Canon Digital Rebel Color Resolution
Page last edited Nov 19, 2005, 1:45 pm
The following images were shot to evaluate the effective resolution of
a Canon Digital Rebel camera (3072 x 2048 pixels) when dealing with
Major result: There is a slight
resolution loss for green light as opposed to white, but a significant
loss for red and blue.
- Lens: Sigma 18-125mm at 18 mm, f/11
- Illumination: fluorescent
- Color filters: Kodak Wratten gelatins, in front of the lens,
Numbers 29 (red), 61 (green), 47B (blue).
- File format and processing: shot in Canon raw (.crw), converted
to RGB using Photoshop CS raw converter with all settings default except Sharpness 0 and
Color Noise Reduction 0.
- For display: colored images (red, green, blue) are converted to
gray scale by replicating the appropriate channel.
- Update (Nov. 13, 2005): histograms of the converted RGB channels
- Update (Nov. 18, 2005): raw
sensor data was extracted with dcraw -d. This data clearly shows
the Bayer pattern. It also indicates that both the red and blue
sensor positions are significantly sensitive to light getting through
the green (#61) filter. This contradicts what was suggested by
the histograms of the converted RGB channels, and helps to explain why
there is only a slight resolution loss for green light, but significant
for red and blue.
However, the nature of the resolution loss is more like blurring than
the block effect described at http://groups.yahoo.com/group/Microscope/message/27425
and illustrated in the links at http://groups.yahoo.com/group/Microscope/message/27430
. Presumably this is due to differences in the raw-to-RGB
Just for context, here is the full-frame view with no filters.
We're going to look at the small block in the center of the resolution
All of the following images have been blown up to 400% with
nearest-neighbor interpolation in Photoshop.
The inset at lower right is the same image at actual pixels (100%).
Here is the central portion of the resolution chart. The order is
1) no filter, 2) #29 red, 3) #61 green, and 4) #47B blue.
To illustrate the effect of Sharpness and Color Noise Reduction
settings, here are two no-filter images. On the left is with
default settings (Sharpness=25, Color Noise Reduction = 25); on the
right is with Sharpness=0, Color Noise Reduction = 0, same as shown
NoFilter Options0 Image.tif
Update Nov 18, 2005:
The raw-format CRW files were processed through dcraw (http://www.cybercom.net/~dcoffin/dcraw/)
using the -d parameter to extract raw sensor data (no de-Bayering, no
interpolation). The resulting files clearly show what appears to
be the Bayer filter pattern. Here is the dcraw -d output for the
red, green, and blue filters, applied as a Photoshop layer mask against
uniform red, green, and blue tones to help visualize them.
Notice that the green filter produces significant response at all
Numerically, the responses in the upper-left Bayer quad are:
Bayer Filter Position
These values are after conversion by IrfanView from dcraw's -d ppm
output to 8-bits-per-color tiff. They may or may not represent
actual sensor values, but should give an idea of relative