# Multiple Exposure - Combining images in photoshop



## benjyman345 (Aug 2, 2008)

hello,

I am thinking of doing some really long exposures of the night sky to get star trails with my new DSLR. To do this I will put it on bulb mode and lock the shutter open.

The only limitation is that my digital camera bulb mode only goes for 8 minutes... so I was thinking after the 8 minute exposure I could take a second exposure for 8 minutes and then combine the images using photoshop. I am NOT talking about combining images to make a HDR image!

How do I combine the images in photoshop to make one single image?
I am fairly new to photoshop so I am looking for easy step by step instructions.

Thanks heaps!


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## reg (Aug 2, 2008)

If you're not talking about making an HDR, I have no idea what you're talking about.

Can you clarify?


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## benjyman345 (Aug 2, 2008)

reg said:


> If you're not talking about making an HDR, I have no idea what you're talking about.
> 
> Can you clarify?


 

For example if I want to take a 16 minute exposure of the night sky.

To do this you would normally set your camera to bulb mode and lock the shutter open for 16 minutes. 
The problem is my DSLR camera only goes up to 8 minute shutter speeds. So to achieve this 16 minute exposure I would lock the shutter open for 8 minutes then, take a second photo for 8 minutes (immediately after the first). I would then combine the two images to achieve the same results as one photo taken for 16 minutes.


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## Garbz (Aug 3, 2008)

Photoshop does not have a logical add option in it's layer blending modes. However never fear. http://www.tawbaware.com/imgstack.htm We have the technology.

For the record, ignoring camera limitations this is a great idea regardless since it totally eliminates long exposure noise from the image and greatly reduces thermal noise as the sensor is reset several times. The other big bonus is that program linked above includes the option of selecting the brightest pixel from each stack which means if you're doing startrails you effectively cut straight through the noise pollution.





120x 1min exposures combined and just 5km from the city. The noise pollution is equivalent of 1min of exposing, the star trails the equivalent of 2 hours.


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## benjyman345 (Aug 3, 2008)

Garbz said:


> 120x 1min exposures combined and just 5km from the city. The noise pollution is equivalent of 1min of exposing, the star trails the equivalent of 2 hours.


 
Thanks Heaps! Out of interest did you take all 120 1 minute exposures one after the other or was there a break between images? You must been busy pressing the shutter for 2 hours!!!!

I guess there must be some way of controlling your camera from the computer and scheduling it to take a 1 minute exposure one after the other....


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## RyanLilly (Aug 4, 2008)

benjyman345 said:


> ? You must been busy pressing the shutter for 2 hours!!!!



Bring along a case of beer and a couple of friends.


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## Garbz (Aug 4, 2008)

The D200 has a timelapse mode, but in this case I used a cable release with a timelapse mode. There are many options available, I have even seen an after market Canon 350D battery grip with a built in timelapse mode (or intervalometer if you want to call it what it is).

Yes computers via USB can control the cameras like this. The Nikon software is called Camera Control Pro. I don't know what the Canon equivalent is called but it does exist. That is assuming you have either a Nikon or a Canon of course 

And indeed RyanLilly's suggestion is very important. There's nothing more boring then sitting in nature for 2 hours babysitting something automated.


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## Bifurcator (Aug 4, 2008)

Well, you did a heck of a job! It looks pretty awesome! Musta been great beer!


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## Garbz (Aug 5, 2008)

Hahahah that was actually using a TV. Shot off my back porch and then went inside. After a movie knowing that my camera was probably well and truly out of battery given the sensor had been on for more than 2 hours all up I got that result. 

In retrospect a beer would have been nicer.:cheers:


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