More Help! - Darks

[erick]

Sorry guys - Photography 101 here! Please be patient with me. :thanx:

So I have previously used ICNR with the Pentax K100D but the time it takes is too much for timelapse - particularly for meteors whizzing by! I want to maximise shutter open time!

So I experimented last night with INCR off.

Here is a 20 sec dark (cap over lens in a dark room), ISO 1600, camera temperature probably 22-25 deg C. White balance set to "Tungsten". And saving as jpegs on the memory card.

I've had to resize and compress but is this what I expect to see - a rainbow of colours, something like the early quantum fluctuations of the Universe?

[toryglen-boy]

Hi mate, no thats not "Exactly" what i would expect to see. replace the purple with black, and your close. My darks (and everyone else's i assume) are usually black, with red or blue pixle noise scattered around, with the amount of "hot pixels" dependant on the exposure time and temp.

:)

EDIT : Is that ampglow or something? seems very bright to me

[Octane]

Eric,

At a guess, the funky colouring may be due to the tungsten white balance.

Leave it on auto and retry.

Regards,
Humayun

[erick]

OK, looks like I go back to auto white balance. Blast! I left the camera at home so it will have to wait until I get home.

Can someone arrange for the Leonids to be delayed for a few days while I get this right! :D

[DavidU]

We should speak to Anthony Wesley:D

[Terry B]

2 things.
Darks don't really work with jpeg files as the individual pixels are not kept separate in the file. You need to use raw files.
The colour balance you choose makes no difference to a raw file itself but does change how it is displayed on the screen.
The colour effect is just because the indivudal pixels that are really still monochrome detectors are having colour added to them in your display as part of the debayering process. This doesn't change the value of each pixel, just how it is displayed. As a dark it doesn't matter what colour they are displayed on your screen.

[RB]

Terry is correct.
It's best to shoot in RAW if you will "Dark Subtract".
And as I stated in the other thread, if you will be using jpgs use Daylight WB, or set your RAW's to Daylight in post process for your time-lapse.

Also if you will be shooting in jpg then ICNR is your better option.
If you're worried about the long delay then just cut down your exposure to around 13 sec so that you end up with a shot every 30 sec or so (I assume you're shooting at a wide Focal Length).

If I'm doing time-lapse I just shoot in jpg and ICNR.

:)

[RB]

BTW Eric, here's a quick little time-lapse I did at IISAC09.

http://www.youtube.com/watch?v=jUr3eUbBUzI

I was only mucking around, so it's nothing special, while I smoked my cigar and went around to annoy the others, LOL.

I plonked the camera down (20Da) on my tripod, set it to 1600 ISO, Daylight WB, 25 sec exposure and ICNR (50 sec total plus 10 sec delay between shots) at 16mm, f/2.8 and let her rip.
So I ended up with 1 shot per minute.

The activity you see is the guys mucking around with the sparklers and red light torches.
Unfortunately we got clouded out soon after.

I used the jpgs, resized them all in VirtualDub down to 800 pixels and assembled them all in Startrails.

This gives you an idea of what to expect.

:thumbsup:

[TrevorW]

Eric thats exactly what I'd expect too see

I have a K100d and they are ... noisey CCD thats why i purchaed a Canon 350d and had it modified to do my astro imaging

suggest with the Pentax dropping ISO and increasing exposure time too reduce noise or use ICNR

[erick]

And here is my first and only effort:-

http://www.youtube.com/watch?v=nL12OhMpY94

I was shooting at ISO 800 there - about 20 sec exposures, I think. Didn't really capture much starlight. I had the lens stopped down a bit. I'm going to leave it wide open this time. I think I'll stay at ISO 800. Meteors should be bright enough. I think the camera is noisy at 1600 - I'd hate to try 3200, which is its max setting!

It's in the down time for ICNR that worries me - you know that the best meteors will go by while the camera is processing the last frame!

Maybe I'll save as RAW with ICNR off - decisions/decisions :confused2:

[rogerg]

I think I'll be going with the following for Leonids..

ISO 800
30 second exposures
Lens @ F/4
ICNR off.

Will take some darks at the start of the night, but noise shouldn't be an issue at 30 seconds ISO800, well, not for my 350D at our 15 degree night time temps anyhow. What noise there will be should be able to be removed acceptably using post processing.

Last time I tried to photographed a meteor shower I made the mistake of too low ISO, too small aperture.

Roger.

[Bassnut]

What do you mean JJ, were your images taken as JPEG, JPEG darks subtracted and then converted to 16 bit?.

If so, then I wouldnt expect any difference between dark subtracted JPEGS (with JPEG darks) and no darks at all. Dark subtraction wont work with JPEGs (compression would mess with that).

Why would you convert a JPEG to 16 bit?. JPEGs are 8 bit, nothing to gain there.

I probably have missunderstood something here.

[jjjnettie]

They come out of DSS as 32 bit Tiffs Fred.

[Bassnut]

OK, so how does this relate dark subracting JPEGs?

[jjjnettie]

I sorry, I must be mistaken.
I thought that we were talking about subtracting darks from jpegs.
So I posted 2 images, one with darks subtracted the other with no darks.
I removed the post.

[Terry B]

Dear JJJ
I'm not sure what you mean.
If you take an image with a DSLR and save it as a jpeg, a compression algorithm is used to store the image. Depending on the amount of compression this will smooth out areas of the image with pixels with similar counts-similar but not the same.
If you take a dark frame and save it as a jpeg then the same process occurs. The problem is that the compression algorithm will not necessarily smooth out the data the same way in both images. This means that if you convert both images to uncompressed files and subtract one from the other then there will be some error as induvidual pixels are not necessarily being subtracted from the same pixels.
A raw (or fits) image file is literally a table of all the counts detected in each pixel. A dark file is the same thing. As a CCD is being exposed as a dark, electrons will accumulate in the induvidual pixels at varying rates depending on the temperature and the individual characteristics of each pixel. This is the dark current and should be fairly constant for each pixel. This is why they can be subtracted from the light images to remove the variation in background. This needs to be on an individual pixel basis to be accurate.
I assume the ICNR method does this before the image is converted to a jpeg. As RAWs you can do it afterwards. The advantage to this is that you can average (or median) many darks to get rid if random noise etc and it wastes less time aiming at your subject.

[Bassnut]

we are getting confused here, sorry JJJ. The thread was about dark subracting JPEGs. Im guessing you dark subracted RAWs correctly, and your comparison was with processed images converted to JPEGs after processing.

In that case, thats a puzzle, darks should work. The conversion to JPEGs after processing doesnt make any difference.

[erick]

OK, I have a lot of RAWs (in Pentax's format .PEF files) - lights and 10 darks when I started and 10 darks when I finished. I shot at ISO 800, 30 sec exposure. Let's see what I can do with these.

Only one problem - they are all of fog! :D :sadeyes:

Well, I can learn basic processing on them.