At 0.1% grey ( logarithmic scale ) the SNR is ca 16dB for the D7000. The 50D´s SNR is just above 6dB. That is very low. What do you think? Would you consider 6dB deep shadows?
John,
I think this point needs further clarification. What is deep shadow is what the eye
perceives as deep shadow, bearing in mind that the aperture of the eye, the pupil, is constantly changing according to the changing levels of reflected light within the scene, as the eye peruses the scene.
If the eye had a fixed aperture like a camera does at the moment the shot is taken, we'd be in deep trouble. We'd be blinded by even moderately bright light, and even moderately dark shadows would appear impenetrably black.
So the answer is, a signal level which is only 6dB above the noise floor of the camera
can be representative of a deep shadow in a scene, especially if the scene is of low contrast and low dynamic range. On the other hand, if the scene is of high dynamic range, a shadow which the eye perceives as normal shade where the light is more than sufficient to read a book easily, may appear in the camera's sensor, and consequently in the processed image, as a deep, noisy shadow with a 6dB SNR or less, which has to be rendered black.
Now, I know it's the case that the sun was last seen in Great Britain in the afternoon of the 23rd of August 1955 (well, let's not exaggerate. I think it might have been seen on a few occasions since then), and I was informed recently by a tourist guide that St Petersburg gets on average about 65 sunny days per year, so I considered myself lucky that the sun was shining for 2 out of the 4 days I was there.
So, for the benefit all the sunshine-deprived people who might never have seen a 'high dynamic range' scene, I searched through some shots I'd taken with my 5D of the temples at Siem Riep in Cambodia, back in 2006, looking for an example with shade which was not at all deep, in which the light was perfectly adequate for reading a book, but which the camera simply cannot render as the eye saw it, because of the camera's limited dynamic range.
Here's that shot which is totally useless because of noisy shadows. The sky and clouds are okay though.
First the ACR Window demonstrating that the shot is a reasonable ETTR, followed by a crop of the sky showing good detail in the clouds.