Asher Kelman said:
In the noise measurements shown, could you explain how these are arrived at? You may well have mentioned that in a previous post. Is there a PC/Mac utility to measure this?
That was my first post here, but you probably recognize my name from the old RG forums. The PC program IRIS
http://www.astrosurf.com/buil can load the RAW blackframe, and then issuing the "stat" command in the console window gives the noise ("sigma").
The points you make are very important. I have always thought I was doing myself good to shoot at 160 rather than 200 ISO!
If the scene is high-key, then you probably are, as shot noise will be the main noise, and there will be less of it metering for 160. If you are concerned with DR, though, 160 is not the place to do it.
What I am saying is that there isn't any monolithic "noise" in an image; there are two basic groups of independent noise; noise that is related to the signal (such as shot noise) and noise that is a blanket over the entire image (readout, or blackframe noise). The former is directly related to real, analog exposure. The latter is dependent upon the ISO setting, also.
I'd like to see some examples of the type of image where either the noise would have an effect on the printed image or where the combing of every 15.5th value in the histogram might degrade the final image too.. (BTW, What do you use to demonstrate this?).
Seeing combing at that level will be very difficult. I'm not saying it's a big problem. In the camera I received the files form, RAW values 107, 123, 138, and 154 were grossly under-represented (the few that were there may be the result of interpolation for mapped-out pixels). Black is 128 in recent Canon cameras. It would take a very slow gradient in the deep shadows at a very low ISO to see the effect. The combing itself is not a big problem, as there is too much noise in all cameras at all ISOs for minor posterization to jump out at you. The posterization is a symptom, though, of some unnecessary math going on; math that should not be done at all IMO; I believe that digitized number should be stated literally in the RAW data, and that all scaling should be done by the converters, by stating figures in RAW metadata, or by external knowledge. That's how the 10D differed it's 3200 from 1600; the RAW levels were proportional to absolute exposure; IOW, turning the ISO from 1600 to 3200 on the 10D in manual mode did not affect the RAW levels; the converters knew to scale the 3200 data by +1 stop more than 1600 data in the conversion, and it worked. Why scale anything, and clip away highlights, or turn two RAW values irreversibly into one?
As far as the increased noise is concerned at ISOs like 160, try shooting the same scene with deep shadows at 160 and 200 (with the same EC), and bring the shadows up and see what they look like. I'm sure you'll see bolder noise and banding in the 160.
On the other hand they are certainly not raw data. What is more when shooting RAW they are not really needed. So I personally do not use them.
