Can stacking multiple exposure images in post-processing give the same effect as a long exposure?

I like to shoot daytime long exposures, where the exposure time is around two minutes. That shows the motion of the clouds in daylight.

I used a Hoya nd400 (which is 9 stops) stacked together with a cpl (around 1 or 2 more stops down) but i couldn't get the 2 minute exposure. So i would like to know can I get a final image with the same result as 2 minutes exposure, by taking multiple exposures and stacking them together in photoshop.

For example, I could shoot 6 images which have 20 seconds exposure each (a total of of 2 minutes exposure) and stack them together in Photoshop.

I don't know much about Photoshop — how would I go about doing it? I want to get same effect of that 2 minute exposure photo by stacking multiple exposure shots with stacking or blending layers — but, how?

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Similar effects yes. Unavoidably different though without special magic. If you just add images you are adding signal and background in the same area. Your processing must differentiate either in space or between images to emphasise the content you are interested in. Take the star trail example as a more clear cut one. If you WANT the star trails then you need to overcome the addition of starless noise to starlight where the trail appears. But if you want the background (why I know not) the star trails are noise that needs dealing with.WEither way you need "differential" processing. – Russell McMahon Sep 25 '12 at 23:19

I am thinking that there might be two ways to do this? I don't have photoshop so...Maybe I'll try python.

Hopefully you used a tripod, a remote, and continuous shooting. I could see variable and long times between images very problematic.

I have never done this, just thinking out loud. I'll have to take a time series and try.

I think mathematically these are the same:
1. Stack all the photos as layers, giving the same transparency to all of them?
trans=100/(# of images), except first image = 1
2. Take the same pixel from each image and average them?

[Update]

Here is a python script to calculate an averaged image from a series of images:

``````from PIL import Image
import glob
import numpy as np

imgList = glob.glob('./*.png')
first = True

for img in imgList:
temp = np.asarray(Image.open(img))
temp = temp.astype('float')
if first:
sumImage = temp
first = False
else:
sumImage = sumImage + temp

avgArray = sumImage/len(imgList)
avgImg = Image.fromarray(avgArray.astype('uint8'))
avgImg.show()
``````

Example: Balls falling, rendered with blender, averaged at different sample rates with the above python code.

[Update 9/26/2012]

I stumbled upon a nice NASA website that has nice image series. I used a set of their pictures (of clouds!) and the above code to generate this:

Images courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center.

More examples and details over here.

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ofcourse bro..i've used tripod,a cable release,stacking nd400 and cpl,(but can't get2mins exposure@day time)..i tried photoshop by blending layers and adjusted transparency but seems to be not change alot(maybe i'm just nervous)..anyhow i haven't try yet to get long exposure by using welding glass.:P..i've saw that tachnic from 1wessite...thanks alot bro. – judass Sep 25 '12 at 18:30
@judass I wrote a python script and added it above. – Onlyjus Sep 26 '12 at 1:46

Yes this is possible.

Image stacking is also commonly used in star trail photography. Instead of taking a single hour long frame you could take one-hundred & twenty 30sec frames and stack them to get the same effect.

Take a look at the website below for a tutorial on how to stack images for a star trail. It is the same process to stack any image.

Automated Stacking of Star Trails in PS CS5

They even give you a nice photoshop action that does a lot of the work for you. I have personally used this tutorial numerous times with great results.

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thanks alot for your answer bro..i'll try again and for the startrail,i think i can't able to shoot as i live in singapore,in singapore i've never seen the starry sky.i've tried to take startrail but not success,i took 3mins exposure of 20frames to stack,can't see anything.i'll try again,thanks alot for ur answer bro. – judass Sep 25 '12 at 18:35
Yeah I have the same problem in Chicago. Maybe star trails aren't possible, but the same workflow should work for any image stacking. – CyberKnoy08 Sep 25 '12 at 20:04
That image stacking algorithm might be specifically written to "enhance" the star trails. It might not work for all occasions. – Onlyjus Sep 27 '12 at 1:47

Yes, you can. I have never tired it for clouds but I have for water falls. My usually way of operating is to use my in camera multiple exposure but doing it in Photoshop works too.

For in camera set your camera to do n multiple exposures and allow it to figure out the auto gain. Shoot n pictures. Done

For in Photoshop take your n pictures as normal. Then bring them all into Photoshop as layers in a single image. Execute the command Edit | Auto Align Layers. This will line up your images to compensate for any variation. Then blend each layer using Normal mode but a lower opacity. This is where some experimenting will need to be done but you can find a opacity that makes things look good. Usually you want that to be the same for all the layers. Start with setting all the upper layers to 100 / n percent opacity and go from there.

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thanks alot bro for your advise.. – judass Sep 25 '12 at 18:20
Due to the way photoshop blending works to average the images you need to set the base later to 100% opacity, then the next layer on top of that to 50%, then the next layer to 33% then 25% etc. photoshop extended allows you to directly average an image stack (or do median filtering, which can be more effective). – Matt Grum Sep 25 '12 at 19:48