Why use mirror lock-up for long exposures, and when does it actually help?
Asked 5/14/2015
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I often see mirror lock-up recommended to improve sharpness, including for long exposures. I don’t understand why a brief vibration from mirror slap would matter much during a 30-second exposure. It seems like that vibration should be a bigger problem at shorter shutter speeds, while on a very long exposure it would make up only a tiny fraction of the total time. Since the mirror comes back down after the shutter closes, that part shouldn’t affect the image. What’s the flaw in my reasoning, and at what shutter speeds is mirror lock-up most useful?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
11y ago
0
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Your reasoning is mostly correct. Mirror lock-up helps by removing vibration caused when the mirror flips up at the start of the exposure, not when it comes back down after the shutter closes.
Where it helps most is typically in the middle range of shutter speeds—roughly around 1/100 s to 1 s. In that range, the vibration lasts long enough to blur detail while the shutter is still open.
At very fast speeds, the exposure may finish before the vibration has much effect. At very long speeds, such as many seconds or 30 seconds, the brief vibration is usually such a tiny portion of the total exposure that its contribution is negligible.
So mirror lock-up is not usually “crucial” just because an exposure is long. It is most useful when the vibration duration is a meaningful fraction of the exposure time and the camera is otherwise steady enough for that vibration to matter.
In short: mirror lock-up can improve sharpness, but it matters most at certain moderate shutter speeds, not automatically for every long exposure.
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