long exposure shots

[Scarecrow]

Ok Im trying to do some long exposure shots at night. I have an awesome tripod and a shutter release switch. Now that I have all the eccentials what else goes into long exposure shots. I shooting with a 5D mk I using a 24-70mm L 2.8. Do I leave the F/ at 2.8 or boost it up.

 

[480sparky]

"Boost it up"?

What you use for an aperture depends on your ISO setting and how long you want the shutter to remain open. If you want to capture long headlight/taillight streaks, you may need 10-30 seconds. For star trails, you may need 2-6 hours.

 

[Scarecrow]

Boost it up...Im scratchin my head trying to figure out where I came up with that. I normally shoot in AV mode. Should I continue with AV mode and what ISO would you suggest?

 

[JG_Coleman]

"Boost it up"?

What you use for an aperture depends on your ISO setting and how long you want the shutter to remain open. If you want to capture long headlight/taillight streaks, you may need 10-30 seconds. For star trails, you may need 2-6 hours.

Yeah, aperture will vary depending on the desired shutter speed. Given equal ISO settings, using f/2.8 will certainly keep your shutter speeds as low as possible... but consider whether or not your subject, and thus the ultimate quality of your photograph, will be negatively impacted by such a limited depth-of-field. For example, I tend to use ISOs between 200 and 800 and apertures somewhere between f/7 and f/11 for long-exposure night shots to capture lightning. But, depending on what exactly I'm trying to accomplish and how much time I'm willing to put into one exposure, these settings can sometimes vary considerably.

You have a whole range of flexibility between varying aperture and ISO and factoring in how long you want to have your camera sitting there exposing.

One helpful tip: sometimes a certain degree of guesswork can go into deciding how long you really ought to expose with a given ISO and aperture. I will oftentimes take a few test shots using big apertures and high ISOs for very fast exposure. Once I find a series of settings that produce a good exposure, I'll quickly calculate how long I'll have to extend my shutter speed in order to get that same exposure with a smaller aperture or lower ISO setting.

This takes a bit of the guessing and luck out of the process. When I first started doing night-time long exposures, it wasn't uncommon for me to let my camera expose for a half-hour only to find that I hadn't collected enough light to get much more than a black frame. Likewise, sometimes I'd let my camera expose for 30 minutes and wind up with a blown-out image that was almost pure white. It can be frustrating to waste so much time for a picture that's going to get a one-way ticket to the Recycle Bin.