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What do I lose by exposing subs beyond 1/3 from the left?

Astrophotography Beginner DSLR Imaging
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#1 Snow2040


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Posted 16 April 2024 - 05:54 PM

Shooting with a Canon rebel t7/2000D with a Samyang 85mm f/1.4 lens @f/2 and a ZWO duoband filter from bortle 8 even a 30 second exposure at ISO 800 with a 50% illuminated moon has the histogram at 1/2 over from the left. I want to ask, what do I lose by doing this over lowering the exposure length to have the histogram 1/3 or 1/4 from the left?

#2 TXLS99



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Posted 16 April 2024 - 06:37 PM

With a lens that fast (f/1.4) you are sucking up a ton of photons very quickly.  When exposing long enough that your histogram is 1/2 way over you are risking saturating multiple pixels and washing out stars as well as adding more gradients (light pollution) to each exposure.

Edited by TXLS99, 16 April 2024 - 06:38 PM.

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#3 BQ Octantis

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Posted 16 April 2024 - 06:52 PM






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#4 Astrola72


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Posted 16 April 2024 - 07:32 PM

As the topic referred to by BQ above shows, "It depends". Read the whole thread. It's worth the time.



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#5 bobzeq25



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Posted 16 April 2024 - 07:47 PM

You're losing a bit of dynamic range.  The right hand part of the histogram starts to saturate faster.


Perhaps more interesting.  A pretty good theoretician who used to be here challenged people to demonstrate that 1/3 over produced a better image than having a smaller gap.


He had no takers.


Bottom line.  If you're going to make an error make it by going further left, not further right.  As stated in the thread above, 20-25% is probably a better guideline for modern cameras.  It's what I use with my Nikon D5500.


But the REALLY important thing is that it's not all that important.  Total imaging time nearly always drives the train, NOT subexposure.  People here are routinely surprised by how good short subs are, if you shoot enough of them.


This is not at all like terrestrial photography.  The best way to capture dim detail is not longer subs, it's total imaging time.


Given that 30 seconds puts your histogram 50% over, 15-20 seconds would no doubt produce better images.


But it's not all that important.  As in:


It could be argued whether 240 X 15" would be better or worse than than 120 X 30".  But the IMPORTANT thing is that EITHER 480 X 15" OR 240 X 30" would beat EITHER of the first two alternatives.


Hands down.  It's not a close call.  <smile>


Shoot more subs.

Edited by bobzeq25, 16 April 2024 - 07:55 PM.

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