Star trails with Digital or Film?

[KirkS]

Back in the 80's, I lived in Northern Idaho, and loved doing star trail photos. Back then I was using a Canon AE-1 program, and actually enjoyed spending the hours it would take to get a decent image.

Fast forward to 2012. I still have the AE-1p, and a EOS 620, and I'm thinking of getting a digital. I would like to do star trails again, along with portraits, nature, and action (live on the Gulf of Mexico, plenty to see/shoot). My EOS 620 will only do 120sec on bulb without the tech back e (more difficult to find then the Holy Grail) and the remote.

My question is, can you do 2-4 hour long exposures with a digital camera (I'm thinking used Canon xti or similar priced), or do you have to do multiple 30sec images and then layer them in PS or startrails? There is part of me that is a purest, and IMHO, a layered and manipulated image looses it's 'photograph' namesake, and becomes more of a cg image (which are fine).

 

[fokker]

The xti or any similar model can take an exposure for as long as the battery lasts - should do 4 hours pretty easily I would think. You'll need the remote for it, but those are cheap if you get the off-brand ones off ebay.
One thing to be aware of is the in-camera noise reduction. This feature will take a second exposure of the same duration immediately after the first one, only with the shutter closed so it will give a 'black' exposure, the idea being that the noise pattern (caused by heat buildup in the sensor over a long exposure) is the same and can be subtracted. If you don't use this feature you can get some pretty bad noise at times, even at ISO100.

 

[KmH]

Most digitals won't do 1- 2 hour long exposure, because the image sensor overheats.

Among astrophotographers image stacking shorter exposures is now the preferred method, even when using cooled image sensor astro cameras. Parsec 10100C Color Astronomical Imaging Camera | Orion Telescopes

 

[fokker]

Keith I've never heard of a sensor overheating (not doubting you, just never heard of it), what happens in that situation? Are you meaning in the sense that the noise just becomes too bad, or it actually damages the sensor or shuts itself off?

 

[KirkS]

^ what he said. If there is a chance of damage, I'll hold onto the AE-1....

 

[KmH]

Some digital cameras have a time limit on long exposures and doing video. The time limit on video is for the same reason, though when doing video the image sensor is turned on and off 25 times a second, or more.

Some users manuals mention that the camera can get hot during long exposures.

One of the main benefits to a having a digital camera that has a magnesium metal-alloy chassis is the big improvement in heat disipation.

An image sensor has electrical resistance from the vast number of transisitors that are on it. Electrical resistance generates heat.

 

[Derrel]

Keith I've never heard of a sensor overheating (not doubting you, just never heard of it), what happens in that situation? Are you meaning in the sense that the noise just becomes too bad, or it actually damages the sensor or shuts itself off?

WHat often happens is something called "amp noise"...quite often, sensors will show fairly large areas of digital noise due to heat buildup, often on one side of the sensor. For example, a comment from Thom Hogan's D7000 review: "Let's start with amp noise: very good. It appears Nikon has finally managed to get rid of the strong amp noise patterns that have plagued previous consumer cameras. (Note: the D7000 has an apparent brightness pattern at the bottom of the color LCD. This appears to be the display backlighting being overdriven just to the center bottom edge of the display. This is not amp noise, though on dark exposures you may see a brightness pattern at the bottom of the display that looks a bit like it on image playback. The images don't actually have that brightness differentiation in them. It's solely a cosmetic issue with the way the color LCD lights.) The D7000 still has some amp noise, but if so, it's more evenly spread over the frame now, which is the way it should be."

Hogan's reviews of d-slrs have always had a test of amp noise over long exposure times. many reviewers never even try a long exposure, and so the subject of amp noise during time exposures often is not brought up.