5D mark III not recording picture after long exposure

[Joseph]

I've been trying to do star trail pictures the last few days. Every time I do, the batteries are almost dead by the time I turn it off, which isn't surprising, but there's no picture. I have a battery grip so I should have twice as much battery power as just one battery. Is there just not enough battery to record a picture after 6+ hours? I don't thing that's it because I can still press the playback button to look at the pictures to see there's no new picture. Why is this happening

[ScottyP]

Are you certain you are doing it right and actually taking a picture at all? Try taking a shorter exposure and see if it is taking a picture.

[Joseph]

Yes, I know the shutter is actually open, the camera is on, theres a card in the camera, the counter on the top screen starts counting seconds when I hit the cable release, ect.. At one point I went outside again to look and the counter showed "---" If that means anything. 
6 hours might just be too long. I'll try shorter exposures.

[hsbn]

You should try to do multiple exposures at short duration (like 5min) then stack them later in post. It will help keep to noise down also. If the battery runs out before the exposure time end, it will not take photo from what I understand.

Check out this link: http://blog.starcircleacademy.com/startrails/

[bratkinson]

I'm surprised your camera didn't shut down due to overheated sensor with an exposure that long.

As indicated above, multiple exposures and stacking is the preferred solution.

Also, if you are shooting in JPG, turn OFF the automatic noise reduction. Leaving that on will cause the camera to do extensive processing after the shutter is closed. And if it doesn't get completed, there is no end product....count = 0.

[TCampbell]

Ahh... star trails aren't made in a single exposure.  If you do that, the sky will almost certainly wash out and become mud due to light pollution.

It's normally a LOT of 30 second exposures that you merge together.

Here's a video:  http://www.youtube.com/watch?v=V6ypRbPzoPM

Incidentally, Canon makes an AC adapter ACK-E6 which will let the camera run off AC power (if you have AC power available where you plan to shoot.).  This is how I run my 60Da (which takes the same adapter as the 5D III) when doing astro-photography (which typically means the camera is taking frames for hours and hours and would normally just kill the batteries.