takes 1/250 second for the shutter curtains to transit the sensor, it's just that the second curtain begins closing 1/8000 after the first curtain begins opening and has only made it 1/32 of the way across the sensor when the second curtain begins chasing it. \$\endgroup\$
    – Michael C
    Commented Mar 24, 2021 at 19:24

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