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Yesterday morning I took a couple of pictures of our neighborhood here in Linz to compare the sensor noise of the Sony Alpha 7 at different ISO levels. Here is the picture at ISO 100 - It also shows the three areas where I made 100% crops for the comparison:
I used the "Minolta MC Rokkor 58mm/1.2" manual focus lens, attached to my Sony Alpha 7 with the Novoflex MC/MD NEX Adapter.
I shot all pictures as RAWs and I set sharpening and noise reduction to 0 in Lightroom, so the result should be pretty close to what the sensor recorded. I used the same aperture for all shots (f/5.6) and let the camera choose the shutter speed for a given ISO value.
Click on the pictures to navigate to flickr, where you can get the sample photos in their original size.
From ISO 100 to ISO 400, noise is barely noticable:
At ISO 800 and 1600, noise begins to show in the wall of the house and the sky (please note again, these images were converted to JPEG with 0 noise reduction). The twigs of the tree, the car and other details still look pretty good. At this level, I think you have a good chance to remove the noise without loosing too much detail. The in-camera noise reduction should still work pretty well too.
At ISO 3200 and 6400, there is noise in every part of the picture. ISO 6400 is the highest level the camera will choose in Auto-ISO mode. It is probably still OK for small prints or photo books.
At higher ISO values, the noise is everywhere. It’s even clearly visible at 800 pixels width. You will lose a considerable amount of image detail when you try to remove the noise from these images.
I really like the sensor noise behavior of the Alpha 7. Even at 3200 and 6400, noise is OK and can be removed to a certain degree. With a fast prime lens (like the Rokkor 58/1.2), the Alpha 7 enables me to shoot handheld virtually in the dark and still get good pictures: