Wow!

[molested_cow]

I was looking for information on long exposure techniques for DSLR and found this:

Pigeon Point Lighthouse | Flickr - Photo Sharing!

The photographer also explained how he took the shot.

Anyways, I got two questions on long exposures for digital:

1. Is stacking more favorable to single-shot? I can see the benefit of stacking beyond just noise, just wondering how long is too long for a DSLR for single-shot exposure.

2. Is noise reduction necessary? It takes as long as the exposure to process, which means I cannot do stacking technique. What's the right way to do it if I want to have 30min exposures without having to wait 30 more minutes and risking the battery life?

 

[kundalini]

Nice shot of the lighthouse.

The one thing you should be concerned with is heat build-up on the sensor. Long enough of an exposure and you can start frying pixels (photocells). It's less of a concern during cooler ambient temperatures, but still needs to be taken into consideration. I would tend to use several short exposures, say 10 minutes each, and stack them.

 

[molested_cow]

So if I want to get a total of 30min exposure and I do 3x10 min continuous shots, won't that heat up the sensor just as much as doing a single 30min shot?

 

[Garbz]

There's a few things at play here. Firstly long exposure noise reduction works really well for getting rid hot pixels which appear rather consistently for given ISOs and shutter speeds. You can take care of this in the stacking process with some programs which will let you shoot a "Dark Frame" which does the same thing, i.e. subtract the hot spots from the final exposure.

In terms of heat you will heat up the sensor just as much, however you don't need to shoot 3x10min continuous with no break. What we do for nighttime shots is something like 30second on 5 seconds off and that dramatically reduces the amount of heat the sensor produces and in extreme cases may save yourself from turning your camera into a heavy brick. How long is too long? I wish someone had data for this. I have seen a 350D which was toasted from an exposure somewhere in the quarter of an hour range, yet I know others who have had no problems shooting a 45min exposure with the same model camera. I have yet to find any hard data on how long is too long, suffice to say that after 20min my camera gets quite warm through the side of the case meaning the sensor is probably very hot.

Stacking is great because between photos the noise is a random gaussian distribution meaning that as you stack multiple noisy frames, the average result of each pixel will tend to eliminate the noise at the expense of one or 2 bits of data. However the REAL benefit comes from statistical games you can play. In astro photography noise is actually your biggest friend as you can use noise to extract details that would otherwise be lost. If you take an ISO1600 exposure of the sky it will look like garbage, but be highly sensitive to light. You won't see anything. However as you take multiple of these exposures, the point where light hits the sensor will cause that pixel to statistically stand out from the rest. By the time you stack 30-60 such exposures the noise will be completely gone and you're left with pixels that would never have shown up in ISO100 shots.

By the way if you're interested in trying some of this look for a free program called "Deep Sky Image Stacker"

 

[molested_cow]

I think I know what you mean.

Another question.
If I do multiple shots for stacking, should each shot be exposed properly or should each shot be under exposed? I guess my question is, is the exposure something that accumulates when stacking, or does the stacking just include the difference between each shot?

 

[tirediron]

F.I.L.M.

 

[molested_cow]

That's what I've been doing for the past 10years until last week.

 

[Garbz]

I think I know what you mean.

Another question.
If I do multiple shots for stacking, should each shot be exposed properly or should each shot be under exposed? I guess my question is, is the exposure something that accumulates when stacking, or does the stacking just include the difference between each shot?

Depends entirely on the algorithm of the stacking program. I suggest you download one and read the manual. They can either add exposures, average exposures, or average exposures into a 32bit file and then adjust curves to suit a final output. Each requires a different input image.

 

[KmH]

F.I.L.M.

Reciprocity failure.

On the image sensor heat issue, ambient air temperature is a factor.

Astronomical image sensors get cooled in various ways so they can do exposures as long as 8 hours at a time.

 

[Trever1t]

I live ~ an hour from this.