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08-03-2020, 02:25 AM   #1
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Pentax Q dark frames or no dark frames hack?

Hi folks,
The previous time I went out and captured the Milky Way, I definitely had to wait an extra 30s for the dark frame with each 30s exposure.

Strange finding during the day, taking a series of pics with the lens cap on, with manual mode 30s exposure, somehow, no dark frame was triggered!?
So question is... Does anyone here know how does the camera decides when it needs a dark frame and when it does not? Hope to find a reliable way to trick the camera into avoid taking DF when shooting a string of intervalometered 30s long exposures without all the pausing for the DFs.


Last edited by 2old4toys; 08-03-2020 at 05:27 AM.
08-03-2020, 03:05 AM   #2
maw
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Hi 2old4toys,

You have to use the manual mode set 30 sec. and after your exposure you shoot a second frame with the lens caps always of 30 sec.

Bye Mario
08-03-2020, 05:30 AM - 2 Likes   #3
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At a guess, the choice of ISO may be the factor involved here. If I remember correctly, I can get 30sec at 400ISO on my KP with no delays, above that there's a choice of in-camera noise reduction, which is OK for single or very few exposures, or turn noise reduction off when taking many frames for stacking and take your dark frames separately.


Inevitably ... YMMV
08-03-2020, 06:46 AM - 3 Likes   #4
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Don't forget - a 30-second exposure is NOT 30 seconds long (assuming that is what you have the shutter speed set for (as opposed to B mode)). It is 32 seconds long: Actual shutter speeds versus "standard" values - PentaxForums.com

So, if you were using an intervalometer set take a picture every 30 seconds, you were actually getting 32 seconds exposures, and then the camera has to wait 28 seconds until it fires again. The shutter press after 30 seconds occurred while the previous frame was still being exposed.

So, set your intervalometer for 33 or 34 seconds (allows an extra second or two for the camera to finish/record the previous frame).

08-03-2020, 09:14 AM - 2 Likes   #5
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When it comes to dark frames I prefer the option to take my own when doing any astro shooting. There are a few benefits of doing things this way. The first is that when the camera does a single shot dark frame subtraction there is a lot of random noise just like there is in light frame. Unfortunatly this random noise by nature is random so while subtracting a single dark from a single light will offer some improvement you will also remove a slight amount of good signal. Instead if you shoot a pile of darks at the end almost all stacking programs will combine and average them to create a master dark frame that has substantially reduced random noise. This gives a much better representation of the actual systematic errors in the system which is what dark frame subtraction is trying to reduce. So a stacking program will create a master dark and then subtract that from each light frame better removing the systematic error and removing even less of the good signal which is what you want. Another benefit is that just taking dark frames at the end (or when clouds roll by) is that you can capture more light frames while you are out which means you can capture more signal. So if we assume you have 30 minutes to capture your shots, if you use the in camera dark frame subtraction you get 30 light frames each of which had 1 dark frame subtracted from each. Instead if you just turned off the in camera dark frame subtraction you could gather 60 light frames which would give you another stop worth of signal which is a huge improvement but you will still have the systematic error which will remain. However if while packing up you set the camera off to the side with the lens cap and view finder cover on and took a pile of dark frames (lets say 20 of them) of the 10 minutes you spend packing up you now have 60 light frames giving you another stop of signal, and you will be able to do a dark frame subtraction in post processing with a master dark frame that has over 4 stops less noise than the automatic in camera correction thus better showing the systematic and correcting it. At this point you will have still spend the same amount of time out in the field but in the case where you take your own dark frames you have the potential to end up with a much better image with another stop worth of signal and a better representation of the systematic error.
08-03-2020, 12:13 PM - 1 Like   #6
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Hi 2old4toys, kypfer is right, it's a combination of ISO and SS in my limited experience with my two Q7 bodies.
  • ISO 800 and lower = no subtraction regardless of SS.
  • ISO 1600 and SS 13 sec or shorter = no subtraction.
  • ISO 1600 and SS 15 sec = mostly no subtraction?? See below.
  • ISO 1600 and 20 sec or longer = automatic dark frame subtraction.
I swear that sometimes dark frame subtraction happened with ISO 1600 and SS 15 sec in the past but not consistently. Maybe just a few flukes. Not sure if high-ISO NR setting changes this, I'm usually on auto or low, but when I took a couple shots with NR high, ISO 1600 SS 15 there was no subtraction.

Mario, AstroDave and MossyRocks, we're talking about automatic dark frame subtraction of Q series cameras that cannot be turned off. If SS is 30 sec and ISO 1600, the camera takes 32 sec for the exposure, closes the shutter, waits for 32 sec and then writes to the card. No option was found to disable it.
08-03-2020, 01:39 PM   #7
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QuoteOriginally posted by kwb Quote
Mario, AstroDave and MossyRocks, we're talking about automatic dark frame subtraction of Q series cameras that cannot be turned off. If SS is 30 sec and ISO 1600, the camera takes 32 sec for the exposure, closes the shutter, waits for 32 sec and then writes to the card. No option was found to disable it.
I did not know that you couldn't turn it off with the Q which seems rather annoying.
08-04-2020, 01:51 PM - 1 Like   #8
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QuoteOriginally posted by kwb Quote
Mario, AstroDave and MossyRocks, we're talking about automatic dark frame subtraction of Q series cameras that cannot be turned off. If SS is 30 sec and ISO 1600, the camera takes 32 sec for the exposure, closes the shutter, waits for 32 sec and then writes to the card. No option was found to disable it.
In the frames appears the value of 30s not 31.6 e.g. are they rounded values for convenience or are they the real values? It would take a very high speed shot to calculate exactly
how open the shutter remains.
But that doesn't mean anything, as the Dark Frame must have the same time as the previous shot. But it's not finished, you should add the Flat's Dark and Bias/Offset....

08-05-2020, 03:43 AM   #9
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Thanks kwb for the figures and others for your replies too! Sticking to ISO 800 and below for 30~32s exposures to reliably skip auto dark frames on a Q is exactly what I needed.

I will be delving into bias and flat frames soon enough. Eek out every last drop of quality from my Q long exposures in the dark skies.
08-06-2020, 09:08 AM   #10
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On the border line exposures that sometimes trigger dark frames, it is possible that heat is part of the equation. Many cameras measure sensor heat and adjust noise reduction strategies accordingly.

Wish I'd shot some dark frames for comet Neowise. Now I realize they can still be taken. Just need to pick same shutter, ISO, temperature and dump them into Sequator with the original pictures. Fascinating.

Thanks,
barondla
08-06-2020, 10:04 AM   #11
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QuoteOriginally posted by barondla Quote
Now I realize they can still be taken. Just need to pick same shutter, ISO, temperature and dump them into Sequator with the original pictures.
Humidity and wind speed would also likely affect things too because of heat transfer so you would want those to pretty close to the same for the most reliable dark frame subtraction. However dark and bias frames are less important than light and flat frames so when given a choice I will always take more lights. Bias frames are really the least important but are also the easiest frames to capture and are not session dependent so can be done at any time so I have them.
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