I have an Sony SLT A57 with a Translucent Mirror. My question is if the mirror is damaged, Will it leave an artifact in the photo? The damage to the mirror looks like a pea sized or bigger bubble.
Almost definitely, yes it will leave an artifact in the photo.
As you get closer to image sensor, defects and damage in the optical path become more visible and prominent. People worry about minor scratches on the front element of their lenses a lot, but defects on the rear elements of lenses typically cause more visible artifacts.
Small dust particles or blemishes on the sensor itself cause visible dark spots several pixels wide, in the same spot on every photo.
Because the pellicle mirror sits in between the rear element of the lens and the sensor, by logical extension you should see effects that are more pronounced than if the defect/damage were on the rear lens element, and less pronounced than if the defect/damage were on the sensor itself.
As Michael Clark helpfully noted, the actual location of the defect on the mirror will also determine how pronounced the produced artifact is. Damage near the bottom of the mirror is physically further away from the sensor, so the artifacts will be similar to those from rear lens element damage. Damage near the top of the mirror, being physically closer to the sensor, will take on characteristics more akin to sensor surface damage.
Because the image produced from the lens is inverted in both the vertical and lateral planes, damage to the bottom of the mirror will appear in the tops of images, and vice versa. And because of the relative distances between top and bottom parts of the mirror to the sensor as described above, you should expect to see:
- larger but less distinct artifacts at the top of your images if the damage is at the bottom of the sensor;
- smaller but more distinct artifacts at the bottom of your images if the damage is at the top of the sensor.
Note that I'm talking in rough generalities here. How acceptable the artifact is is highly dependent upon the size, location, and nature of the damage/defect.