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Sony A55 bahaviour with and without mirror
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    1. The mirror reduces the amount of light reaching the sensor by about 1/2 stop. This is what we've been told all along, and my tests confirm it. Although it's probably hard to see in the images above, the shot with the mirror is actually slightly underexposed compared to the one without the mirror, even though the shutter speed in the second shot was 1/3 stop slower. If the A55 provided manual shutter speed increments of 1/2 stop, I believe a setting of 1/2800s would have produced exactly the same overall exposure as the first shot.

    2. The mirror reduces image detail. It's logical to expect this... but again, many discussions have been posted about exactly how much image degradation occurs. The 1:1 pixel crops that I've included help to show how much. Another clue is provided through the relative file sizes of the two versions of the scene. A55 RAW and JPG files are compressed when saved from the camera. Both files from the non-mirror version are slightly larger in size than the files from the mirror version by a few percent. This indicates that the non-mirror version must contain more detail and/or more noise. Since noise should be essentially the same in both shots, we can attribute the larger size to more detail.

    I am going to add something important here. Remember that in this recent test I used an aperture of f/2.8 to obtain the worst possible purple fringe effect. But I have shot similar tests at other apertures, and in many of those tests the results without the mirror are nearly indistinguishable in terms of detail rendition.

    So how significant is the reduction of image quality in real terms? I can't be scientifically precise, but I'm going to say that the mirror may produce up to about a 5% loss of detail, but it depends on the aperture, and perhaps on other factors as well. This figure comes from comparing the file sizes in the worst case examples as mentioned above, and also from doing separate experiments not shown here. I downsampled the f/2.8 non-mirror shot to 95% of its full size, then upsampled it again to its original size, then compared that result to the mirror shot and found the visible detail to be roughly the same. So until I learn of a more scientific way to do this evaluation, I will say the mirror can be expected to retain at least 95% of the image detail of a non-mirror shot.

    3. Purple fringing is not influenced in any way by the SLT mirror. This is immediately obvious by looking at the 1:1 pixel crops.

    4. I have nothing to add regarding the 'ghosting' effect of the mirror, which has been shown to cause tiny duplicates of point source lights in night shots. I believe that has been adequately documented.

  • 3 Replies sorted by
  • looks like vitality is in USA Lol. I thougt all the way you where in russia. Even though you make grate hacks that help me on my day to day work. Thanks for that sony shity mirror aproach. Looks much better without it.
  • Why don't they make it so the mirror flips up like any other DSLR.
  • Nikon I think had a patent like that. Would be good if you are shooting at low iso and what to use phase detect auto focus for video, and filp it up in low light.