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View Full Version : Trying to recall advice re: blown pixels and night photos



mirrodie
08-05-2006, 10:17 AM
I recall ages ago, someone here, and for some reason I thought it Tom or Mike or Josh, mentioned that :


night shots with Canon DSLRs can sometimes cause blown pixels to occur. Or something to that end. Am I imagining that? If not, what is the scoop?

pgengler
08-05-2006, 11:34 AM
I don't know about "blown" pixels, but with DSLRs, long night exposures often end up with "hot" pixels that end up showing up as, for example, a small blue dot on a night sky.

I've heard that cameras like the 20D have a custom function designed to deal with this, where after taking the initial shot it then records on the sensor (with no light getting through) for the same length of time, so that it can "subtract" the hot pixels from the final image.

PhilDernerJr
08-05-2006, 05:13 PM
I'm not home right now, but I'll show you some nighttime moon photos that I have with that same thing, except I know them as "dead" pixels.

I don't know much else about them, however.

Derf
08-06-2006, 08:13 AM
I recall ages ago, someone here, and for some reason I thought it Tom or Mike or Josh, mentioned that :


night shots with Canon DSLRs can sometimes cause blown pixels to occur. Or something to that end. Am I imagining that? If not, what is the scoop?

Actually it is not a Canon problem, It is a problem with all digital
cameras. The Canon is great on hot pixels because it uses a CMOS
instead of a CCD. Generally, the CCD gets hot from taking log
exposures. The longer the exposures, the more hot pixels you will see.
As the camera gets older, the more that will appear with age. These are
the general rules. A hot pixel can be white, green, red or blue and a dead
pixel will always show up as black. With this said, there are things you
can do to take care of the problem with no photoshop work on your end.

1. NOSE REDUCTION (IN CAMERA)
Noise reduction will NOT make an image soft. Lets say that you take a
picture for 5 seconds. The camera will close the shutter after the picture is
taken and record the same amount of time (5 sec in this case) with NO
LIGHT at all and use the second image to see any Hot or dead pixels. It
will then FIX the dead or hot pixels in the image making a MUCH cleaner
image. The only downside is that the shot we just took of 5 seconds took
5 more seconds to process. Not bad but when you leave the shutter open
for 20 minutes and then have to wait 20 minutes before you can do
anything with your camera....well, the image quality is still worth it...BUT
FRUSTRATING!

It has absolutely no effect on sharpness.

2. Noise Reduction (out of camera)
I have a program (image stacker) that will allow me to use many images
to make one image....IE, stacking 50 shots to make the stars in the sky
swirl and keep the foreground subject from being overexposed. This
program also has what is know as "Dark frame sublimation?" where it
will let you include a picture that was taken with the same camera setting
as the other images but with the lens cap on... It basically does the same
thing as the in camera noise reduction and works just as well. It will see
any lit pixels and alter the blown pixels in the actual picture so dead and
hot pixels are fixed.

FYI I just rolled out of bed and this may just be rambling nonsense,
if you have more questions, please ask me after 11am to get an
answer that makes more sense..

Derf
08-06-2006, 08:14 AM
My rebel XT will use noise reduction for 30
seconds or more OR if the ISO is set to 1600.

My old Olympus was anything over 1 second.

It is really dependant on the camera, camera maker and how bad the
noise on the CCD or CMOS is. Take the longest exposure you can without
using bulb and then do it again without nose reduction....prepare to be
amazed when you blow it up on your monitor!


(((NOT ALL CAMERAS HAVE NOISE REDUCTION)))