a77 and noise in skin tones

[MarcoC]

I am generally enjoying my a77 but am finding it a bit frustrating that even when shooting at ISO 100 and being careful to avoid under-exposure I am finding some luminance and chroma noise in white caucasion skin tones when a model is not looming large in the image - e.g. in what you might call "environmental portraits". I know that 24MP is crammed onto an APS-C sensor, but this has surprised me a bit that I am getting this at ISo 100. Anyone else ?

[David Kilpatrick]

Well, you have had me checked over files from the A77, A99, A55, A580 and finding comparable faces at ISO 100. The result is quite disappointing, I disabled NR and examined details at 100/200/400%. The A77 is the worst by a fair margin and the noise doesn't look good. The A99 can be used at ISO 400 with less noise.

Then I thought, let's check the RX100 and see what its ISO 125 images are like on skin tones. Just don't bother with DSLRs. I have no idea how Sony managed to get what they do from 20 megapixels on a 2.7X factor sensor, but the RX100 results are in every way so much better than the A77. I used ACR to output RX100 at 25 megapixels - 72MB final file - and you honestly could not tell this was not taken on the A99 with a CZ lens.

[MarcoC]

Thanks David - it's what I suspected and a bit of a shock as a former a700 user - I didn't have a problem with a700 skin tones at ISo 200 in a well exposed image. Is it just the MP on the a77 sensor or something about the way the RGB channels are set up and the color response do you think?

As usual, when I posted this on DPR and Dyxum (more so on DPR) it set off a whole flame fest of a77 versus a77 bashers and a regurgitation of the same old, same old, debates about whether you must upscale, downscale etc before comparing 24MP images to 16MP images...

Thanks again

M

[David Kilpatrick]

I'm not sure why the A77 low ISO files look so noisy on skin tones, because within the same images, there are other neutral tones which seem less noisy. I find it difficult to locate any A77 file which has the same sharpness at pixel level as A99, A580, RX100 and even A900 at ISO 100. It's almost as if the noise is exaggerated by a slight softness to the micro-structure of the image; noise always appear less when perfectly sharp detail is rendered.

ailsa-henna.jpg (162.13 KiB) Viewed 3982 times

This is a very quick autofocused (of course) snap at ISO 125 on the RX100, 1/800th at f/5.6 and widest angle.

skindetail-rx100.jpg (175.64 KiB) Viewed 3982 times

This is a 100% clip of the raw conversion in ACR with all sharpening all noise reduction set to zero (normally, I would sharpen 25/0.5/20/0/0 and I have colour NR set to 25/50 to help remove moiré effects and single pixel colour fringes, on all camera file types). I know this is crunched up badly by Save for Web and then by ImageMagic compression on the forum system, but you should still be able to see how the RX100 captures every nuance of skin colour variation and texture and also records single pixel dots of the glitter. I do not have any A77 shot directly comparable to this, but A77 results are simply nothing like this. A900 or A99 at lowest ISO can look similar, the NEX-5n can look similar, and the A55 which has an SLT mirror is also better.

See my next post for a bit more.

David

[MarcoC]

Thanks for that. Your comment about a sharp shot always looking less noisy made me think because I had previously been sceptical about claims some DPR posters made about higher quality lenses giving results that appeared less noisy - I had argued that there is no law of physics that connects lens resolution to sensor noise, but what you said in your reply makes me think I may have been hasty in dismissing the idea that better glass means less apparent noise. I guess the key word here is "apparent".

The RX100 IQ looks stunning by the way. I had been holding off waiting to see if the price drops but it hasn't much in the UK, and I'm pondering a grey import but hesitant as there have been a few reports of owners returning cameras due to lint/dust stuck inside the lens (an issue that plagued three Canon G9s I owned).

[David Kilpatrick]

Moving on to the A55 - here is a quick candid I snapped of two butterfly farm visitors. I was using the 30mm SAM macro, one of the sharpest lenses for all round use you will ever buy for about £130.

camerausers-A55.jpg (271.93 KiB) Viewed 3979 times

And here is a 100% crop:

camerausers-100pc.jpg (157.59 KiB) Viewed 3979 times

OK, but you need to know the rest of my information. First of all, both the 100% crops I have shown here are not from the native file size. In both cases they are enlarged using ACR to 25 megapixels - 72MB final file size.

Secondly, the A55 shot is not at ISO 100. It is at ISO 1000, with -0.5 EV correction because it was a bit light overall; with 20/0.5/25/0/0 sharpening and normal 25/50 luminance NR plus 25/50 colour NR.

The A77 can, at minimum ISO and with excellent glass, produce very sharp clean images but it simply can't match the 16 megapixel APS-C sensor or the RX100 sensor for flexibility. It is difficult camera to rely on across a wide range of conditions day and night.