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08-16-2009, 02:24 PM   #91
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By way of example here are two shots of everyone's favorite Orion's Nebula M42 and M43. Taken last Feb. with my K10D through a 10inch Newtonian on an equatorial motor driven mount.

The top shot is 225 seconds long, while the second shot is a stack of 4 shots combined in RegiStax for a total of 809 seconds. Notice that the SNR of the 809 second shot is much better. However, there is NO more information (look for more faint background stars). And yes I know they are slightly out of focus and there was a dust speck present (grrrr - too many things to coordinate at once in astrophotography).

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Last edited by JackBak; 08-16-2009 at 08:13 PM.
08-17-2009, 02:19 AM   #92
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Since I don't do astrography I'm probably not the best person to comment on this, but I can walk through the physics and the logic.

In astrography you would be dealing with very minimal amounts of light. If, over a 255 seconds exposure, a very faint star still doesn't provide enough light to register on the camera sensor, it would appear "black". 4 x Black is still black. So it doesn't work there.

Its a different game in most terrestial low-light long exposure shots. Even in dim light from a crescent-moon, or from reflected "city-glow", there is some amount of light which will register with the camera'sensor in say, a 30 second exposure. Still way below what's needed for a decent-looking photo, but the information is captured there in the RAW file.

If I take 10 sequential shots + 1 dark frame for noise reduction, I can do an additive stack in PP, which will accumulate the light from the 10 shots, and give me a reasonably exposed night-shot. Use the Screen Blending mode in Photoshop, each shot becomes a new layer in the file.

Unlike an artificially boosted shot in Post-Processing, this method produces detailed and low-noise nighttime photos.
The example I used earlier was done in-camera with the K-7, and the output is only available in JPG, but its the same principle.
08-17-2009, 01:35 PM   #93
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I know I'll regret sticking my nose in the discussion between kittykat46 and JackBak.

I think you are both right but using a different "stack." If you average, which is what most astrophotographers do to reduce noise, the brightness does not change but the signal to noise gets better. If you add (which I never do) then the brightness does increase. Normally lit objects get brighter. Dim objects which can't be seen in the short exposure will not magically appear when added as kittykat stated.

Note: Adding in camera requires a dim subject so the sum does not exceed the maximum pixel value. Adding in software just requires the program be able to scale large sums properly. Usually that means 32 BIT math routines.

There is also BIT resolution to consider. Let's say the short exposure only has 2 bits between the brightest and dimmest parts of the nebula. It will look pretty awful with only 4 brightness levels. No amount of adding multiple frames will fix that. GIGO!

Using an exposure 16 times longer gives the nebula 6 bits between the brightest and dimmest parts of the nebula. This allows smooth transitions between the wispy parts of the nebula.

My conclusion? Use the longest exposure you can reasonably take and take lots of them. The better amateur astrophotographers take 10-20 minute frames for many hours.

And I just have to add one of my images of Nebula M42. K100D through FL 2000mm telescope 2 minutes * 42 frames.
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Last edited by LeoTaylor; 08-17-2009 at 01:36 PM. Reason: Fix wording
08-18-2009, 05:55 AM   #94
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Thanks for weighing in Leo. Yes the more I thought on this the more I was wanting to know what Pentax's stacking function is. For anyone listening in there are as many stacking algorithms as there are NASA mathematicians (quite a few). I have reworked the images I posted and with different stacking alorithms I can bring more wispy gas out of the image always at a price though. You can't bring out information that was never recorded.

Leo nice balance of range in that shot. Most problems with processing M42 come from trying to capture the wispy gas without blowing out the Trapezium. As you can see my shots overexposed that area (those shots were taken on an equipment run through - not an excuse but hey). Any way nice to see there are others in the Pentax land doing astrophotography.


Last edited by JackBak; 08-18-2009 at 06:16 AM.
08-18-2009, 07:10 AM   #95
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QuoteOriginally posted by Jonson PL Quote

I thought my K10 had automatic DFS ? I guess I would prefer that it had. I seldom use bulb mode, and when I do, I would prefer that I canít go wrong and the camera will help me out.
Anyway, if some shooting situation is important to me, I would usually plan on bringing along two bodies.
The K10 does have automatic DFS, and it is on by default.

The problem is that in the K20 and later, it becomes *forced* automatic DFS. For K10, the options are something like "Auto/off" or "On/Auto/Off".

In K20 and K-7, it becomes "On/Auto". The issue people have here is that there is no "Off" any more.
08-18-2009, 07:35 AM   #96
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I'm not sure if this qualifies as a true long exposure shot, this was made up of about 16 30" exposures + a dark frame shot at 500iso, other than the stacking and enhanced foreground there was actually no post-processing NR applied to either the stars or the foreground.

08-18-2009, 07:50 PM   #97
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QuoteOriginally posted by JackBak Quote
Thanks for weighing in Leo. Yes the more I thought on this the more I was wanting to know what Pentax's stacking function is. For anyone listening in there are as many stacking algorithms as there are NASA mathematicians (quite a few). I have reworked the images I posted and with different stacking alorithms I can bring more wispy gas out of the image always at a price though. You can't bring out information that was never recorded.

