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

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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()  

I explained/documented it over here.

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.

Links:
Instrutables low light multiple images

Answer 4

Edited because i re-read your question!:

You may well be able to produse a similar effect yes, However you may experience artifacting, IE the edges of clouds may repeat.