This is an in-depth review of the Sony SLT-A77 digital SLR camera that was announced together with the Sony SLT-A65 in August of 2011. I had a chance to test both cameras, along with a number of Sony / Zeiss lenses for the Sony mount, while reviewing the Nikon 1 camera system in late 2011. While I concentrate most of my gear reviews around Nikon cameras and Nikkor lenses, I got really excited about these Sony cameras after seeing the press release and decided to try them out.
NOTE: A full Sony A77 Review has been published.
Yesterday Sony released two brand new cropped-sensor DSLRs – the Sony Alpha A77 and Sony Alpha A65. Actually, it is not right to call these cameras DSLRs, because they are not equipped with a traditional DSLR mirror. Instead, the A77 and A65 use a “translucent mirror“, so the correct terminology is “Single-Lens Translucent” (versus Single-Lens Reflex), or “SLT”. On traditional DSLRs, the camera mirror reflects the light coming from the lens into both the viewfinder and the AF sensor, allowing the camera to quickly acquire focus through the phase-detection system. When a picture is taken, the mirror gets raised, thus blocking the viewfinder and preventing the light from reaching the AF sensor. Because of this, the camera can only focus using contrast-detect, which is much slower than phase-detect. Sony’s translucent mirror, on the other hand, allows the light to pass through the mirror and hit the camera sensor, simultaneously reflecting some of the light off the mirror on to the AF sensor. The mirror never moves and stays in the same spot. This allows the camera to acquire focus with the phase detection system even when shooting video. Because the shutter is the only moving component inside the camera, images can be captured at crazy fast speeds. For example, the top-of-the-line Nikon D3s can loudly capture 9 frames per second maximum, while the new Sony Alpha A77 SLT can take 12 frames per second and the only thing you will hear is the sound of shutter opening and closing. There are many advantages to SLTs like smaller size, less camera shake, etc.
The translucent mirror obviously has its own problems. The first problem is that only a small portion of the light gets reflected off the mirror into the camera viewfinder. This results in very dim viewfinder that is extremely hard to see. The workaround is to use an electronic viewfinder instead of an optical one (an electronic viewfinder is basically a small LCD inside the viewfinder). The difference between the two is huge – an optical viewfinder is just a mirror of what the lens sees and it cannot be altered, while an electronic viewfinder can be customized to display whatever the manufacturer wants. For example, if a photograph is going to be underexposed, the viewfinder could show a dimmed image with warnings. Or the histogram could be placed right on top of the image as an overlay. Or you could view your photos right inside the viewfinder after you take them, which is very useful in daylight conditions. You could do many different things with an electronic viewfinder. If it is not implemented properly, on the other hand, an electronic viewfinder could cause a lot of grief. The second problem with the translucent mirror is the fact that it is another piece of glass in front of the camera sensor. Any dust or other foreign particles could end up on this mirror, which would obviously mess up photographs. So now you have to worry about keeping both the camera sensor and the translucent mirror clean. On traditional DSLRs, you don’t have to worry about dust on the mirror. It will never show up in photographs, because once the mirror is raised, nothing stands between the lens and the sensor. The third problem is the amount of light that actually reaches the sensor – the translucent mirror blocks about one third of a stop of light, putting more stress on the sensor. Lastly, continuously looking at an LCD inside the viewfinder could put a lot of strain on eyes and the actual captured images would look better, because the viewfinder has much less dynamic range than the sensor.