Leo nice balance of range in that shot. Most problems with processing M42 come from trying to capture the wispy gas without blowing out the Trapezium. As you can see my shots overexposed that area (those shots were taken on an equipment run through - not an excuse but hey). Any way nice to see there are others in the Pentax land doing astrophotography.
There are two broad ways you can stack shots - averaging or additive. Many different flavours to doing it, but the basic principles revolve around these two.

An averaging stack will produce an end photo of the same brightness , with very good control of random noise. Average (A+B+C+D.....)

An additive stack produces a brighter end photo (like adding light). If the individual photos are low-noise, the end result will be quite good. If one or more of the individual photos are noisyl the end result may be bad. Sum (A+B+C+D.....)
I dont' think this works well for astrography because you have very little light to start with, but its very usable for moonlight and city night-scapes.
08-19-2009, 04:45 PM   #98
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C.W Tsorotes

Beautiful star trails over a city! The Green, Orange, and Blue city looks eerily colorful.

My wife always included some foreground (usually trees) on her star trails. I always forget to do so.

08-19-2009, 06:53 PM   #99
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QuoteOriginally posted by JackBak Quote
Interesting. I have never tried stacking non-astro shots.
[...]
Not because we are increasing the signal but because we are decreasing the noise.
You may want to have a look at
PhotoAcute ->
Take Better Photos, reduce noise, increase quality and resolution of digital photographs. Leading superresolution technology.

They do not only stack, they try to increase the effective resolution as well. To some success, actually


BTW, there is no need to differentiate between adding signals or averaging noise. Think of stacking N images as dividing ISO by N.
08-19-2009, 10:40 PM   #100
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Hi, Falk. Yes, I believe the "Auto EV" mode of multiple exposure in the K10D and newer is a great way to do stacking...albeit for only 9 exposures. Effective ISO at ISO100 then becomes 100/9 = 11 and change. And some people wonder why Pentax doesn't offer ISO 25?

Jack
08-20-2009, 02:45 AM   #101
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QuoteOriginally posted by jbinpg Quote
Hi, Falk. Yes, I believe the "Auto EV" mode of multiple exposure in the K10D and newer is a great way to do stacking...albeit for only 9 exposures. Effective ISO at ISO100 then becomes 100/9 = 11 and change. And some people wonder why Pentax doesn't offer ISO 25?
Like with HDR, it is a pitty that the firmware cannot align.

So, extremely low ISO shots / high DR shots on a tripod are all we got (actually, one must use the feature in RAW and slightly underexpose (with Auto-E/V, up to 1.5 stops in extreme high contrast scenes) to fully exploit the high DR advantage).

Otherwise, I would have liked to shoot night scenes at ISO 14400 with small grain like in ISO 1600 (I still do it but use PhotoAcute and 16 images then).


BTW, Pentax considers the multi exposure feature to be an artistic offering: The K-7 displays the partially developped image (from all preceding exposures) as a live view overlay to enable multi exposure composition ...
08-20-2009, 03:23 AM   #102
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Thought i'd throw this into the mix, I left the camera locked in bulb mode to drain the battery.

This is a 19min exposure show at F11, 100iso in a room that was VERY dark, the only light was a bit of the street light coming through the blinds which barely illuminated the room enough to see the tv.

What suprised me was the lack of noise and lack of hot pixels.

Call it my imagination but I feel more the I use the K7 the less apparent noise is, it's either because I am learning how to use it better or making because I am breaking it in? Or I am just imagining.

Otherwise I'm more and more confident with them camera.
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08-20-2009, 07:16 PM   #103
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QuoteOriginally posted by C.W Tsorotes Quote
Thought i'd throw this into the mix, I left the camera locked in bulb mode to drain the battery.

This is a 19min exposure show at F11, 100iso in a room that was VERY dark, the only light was a bit of the street light coming through the blinds which barely illuminated the room enough to see the tv.

What suprised me was the lack of noise and lack of hot pixels.

Call it my imagination but I feel more the I use the K7 the less apparent noise is, it's either because I am learning how to use it better or making because I am breaking it in? Or I am just imagining.

Otherwise I'm more and more confident with them camera.
You're definitely not imagining it.
Its a new moon last night, so I'll have a good opportunity to take some really low-light shots this weekend, weather permitting...
I've taken the K-7 up to 10 minutes exposure so far, and ISO100 is very clean.
Unless you look at the EXIF, you won't know its a long exposure.

IMHO, it beats the Canon 50D, easily.
I know......we're still stuck on the long exposure forced DFS issue.
Other than that, I find the K-7 is a very good long-exposure tool.
08-21-2009, 05:34 AM   #104
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Another experiment.

F20, ISO100 and 71mins in pitch black. Shot in raw, suprisingly quite noise free, camera was very warm.

There was no extra NR applied.

08-21-2009, 06:54 AM   #105
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I too was unimpressed that you couldn't turn off the dark frame noise reduction in the K-7 but I bought one anyways. I do a great deal of long exposure night photography and waiting is a bit annoying, but the results are still amazing. If you can live with it, the K-7 is an excellent camera for night time shoots.

Here's a couple recent examples:



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