Noise reduction redux

[Jamon Camisso]

On 30/05/13 11:24 PM, Walter Dnes wrote:
>   Earlier this month I asked about de-noising software.  This goes off
> on a tangent, but is related, so here goes...
> 
> * On bright sunny days, the JPEG image seems washed out, i.e.
>   overexposed, and I have to back off approximately a full F-Stop when
>   importing manually with the UFRAW plugin in GIMP, to get a decent
>   image.

Using UFRaw on a jpeg is a little like taking an MP3, editing it and
saving it at a different bitrate than the original as a lossless FLAC.

You have the option with the S100 of shooting in raw - if you're after
quality you absolutely must use that. Try shooting in RAW+jpg mode if
that's available, then you have a really good way to see how bad in
camera jpeg processing is.

Also, have you checked the white balance settings in UFRaw? It uses
DCRaw in the background and I think there's a profile for your S100 in
that tool, so you might want to start with auto white balanace as a test
to see if that washed out look goes away. But, I'll get to this, doing
this on a jpeg is a lost cause, since the data is already permanently
encoded into the image. Essentially this is just playing with the
composite of all RGB levels, not controlling each individually like with
a raw.

General white balance tip: when in doubt, make it warmer (closer to
zero), even with jpgs.

> * A few days ago I went and took some test shots on a sunny day at a
>   local park, with various exposure compensations.  At -1 F-Stop
>   compensation, the JPEG images were finally OK, with the sky actually
>   being blue rather that off-white.  Plus the rest of the image was
>   properly exposed.

You really can't get the best performance from your camera without
shooting in raw. JPEG settings in cameras are all programmed at the
factory to give too much sharpness and contrast in general (so that it
looks punchy on an LCD screen on the camera). Each manufacturer will
tweak their exposure settings too, some preferring to under expose (left
histogram) for fear of the dreaded blown highlight and some preferring
closer to overexposure (right histogram) to capture more data.

>   There are 3 ways of cutting down the amount of light sensed...
> 1) Faster shutter speed; let's avoid that option for now
> 2) Tighter aperture; but the Canon S100 only goes to F8
> 3) Lower ISO; which automatically reduces noise... tada

Cutting down the amount of light is not exactly what you want to do. I
can guarantee that if you do these two things your photos will turn out
more consistently with no blown highlights (even with jpegs) and you'll
have much cleaner looking images in darker areas:

1) Shoot in raw like I mentioned
2) Turn on the live histogram feature of your camera and learn to see
what it sees. It is a graph of light values, brightest on the right,
darkest on the left. About 18-20% gray is in the middle.

You want to have a graph that is pushed to the right, without any peaks
disappearing off the edge. Don't crowd your exposure all the way right,
but get it over there until you're comfortable seeing an image and a
scene like this. Use aperture, or shutter speed to change this. ISO can
be used but you will affect the noise profile of the final image.

The reason for shooting to the right as it is termed is that this area
of the graph correlates to how camera sensors capture light. They are
electrically better at capturing brighter levels than dark. As you move
across the histogram to the left, you start to have darker areas of the
image with spikes, but there is less light being captured, which leads
to more background sensor noise in dark areas.

Try it out. Shoot the same image in raw with the histogram all the way
left, and then all the way right using shutter and aperture only to
compensate. Then in UFRaw increase the darker image's exposure until it
matches the brighter image and then compare the dark areas. The image
shot to the left will have significantly more noise, that is not the
result of ISO settings since you controlled for that.

Now, back to raw versus jpg. If you keep your histogram to the right,
without going over the edge, you can then edit with UFraw to properly to
get correct exposure, increase sharpness & contrast, tweak white balance
properly, and generally have the best possible quality of image *before*
you take it into the Gimp and the land of lossy jpegs. The key is that
you process the raw into a jpeg yourself and maintain quality and
control through the process.

Now I'm thinking that maybe I should do a TLUG talk about Linux and
digital photography tools, workflows, and editing techniques...

Cheers, Jamon
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