Hot Pixels?

[kyzen]

I got a Rebel XSi back in October - it was great, till I started taking longer exposures for some night shots, and noticed I had some stuck/hot pixels on the sensor. There was a rather giant spot, spanning several pixels, in the upper left corner of my images, that was white in the center, and red on the sides. There were a few more decently sized hot spots in the lower right of the image as well. And then there were a few smaller, one-pixel spots along the bottom and top. Luckily, none in the center.

So, I figured I was under warranty, and shipped the camera off to Canon for repairs. Got it back last night, and in a way, it's actually almost worse.

There are no more giant spots like the red one, however there are significantly more overall. And while before the pixels were all out of the way, these are all over the frame. Even worse, these spots start appearing at 5 second exposures, whereas prior to repair, the only spot that would appear before a 15 second shot was the big red one.

Now, I understand that hot pixels are normal, and can be cloned out, and whatever - but I'm still frustrated by this - especially after spending $30 to send the camera to them, and waiting for it to come back for about 3 weeks.

What's your opinion - am I being too picky here? Will trying to get it repaired again just result in more frustration?

 

[ghostman]

Difficult to say without seeing an example of the full image. Have any you can post?

 

[ElFenix]

turn on long exposure noise reduction.

 

[kyzen]

Uploading RAW images now to my website, don't have the software here at work to convert to JPEG.

Fenix - the problem with that is that I take a lot of pictures of constellations and stars... it's easy for a star to look like a hot pixel

 

[kyzen]

The pre-repair image is at: http://www.chrispiekarz.com/files/IMG_1912.CR2
Post-repair is: http://www.chrispiekarz.com/files/IMG_2204.CR2

 

[Flipped Gazelle]

Hmm, that's a tough issue. Many cameras are prone to hot pixels, because the longer the exposure, the hotter the sensor gets.

 

[soydios]

Originally posted by: kyzen
Fenix - the problem with that is that I take a lot of pictures of constellations and stars... it's easy for a star to look like a hot pixel

no, it isn't, because a star doesn't shine through a closed shutter.

long exposure noise reduction works by closing the shutter after the first exposure, then taking a black exposure of the same length, then subtracting the black exposure from the first one. in effect, it should subtract away any hot pixels.

EDIT: I looked at your files. I saw the hot pixels, but a second later after Lightroom did its rendering they disappeared and I haven't been able to get them back, even after turning off noise reduction and sharpening, and playing with levels. So I guess that ACR deals with such things.
I did notice, however, that your new sensor is much worse at low-light noise and has much more noticeable banding.
Again, try long-exposure noise reduction (aka dark noise subtraction), and let us know how it goes.

 

[kyzen]

How were you able to see that? I'm really not at all knowledgeable about post processing.

Hell, how were you able to see anything period? Those images were taken with the lens caps on; I wouldn't think you could see anything at all. I don't suppose you could post example pics?

My camera is still under warranty, so if it was returned to me in arguably worse shape than it was when I got it, I can still try again

EDIT: Also, is the banding you're talking about a sensor issue, or could it be related to the lens? When I took the second black shot the only lens I had handy was a crappy 28-90mm lens; the first one was taken with the 18-55mm kit lens I believe. I didn't think the lens would matter much for a shot with no light.

 

[ElFenix]

Originally posted by: kyzen

EDIT: Also, is the banding you're talking about a sensor issue, or could it be related to the lens? When I took the second black shot the only lens I had handy was a crappy 28-90mm lens; the first one was taken with the 18-55mm kit lens I believe. I didn't think the lens would matter much for a shot with no light.

banding would be a sensor issue

 

[kyzen]

Good to know, thanks.

 

[kyzen]

Well, after taking a few test shots around my neighborhood, them seem slightly noisier in general compared to shots I've taken before; though that could just be paranoia at this point.

I think I'm just going to try a second repair.

Thanks for the help as always

 

[soydios]

Originally posted by: kyzen
How were you able to see that? I'm really not at all knowledgeable about post processing.

Hell, how were you able to see anything period? Those images were taken with the lens caps on; I wouldn't think you could see anything at all. I don't suppose you could post example pics?

My camera is still under warranty, so if it was returned to me in arguably worse shape than it was when I got it, I can still try again

EDIT: Also, is the banding you're talking about a sensor issue, or could it be related to the lens? When I took the second black shot the only lens I had handy was a crappy 28-90mm lens; the first one was taken with the 18-55mm kit lens I believe. I didn't think the lens would matter much for a shot with no light.

I pushed the heck out of the files, something like +4 exposure compensation (the limit), +150 brightness (the limit), and +100 contrast (the limit). It shows the base noise of the sensor, which is the noise that is there even if no light is collected. Sorry but I'm away from my desktop for the week, and thus won't be able to post examples because I don't have Lightroom on my laptop; but try the settings I gave in the first sentence and you should see something similar.

Banding is a sensor issue, but extra light leaking through a different lens could have brightened it. Try doing the 30-second exposure with the body cap on (no lens) and no long exposure noise reduction, then with the body cap and long exposure noise reduction. Temperature also affects such things, so if your camera body was warm, it would result in more noise (remember: the sensor is analog; it's not a digital image until after the Analog-to-Digital converters, aka ADCs).

 

[jpeyton]

http://www.pixelfixer.org/