Invisible wrote:

> 
> All I know is that when I take a dark image and try to make it brighter, 
> it comes out hopelessly noisy.
> 

As you push those low RGB values up, values in-between maintain their 
relationship by scaling. You're also scaling the baseline noise of your 
camera. S/N ratio is still the same, the only difference is that there 
was very little signal to begin with. You might be able to stretch this 
a small amount, using a noise reduction algorithm, but this will eat 
details, which you have very little of, so the image will be soft and 
begin to show artifacts. You could also take advantage of human 
perception of color and smooth the color channels, which will reduce the 
apparent color noise, you'll still have luminance noise, but this is 
generally regarded as more pleasant than chroma noise.

Also, with 8 bit images, the risk of posterization becomes greater the 
more drastic the changes. You only have 32 levels to work with in an 
image that is underexposed by 3 stops. (which would be rather dark) as 
opposed to 256 levels had the image been properly exposed in the first 
place. Now, lets say you have an image that had been recorded in 12 
bit-- which is common in most digital SLR's, though 14bit is becoming 
more common-- , now that same image has 512 levels, therefore on current 
displays and print technologies, there really is no posterization. This 
gives a much greater lattitude when performing color adjustments.

Post a reply to this message