I’ve known that my SLR cameras have had the capability of locking the mirror in the open position. This feature is intended to allow any vibration caused by shutter movement to die away before the shutter is opened. However, I always believed that the effect would be subtle and have no practical impact on the type of photographs I take.
I was working on a project that required a full-frame photograph of a pine cone. I set up my Canon 5D Mark II with Tamron 70-300mm lens on a Bogen 3021 tripod with a Really Right Stuff BH-40LR ballhead & quick release. The pine cone was placed in bright sunlight about 6 feet from the camera with some reflectors to control contrast. Using autofocus to focus on the center of the subject, I used a remote shutter release to take several exposures, each with different positions of the reflectors.
When I viewed the images on my computer screen, I was quite disappointed. They simply were not sharp. I was using a fairly robust tripod and remote shutter release and had been careful with focus, so I was baffled. The depth of field was just a little over 1 inch under the exposure conditions, and thinking this might be the problem, I closed down the aperture and tried again.
The results were worse.
Could diffraction now be the problem? On the computer I zoomed in on a central region of the pine cone, as shown in the top image of figure 1 below.
What was readily apparent was that side-to-side motion was blurring the image. This is most easily recognised by looking at the feature to which the arrow points.
I would have thought that 1/80 sec would be adequate to capture a stationary object on a calm day with a tripod-mounted camera, even with a 300mm focal length lens.
Then I remembered some work a friend of mine, David Horn, had done recently. He set out to compare the resolution of the Tamron 70-300 mm lens to the equivalent Canon lens at a variety of apertures. The results did not make any sense. Until he performed the experiment with the mirror in his Canon 5D locked up. David graciously shared his work with me and some selected images are shown in figure 2.
Comparing the top two images in figure 2, you can see a dramatic improvement in image sharpness when the mirror is locked up. The exposure conditions, aperture and shutter speeds are otherwise comparable in these two images. As shown by the third image from the top, even increasing the shutter speed to 1/400 sec is not adequate to completely eliminate the effect of camera shake due to mirror motion. In fact, better results are obtained at slower shutter speeds, as shown in the bottom photo. Apparently at 1/4 sec most of the exposure takes place after the mirror-induced vibrations have damped out.
If you think that this effect only is observable at high magnification, as I once did, compare in figure 3 approximately full frame images of the pine cone as originally shot and as re-shot under conditions to minimize motion blur.
The small, jpeg images that can be displayed in this blog do not have the same impact as a print or a full-sized tiff image on the computer screen, but to my eye, the image on the right is significantly sharper.
What shocked me is that motion blur caused by mirror motion is not something that appears only under exotic, seldom experienced conditions Instead it has its greatest impact under what I would call “normal” exposures. It is less of a problem for exotic (i.e. very short or very long exposure) conditions! As a result, I have begun using mirror lock up as standard practice whenever my camera is mounted on a tripod. Which also suggests an advantage to LiveView.
But that is a topic for another day.