It’s the reflex in Single Lens Reflex – the ubiquitous mirror that allows SLR cameras to use the same optical path for composition, focusing, and metering. The mirror action is a marvel of mechanical genius, but does the mirror movement cause vibrations that subtly degrade the image? And does the mirror lock-up feature solve the problem?
The image capture process in an SLR camera unfolds as follows. 1) The shutter button is pressed 2) The mirror flops up 3) The aperture closes down to the desired setting 4) The shutter curtains expose the sensor 5) The aperture returns to the wide open position 6) The mirror flops back down.
This flurry of mechanical activity happens before you can blink your eye. If there are any vibrations induced by this process, it is almost impossible to tell without some creative analysis. One way to quantify the impact of mirror induced camera shake is to use a DIY vibration detector. Such a system is pictured below. To make this device, a laser pointer is hot glued to a long, narrow plastic base. This base is then slid into the hot shoe of the camera. When the laser pointer is then aimed at a far wall, any vibration in the camera is amplified into a dancing laser dot.
Most DSLR cameras have a feature called “mirror lock-up.” As the name implies, this feature presets the mirror in the up position when the shutter is pressed. Once the mirror is “locked’ and free of vibrations, the shutter is pressed again to capture the image. Although this feature should remove any question about the influence of mirror vibration, it is a pain to use. The feature is normally buried deep in the camera’s menu and enabling it will slow down the image capture substantially. Is it worth it? Our goal was to find out.
The video below shows three case studies using our DIY vibration detector. In all three cases the camera was mounted firmly on a tripod and focused on a wall twenty feet away. In the first experiment, a hand was placed on the camera and the shutter button was pressed manually. In the second experiment, the shutter was fired remotely and the mirror was allowed to flop normally. In the third case, the shutter was again fired remotely but with the mirror lock-up feature enabled. In each case, the resultant laser beams were filmed using a separate camera.
The results are fairly telling. In the first case, despite all attempts to press the shutter button smoothly, erratic vibrations sent the laser dancing in all directions. The addition of the remote shutter in case two improved things substantially, but there was a surprising amount of movement induced by the mirror. In the third case, it became clear that allowing mirror vibrations to dampen during mirror lock-up helped the situation substantially.
As for optical evidence, here are two comparison images taken at slow shutter speeds with mirror lockup disabled (left) vs enabled (right).
At fast shutter speeds, however, the difference is negligible:
- Manually pressing the shutter button is by far the most significant cause of vibration during image capture.
- In order for mirror lock-up to be worthwhile, the camera must be on a tripod with remote shutter attached.
- Mirror vibration can be significant in some circumstances. For slow shutter speeds, significant degradations in image quality were noted.
- For fast shutter speeds, mirror vibrations seem to have negligible impact.
- For some newer cameras, live-view mode may be a better alternative to mirror lock-up. By definition, the mirror will already be locked up to enable a live view of the light hitting the sensor.
EDITORS NOTE: Due to the overwhelming response to this article, we have added a follow-up post with more information on the original setup and some new enhancements. Please see the follow-up post here.
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