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04-29-2012, 01:55 PM   #1
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645D Slow Shutter Speed NR and really long exposures

I'm curious what other 645D users think of the Slow Shutter Speed NR and how neccessary it is to do in camera vs. doing in in post afterwards and which provides best results or if both are required for the best results. The processing time ends up being really long on the longer exposures and it starts to test my patience

I was thinking of doing some night sky exposures and getting 'star trails'. So, exposures from 10 mins to a couple hours possibly. Anyone have experience doing this with the 645D?

04-29-2012, 02:11 PM   #2
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Stars will start trailing within 10 sec or so. I have done some night work with exposures up to 5 minutes, but not longer. You probably want to then do separate dark exposures at that point. If you are also including foreground elements, you may want to pick up a green laser so you can focus.
04-29-2012, 02:17 PM   #3
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QuoteOriginally posted by Yamanobori Quote
You probably want to then do separate dark exposures at that point.
What do you mean by this?

Typically, I've done my star trail work with wide angles, so on the A 35mm I'm able to just set to infinity, but the laser idea sounds interesting. Might be useful... Where did you get yours?
04-29-2012, 03:00 PM   #4
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For dark-frame subtraction, which is what long-exposure NR is, you take a frame that is shot with the lens cap on at the same exposure duration and at the same temperatures--your camera automatically makes this frame which is why you wait as long as the exposure. That is your dark-noise frame which you can subtract from your actual exposure and can be done in post processing. You can even make a database of those dark frames.

I picked up a laser on Amazon for about $10 or so. I got green as it is easier to see than either a blue or red.

04-29-2012, 05:26 PM   #5
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Hi guys, just on this topic, you say take a dark frame with lens cap on ? When i go to bulb and take anyhting over 30 secs, it takes another NR capture, at similar exposure times, should i be putting the lens cap on at this point ??

Also i understand there is no way to turn this off in the camera (the NR) is that correct ?

Cheers.
04-29-2012, 08:22 PM   #6
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QuoteOriginally posted by Yamanobori Quote
you take a frame that is shot with the lens cap on at the same exposure duration and at the same temperatures--your camera automatically makes this frame which is why you wait as long as the exposure.
How and why is this different than just using a completely black fill layer in PS...or is there something larger here that I'm missing? I've never tried that before.

Thanks for the tip on the laser!

Bigvern1263, you can turn long exposure NR off in camera (on 645D anyways). It's under C3 19). Slow Shutter Speed NR (ON is default)
04-29-2012, 08:39 PM   #7
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Hi Ryan, not on mine, i tried.... still does the NR exposure, regardless of what setting is made, i also found another thread a while back , cant remember where it was though, that said same thing, it cant be turned off. What SW version do you have ? maybe i have old SW/FW etc .. Camera < 10 months old. i will check and try once more.

Cheers.
04-30-2012, 09:01 AM   #8
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QuoteOriginally posted by Bigvern1263 Quote
Hi guys, just on this topic, you say take a dark frame with lens cap on ? When i go to bulb and take anyhting over 30 secs, it takes another NR capture, at similar exposure times, should i be putting the lens cap on at this point ??

Also i understand there is no way to turn this off in the camera (the NR) is that correct ?

Cheers.
No, the camera does this automatically by closing the shutter. The lens cap idea is if you just want to make your our dark frames manually.

04-30-2012, 09:03 AM   #9
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QuoteOriginally posted by Ryan Tischer Quote
How and why is this different than just using a completely black fill layer in PS...or is there something larger here that I'm missing? I've never tried that before.
The dark frame is an image of random noise created by your sensor, it is not simply black. The random noise image is subtracted from the exposed image to try to cancel out the random noise.
04-30-2012, 09:04 AM   #10
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QuoteOriginally posted by Bigvern1263 Quote
Hi Ryan, not on mine, i tried.... still does the NR exposure, regardless of what setting is made, i also found another thread a while back , cant remember where it was though, that said same thing, it cant be turned off. What SW version do you have ? maybe i have old SW/FW etc .. Camera < 10 months old. i will check and try once more.

Cheers.
Update your firmware to the latest version.
04-30-2012, 01:26 PM   #11
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QuoteOriginally posted by Yamanobori Quote
The dark frame is an image of random noise created by your sensor, it is not simply black. The random noise image is subtracted from the exposed image to try to cancel out the random noise.
Thanks so much for explaining...this makes much more sense now! It would be much easier to do it afterwards at my convenience if I decide the image needs it vs. having to wait for, say, TWO 30 minute exposures just to get one.

What software do you use for NR in post?
04-30-2012, 04:43 PM   #12
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Thanks Yamanobori, will go look for latest firmware.

Ryan i use ImageNomic, i think its called, does a pretty good job and you have quite a few sliders to play with to adjust things and make personalized settings.

i have found with the 645D i use it WAY less then with my Canon 50D, it was every shot on the canon, now likely 1 in 10 on the D.
04-30-2012, 05:34 PM   #13
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QuoteOriginally posted by Ryan Tischer Quote
Thanks so much for explaining...this makes much more sense now! It would be much easier to do it afterwards at my convenience if I decide the image needs it vs. having to wait for, say, TWO 30 minute exposures just to get one.

What software do you use for NR in post?
The astrophotography community might be able to point you in the right direction. Try the Cloudy Nights forums as they have an astrophotography section. Very nice folks. I have not been doing dark frame subtraction manually, but for similar type stuff i have used Photoshop. For freeware, ImageJ may be able to do it--it is open source software for the scientific community, mostly microphotography.
05-01-2012, 02:25 AM   #14
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Ryan,
I experimented with really long exposures this winter, and found that the 645D sensor is sensitive to what the astronomers call "spurious noise". The most noticeable artefacts look like curved or curled streaks, and are indirectly caused by cosmic radiation, no less, hitting the sensor. When rays hit during exposure, you get bright streaks. When rays hit during the dark-framing in "B" mode, you get dark streaks.
There's a science research paper describing spurious noise in CCD telescopes here (PDF). Or if you take my word for it and can bear with my drivel, I wrote a blogpost on my find here. :-)
05-01-2012, 03:46 AM   #15
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<<That is your dark-noise frame which you can subtract from your actual exposure and can be done in post processing.>>

How do you actually subtract the dark frame in Photoshop? Thanks....
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