Can 30 1-second exposures be combined to a form the equivalent of a single 30s exposure?

Question

I need to take 30 1s exposure shots in raw at night. The image becomes ALMOST black... Is it possible to take this tiny amount of light/information and sum them all toghether and get as a result a single image that equals a longer expoaure? I am not talking about brightening an image, I am talking about summing the single light information of the pictures together. Would that be possible in photoshop? Would the result be any good?

Answer 1

Fundamentally you're capturing the same amount of light in either case so the results should be the same.

Practically, there are 2 differences between stacking 30 one second exposures and shooting one 30 second exposure. The first is the light lost between each one second exposure after the shutter closes before it reopens for the next exposure. This can cause problems with light trails of fast moving objects (such as cars), but is otherwise unlikely to cause significant problems.

The second is that doing 30 one second exposures requires 30 reads from the sensor, so you will get 30x the read noise (errors that are picked up as the signal is read off the sensor before it is converted to a digital value).

The fact that you state each exposure is "almost black" suggests that this could be significant problem, if each individual exposure isn't bright enough to rise above the read noise then your image stack will just contain noise, whereas a 30 second exposure would be brighter than the read noise.

There are some things you can do in this situation, push the ISO up as high as it will go (disregard what you have been told about increasing ISO causing noise, it doesn't apply in this case). Read noise occurs after ISO amplification, so amplifying the signal raises it above the read noise.

You could switch to a camera with lower read noise, the latest sensors from Sony (which have found their way into many camera brands) are the leaders in ultra-low read noise.

Finally you could increase the brightness of each exposure by using a wider aperture lens, or switching to say, 15 two-second exposures, or 10 three-second exposures.

Answer 2

I didn't have enough "reputation" to address some of these answers as comments.

AJ Henderson is wrong, 30 1s exposures will (for the most part) be identical to 1 30s exposure. If it shows up in a 30s exposure, then stacking 30 1s exposures will also show it.

I am actually the author of the article that Trengot linked (thanks!). In fact, unless you are specifically concerned about fast-moving light trails, as already mentioned, then doing 30 1s exposures is actually a better method of doing a long exposure.

Why? Partially due to what Matt Grum already mentioned - each exposure is a different read. What does that mean? Well, excepting the case of hot-pixels, as noisy as each read may be, it is random in nature.

This means that if you do a mean or median stacking on the 30 images, you will be denoising the image significantly. In some cases you'll be able to push the noise in the image below what you could ever capture with the camera in a single shot.

Of course, there are practical concerns to doing this, but that's not the question.

Of note, this is something that astrophotographers do all the time, and for good reason.

Answer 3

This explains how to merge multiple short exposures to mimic the effect of a longer exposure. It's aimed at emulating ND filter photos but the principal should be the same.

The basic premise is to take multiple shorter shots and then use a tool like Hugin to align and ImageMagick to convert them into in a single image. The result is effectively the same as a single long exposure.

The main commands from the linked blog are

C:\Program Files\Hugin\bin\align_image_stack -a OUT FILE1 FILE2 FILE3

to align the images (eg if the tripod wasn't quite stable) then using

convert *.tif -evaluate-sequence mean -alpha off OUT.tif

to create the single image.

Answer 4

No, 30 one second exposures is not equivalent to a single 30 second exposure. You do gain a lot of information from doing 30 1 second exposures, but you are not able to detect anything that is too faint (which might have shown up on a 30 second exposure, but still registers as 0 on the 1 second exposures). It is a good technique to avoid noise, but does not produce the same overall exposure as a 30 second single exposure.

Answer 5

Yes it is possible. Put all images as layers and choose Add to on every layer in Photoshop.

For night sky images, there is a software called deepskystacker. DeepSkyStacker is a freeware for astrophotographers that simplifies all the pre-processing steps of deep sky pictures.

http://deepskystacker.free.fr/english/index.html

This image is stacked from 4 images in photoshop. Every image was exposed 30 seconds. https://www.flickr.com/photos/schau-mal-einer-an/8723312050/

Update: A nice itroduction to photo stacking: http://www.shutterphoto.net/article/photo-stacking-and-long-exposures-part-1-introduction/

Answer 6

i don't think it is possible. amount of light on every photo is the same, it means no additional information in details you can add, so it's like you have only one photo