# Stacking - need help

Started Jan 7, 2013 | Discussions thread
Re: Stacking - need help

DuncanDovovan wrote:

BlueFightingCat wrote:

I recently discovered that I can stack several exposures of the same starscape to reduce the effective ISO. Essentially I just want to get a photo of the stars (not star trails) with as little noise as possible.

Scenario: I take 32 exposures at 30 seconds each with ISO 6400 and F2.8.

Questions:

1. Somebody said that by taking 32 exposures at ISO 6400 and stacking the images I would end up with an effective ISO of 400. Can somebody explain the math behind that. I don't quite understand the relationship. Likewise the same person said if I took 128 exposures I would get an effective ISO of 100.

2. How does stacking work taking into account the earth's spin. If I take 32 images with a static foreground won't the stars be in a slightly different location in each image compared to the foreground? Does the stacking software take care of this?

BFC

1) Math =

1 times 480 sec @ ISO 400 =
1*2=2 times 240 sec @ ISO 800 =
2*2=4 times 120 sec @ ISO 1600 =
4*2=8 times 60 sec @ ISO 3200 =
8*2=16 times 30 sec @ ISO 6400

But this would not be entirely true, because the higher the ISO, the more worse noise becomes (not linear). Therefore to get that rule of thumb, you double all:

1 times 480 sec @ ISO 400 =
4 times 240 sec @ ISO 800 =
8 times 120 sec @ ISO 1600 =
16 times 60 sec @ ISO 3200 =
32 times 30 sec @ ISO 6400

1 times 480 sec @ ISO 400 =
4 times 240 sec @ ISO 800 =
8 times 120 sec @ ISO 1600 =
16 times 60 sec @ ISO 3200 =
32 times 30 sec @ ISO 6400 =
64 times 60 sec @ ISO 3200 =
128 times 30 sec @ ISO 6400

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I'm a peaceful person looking for clarity and harmony and willing to help. If you believe my post had the intention to harm you in some way, you are probably wrong. =;-) Please verify first before assuming something bad happened to you. I can be a bit too brief at times. =;-)

There is a mistake in the 2nd table here is the corrected version:

1) Math =

1 times 480 sec @ ISO 400 =
1*2=2 times 240 sec @ ISO 800 =
2*2=4 times 120 sec @ ISO 1600 =
4*2=8 times 60 sec @ ISO 3200 =
8*2=16 times 30 sec @ ISO 6400

But this would not be entirely true, because the higher the ISO, the more worse noise becomes (not linear). Therefore to get that rule of thumb, you double all:

1 times 480 sec @ ISO 400 =
4 times 240 sec @ ISO 800 =
8 times 120 sec @ ISO 1600 =
16 times 60 sec @ ISO 3200 =
32 times 30 sec @ ISO 6400

1 times 480 sec @ ISO 100 =
4 times 240 sec @ ISO 200 =
8 times 120 sec @ ISO 400 =
16 times 60 sec @ ISO 800 =
32 times 30 sec @ ISO 1600 =
64 times 60 sec @ ISO 3200 =
128 times 30 sec @ ISO 6400

-- hide signature --

I'm a peaceful person looking for clarity and harmony and willing to help. If you believe my post had the intention to harm you in some way, you are probably wrong. =;-) Please verify first before assuming something bad happened to you. I can be a bit too brief at times. =;-)

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