High ISO Raw Noise Compared...
We've already seen that the A77 delivers rather high noise levels, both measured and visible when files are viewed at 100%. This test is designed to establish whether the A77 is any noisier in normal use than its nominal predecessor the SLT-A55. To make the comparison as useful as possible we've opted for low-intensity tungsten lighting (the sort of lighting you might be faced with when you reach for high ISO settings) and selected an ISO sensitivity of 12,800.
The Sony SLT-A55 uses Sony's last-generation 16MP CMOS sensor (a varient of the one used in the NEX-5N, Nikon D7000 and Pentax K-5). What we want to know here is whether - if you're used to the output from the A55 - the A77 will give you substantially different image quality when you use the files in the same way. We chose the A55 as a comparison partly because it is likely to be a camera from which a lot of people will be considering upgrading to the A77, but also because like the A77 it is an SLT design, so we are comparing 'like with like'.
To see how much difference the slight neutral density filter effect of the SLT mirror makes to the A77's noise levels, we've also shot the same scene on the NEX-7, which features the same 24MP CMOS sensor as the SLT-A77 but minus the mirror. You can see images from the NEX-7 at the bottom of this page and we will investigate the NEX-7's image quality more thoroughly in our forthcoming full review.
Compared to SLT-A55
The first set of images on this page (below) show how the A77's raw files look at 100% next to those captured by the A55 at the same ISO setting of 12800. We've run the files through two processes - Adobe Camera RAW 6.6 (Beta) with all noise reduction turned off (the same treatment that we use in our noise comparisons page), and again with noise reduction at Adobe's defaults.
It is evident from these images that the A77's pixel-level noise is slightly higher, on visual inspection, than the A55, but not by much. Both chroma and luminance noise are more intense, but after default levels of noise reduction are applied in ACR, the difference is not of much practical significance. So what happens if we downsample the A77's 24MP files to 16MP, to match the output resolution of the A55?
Compared to SLT-A55 after downsampling to 16MP
The A55 images below are the same as those shown in the table above, but the A77 files have been downsampled in Photoshop (using the 'bicubic smoother' resizing algorithm) to an output resolution of 4912 x 3275 (16MP). The purpose of doing this is to establish whether the A77 is a noisier camera than the previous-generation A55 when its greater pixel count is taken out of the equation.
|A77 @ 16MP (ACR - NR 0)||A55 (ACR - NR 0)|
|A77 @ 16MP (ACR - NR default)||A55 (ACR - NR default)|
Even when its files are downsampled to 16MP, images the A77's 24MP sensor still display slightly higher noise levels at ISO 12800 than the 16MP sensor of the A55, as you can see from the images above with noise reduction turned off. Chroma noise is slightly more intense, with larger chroma 'blobs', although this is muted by ACR's default noise reduction settings. After ACR's default NR, the differences between the A77's resized output and that from the A55 are minimal, to the extent that we would consider them effectively on a level (remembering that much better results than the ones shown here can be coaxed out of files from both cameras with more considered post-processing work).
What this means is that if you're an A55 user habitually making prints of a given size, the A77 will give you near-identical image quality in low light, high ISO situations. The extra pixels on its sensor won't appreciably increase the quality of the output you're used to, but they won't make things noticeably worse, either.
Compared to NEX-7
The NEX-7 has the same sensor as the SLT-A77, but it doesn't have a mirror in the light path. At the same exposure settings therefore, the A77's sensor receives slightly less light than the NEX-7's (thought to be around 0.5EV less). This quick test is intended to show how much difference the design of the NEX-7 makes in a typical high ISO, marginal light situation.
|A77 (ACR - NR 0)||NEX-7 (ACR - NR 0)|
|A77 (ACR - NR default)||NEX-7 (ACR - NR default)|
As you can see from the 'NR 0' images, above, the A77's ISO 12,800 output is slightly noisier than that of the NEX-7, especially in terms of chroma noise. The A77's luminance noise is more intense too, with a noticeable extra 'grittiness'. We will explore the NEX-7's image quality in more detail in our forthcoming in-depth review.
Raw files for download
Here, we've provided the original raw files of this scene from the SLT-77, A55 and NEX-7 for you to download and run through your own preferred processing.