One of the first things I did with a
brand spanking new Canon 7D
was to check for hot pixels as there are 18 million of 'em!
Testing involves shooting a series of long exposures at various
ISO's in a dark room with the lens cap on, and then looking for any
pixels that are overly bright in the exact same location.
Keep in mind there will be noise at
higher ISO's as seen in these black frames.
I was pleasantly surprised to only find three hot pixels, and those only
became visible at high ISO's and long exposure speeds.
These can be annoying, but are not difficult to remove in
post-processing and in fact, Adobe Camera Raw auto-removes 'em.
You can optionally enable Long Exposure Noise Reduction on the camera
which also removes them. I've heard using the camera's sensor cleaning
for an extended time can remove these, but this has not worked for me.
I've usually seen hot pixels manifest themselves as red dots, but
when looking via extreme magnification at the three I found on my Canon 7D,
there's one near the edge that looks noticeably different - must be some
sort of other defect. Similar to the red hot pixels (and on both
the as-shot JPEG's and RAW files), it is
not visible until ISO 200/400 with a 30 second exposure.
Canon 7D 30 second exposure at ISO 1600 - full image resized to 900x600
Same image cropped to actual pixels
Same image shown 5x actual size with three hot pixels overlaid in 1-2-3 order
Same image shown 10x actual size with three hot pixels overlaid in 1-2-3 order
Click on ISO or shutter speed to see how the hot pixel changes
Actual Pixel crop showing Canon 7D Hot Pixel - ISO 800 - 30 second shutter speed
Conclusion: This particular Canon 7D is pretty clean and
only has a few hot pixels ... plus
very little sensor dust.
It will be interesting to compare these baseline images in a year or so
to see if any hot pixels will develop over time.
Here's a real-life example of hot pixels.
This image is an original as-shot JPEG (full-res crop) from
rafting the Green River. The Hot pixels show up as
red dots and then
mouseover the image to see it as-generated from the RAW version using ACR defaults & Standard Beta 2 Profile which removes them.
Note that while the Canon 50D in-camera noise reduction does clean up the
image a bit, there is a loss of detail which is expected compared to ACR which
had no noise reduction applied.
Also expected is the slight tonality difference even though manual
white balance/exposure was selected on the camera.