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Old Apr 12, 2005, 4:16 PM   #1
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I was trying a few night shots last night with the 7D, and I was getting TONS of hot pixels. I was shooting at ISO 100.
Here is a 100% crop of a 192 second exposure (those are not stars!)
Is this normal?

note: that little bit of green on the right, is northern lights in the sky, not the camera :?
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Old Apr 13, 2005, 1:50 PM   #2
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Do all 7Ds do this, or did I get a bad camera?
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Old Apr 13, 2005, 4:07 PM   #3
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I don't know of anyone that's tried exposures that long with one.

I'd suggest making sure the camera is cool before you start your exposure (and also make sure anti-shake is turned off). Heat can impact noise levels/hot pixels.

Is this with the camera's noise reduction turned on?


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Old Apr 13, 2005, 5:18 PM   #4
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JimC wrote:
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I don't know of anyone that's tried exposures that long with one.

I'd suggest making sure the camera is cool before you start your exposure (and also make sure anti-shake is turned off). Heat can impact noise levels/hot pixels.

Is this with the camera's noise reduction turned on?
Yup, noise reduction was on, and anti shake was off. It was about 40ºF outside at the time. . .and the camera had not been on for a few hours.
This was just part of a "test run" with a borrowed 7D-as I am trying to decide between the 7D and the canon 20D-and I must say that I am not too impressed by the 7D on long exposures.
I will see if I can find a 20D to borrow, and run the same tests.
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Old Apr 13, 2005, 5:23 PM   #5
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Aaah... I though it was your camera (especially given your chosen forum member name). ;-)

I am asking around now, and will let you know if I get some responses from 7D owners that have tried longer exposures with this model.

If this is indeed normal behavior, there are techniques that can be used to simulate longer exposures by stacking multiple images taken at shorter exposure lengths.



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Old Apr 13, 2005, 5:31 PM   #6
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I have shot many long exposures ( over 30 sec) with my 7D and I have no hot pixels.

Strange, I would venture to say it looks like a defective ccd, but I am by far not an expert.

Interested to find out what the rest of the board says?:?

Good luck

Charlie
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Old Apr 13, 2005, 6:13 PM   #7
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A 7D owner just did a quick test of a longer exposure (197 seconds at ISO 100) and posted the image. No hot pixels were noted. Here isthe resized sample (complete with EXIF indicating the exposure length):

http://www.dyxum.com/photogallery/tmp-mtf/bulb.jpg

Are you sure the camera was cool for the test, with noise reduction enabled via the camera menus?


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Old Apr 13, 2005, 6:46 PM   #8
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JimC wrote:
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A 7D owner just did a quick test of a longer exposure (197 seconds at ISO 100) and posted the image. No hot pixels were noted. Here is the resized sample (complete with EXIF indicating the exposure length):

http://www.dyxum.com/photogallery/tmp-mtf/bulb.jpg

Are you sure the camera was cool for the test, with noise reduction enabled via the camera menus?

I am sure that noise reduction was on, and that was only my second shot outdoors (at 40F, with a slight breeze). The first shot (171 seconds @iso 100) has just about as many hot pixels as the crop from the one I posted. Is it just from a bad CCD? Also, in Steves review sample pics, he has one pic @iso100 with a 30 sec. shutter speed. http://www.steves-digicams.com/2005_...s/PICT0615.JPG
that image also shows several hot pixels (maybe 10?). Perhaps this is a very common problem with the 7D:sad:
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Old Apr 13, 2005, 7:09 PM   #9
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Hot pixels on longer exposures is a common problem with most digital cameras. The Sony sensors used in models like this Minolta (as well as similar Sony sensors used in Nikon and Pentax models) are not going to be an exception.

Heat can be a big contributing factor (which is why I asked about anti-shake, which adds heat). I remember these same types of discussions when owners of the Ax series cameras started doing tests for hot pixels.

KM specifically recommended leaving anti-shake off to prevent overheating the sensor during longer exposures with their Ax series cameras (so I doubt the D7 would be any different). That's why I wanted to make sure the camera was cool with anti-shake turned off.

The impact of heat on noise and hot pixels iswhy you'll find that digital cameras designed for use in astronomy are using cooling systems for their CCD's.

The reason you don't see hot pixels as much with newer models is due to dark frame subtraction noise reduction.

The camera is actually taking two photos with noise reduction enabled (one of your subject, and another with the shutter closed). It then identifies the locations of hot pixels in the dark frameexposure (these are the only pixels that will be bright because the shutter was closed),and mapsout pixles in the same locations in the actual exposure of your subject. This is know as "dark frame subtraction"

It's not uncommon for a non-DSLR model digital camera to have hundreds of hot pixels on a relatively short exposure (i.e., 15 seconds or so). You just don't see them as much since the dark frame subtraction system maps them out. DSLR models aren't as bad.

But, the longer the exposure, the less accurate this type of system is. It's relying on hot pixels to be in the same place in both the actual and dark frame exposure, since the exposures are being taken at approximately the same time, with the sensor at approximately the same temperature. But, when you get into very long exposures, you sometimes end up with pixels that are hot in the actual exposure, that didn't show up hot in the dark frame exposure. So, the system misses these.

Not too long ago, digital cameras did not have this type of system (you had to take your own dark frame exposure with the lens cap on, and map hot hot pixels yourself using software).

As a general rule, larger sensored models (DSLR's) arebetter with hot pixels compared to smaller sensored models. But, they are not immune to it either.

Hereare someproducts you may find useful if this is something you plan to do often:

Image Stacker (allows you to combine multiple exposures into one, in order to simulate a longer exposure for star trails, etc.). It can also perform a dark frame subtraction on images, using a dark frame image you provide taken with the lens cap on.

Here are some examples:

http://www.tawbaware.com/maxlyons/cg...e.pl?gallery=8

PixelZap (removes hot pixels, with or without a dark frame image as a reference).

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Old Apr 17, 2005, 9:10 AM   #10
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With long exposures - it's not hot pixels

Each sensor element can also saturate which is likely the case (exposed too long). There's also an integration time and you could be operating outside the sensor limit!
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