Re: Nikon D200 and Canon 350D/Rebel XT Noise Tests

In message <SxkZf.62626$H71.55997@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
Paul Furman <paul-@xxxxxxxxxxxxx> wrote:

You can
set the WB on this dialogue box with RGB values but I have no idea how
to determine the right numbers!

That slider is a bit slow to adjust in realtime; the program is broad in
scope but simple in implementation - it seems to adjust a working copy
of the entire image before updating, rather than updating the display on
the fly.

File menu > Settings to set the 'Working Path' where files will be saved
out to & opened from.

I've never noticed that.

File > Load Raw brings up the RAW file in grainy B&W bayer pattern grey
scale, zoom in to see the RGB pattern in grey.

Yeah, those look very noisy, because of the different sensitivities and
levels of colors. They can look like checkerboards. You get a nice 50%
greycale, however, if you use "binning" (2x) from the geometry menu, and
the checkboard goes away and you wind up with a 1/4x MP, 13-bit
greyscale without the grid.

The Threshold dialogue should be visible, set the top box to 4095 & the
bottom to 0. For astronomy work, you can figure out a dark frame
subtraction thing to set zero higher in some cases but I'll ignore that
for now. You can fiddle with these for the proper look of brighness &
contrast but since we're doing a comparison of raw values, don't worry
if it looks dark, that's the linear conversion. I have no clue what
color space this works in (ie sRGB) or how to control that.

Those are just literal values from the channels, displayed on your
monitor. They have a color space, but are not treated as if they do;
they are just treated as if they are in the colorspace of your system.

Wave the cursor around & in the lower right status bar you see the raw
values from 0-4095 so you can see here if there is anything at all to be
recovered in highlights & shadows. I think you can split the image into
RG&B files to see which channel is blown or not. This is all before the
whole thing gets muddied up by merging the bayer pattern RGB.

Well, if you use the "linear" option you mentioned in the beginning of
your post, all interpolated color pixels will be within the range of the
original RAW pixels. It's not the most aesthetically pleasing option,
but the most honest one.

From the Digital Photo menu select 'Convert a CFA image' to interpolate
the RGB bayer pattern into a normal color image. It probably looks too
green because there's twice the green pixels...

That's what a lot of people assume, but once you do the color
conversion, the amount of pixels of any given color has *NO* effect on
color balance at all. The image is green because that is what the
camera is really seeing - the green pixels let more light through. For
a white subject in white light, most DSLRs let about a stop more light
through the green filters than the red ones, and the blue falls
somewhere in-between. The natural light that the camera wants to see is
somewhere in the magenta-ish purple range. Daylight needs to be
white-balanced quite heavily!

I have no idea how to
correct this meaningfully, see WB comments above. From the Digital Photo
menu select 'RGB Balance' if you want to try... no undo that I can see
so it's frustrating.

That seems to be something that is used upon loading. Only useful if
you use the same camera all the time, and want to WB for one specific
light color all the time.

From there just save as a tif & when opening in photoshop, sRGB seems
as good as any color space to assign. Then I do a levels adjustment to
set the brightness of the middle gamma level & make it look like a
normal exposure.

I probably got it all messed up but you won't find a more
straightforward explanation <g>. Most of the tutorial info on the web
site is geared toward processing multiple exposures of the same scene
for astronomy stuff and will make no sense to a regular photographer.

For your comparison photos, it seems important that they be comparable
exposures & I see a significant difference in brightness & contrast. A
brighter exposure will have less noise and more blown shadows. The
histogram on the camera is jpeg not raw but probably close enough to get
things real close. The two cameras histograms should have the hump
placed in the same spot & not be going off either side more than the
other to get comparable exposures. If that takes a +.3EC or -.3EC that's
worth noting but the exposures need to be the same.

There's really 3 different ways of measuring noise between two cameras
at an ISO:

1) by camera's metering

2) by external metering (same real exposure - f-stop and shutter speed,
on both)

3) Target the same RAW levels (adjusting for blackpoint) in the images.

All three are probably different in most cases. Without RAW data, you
can't even begin to attempt #3. #3 is where dynamic range and exposure
latitude are happening.

Again, about blackpoint - your Nikon probably has a blackpoint of 0, but
other cameras may have black at another RAW value, as the digitizer in
the camera leaves the readout bias in the signal (all Canons do this,
AFAIK; on the 10D, it varied with exposure time, temperature, and ISO -
on the 20D it is always about RAW level 128). With the 20D, as an
example, you can subtract 128 from the image upon loading, or you can
see the image blackpointed by putting the bottom threshold slider at
128; it all depends on how you want to work. Not subtracting first is
good if you're going to do any binning - positive/negative noise bins
more cleanly than noise that is half zeros, and half positive numbers,
as is the case in the extreme shadows when the RAW data is already
blackpointed in the camera. Binning by 2x, of course, means that the
blackpoint has quadrupled.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <JPS@xxxxxxx>
<<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
.

Relevant Pages

  • Re: Adventures in RAW
    ... It was a lesson in how RAW works. ... >>> settings such as exposure and white balance. ... >>> though I was increasing the ISO on my camera. ... >>> Properly exposed, no noise. ...
    (rec.photo.digital)
  • Re: ND filter
    ... To get good exposure of the black train, ... camera ... And here is what I got by converting from raw and lifting the shadows ...
    (rec.photo.digital)
  • Re: ND filter
    ... To get good exposure of the black train, ... camera ... And here is what I got by converting from raw and lifting the shadows ...
    (rec.photo.digital)
  • Adventures in RAW
    ... The first thing you notice is that the import program will make its own "adjustments" to your foolish camera settings such as exposure and white balance. ... When I printed some of the pix, I noticed an unusual amount of noise in the sky and smooth colors. ... Through experimentation, what I discovered was happening was that if I underexposed the original RAW image, the import program was adjusting the exposure up, just as though I was increasing the ISO on my camera. ...
    (rec.photo.digital)
  • Re: Raw File Formats
    ... would you say that the crucial difference between an "exposure" ... , and an "image" (a JPEG or a developed ... JPEG files almost certainly do contain RGB values. ... deciding how to convert raw data. ...
    (rec.photo.digital.slr-systems)
Loading