Night time exposure

[tinyIsland]

Dear all - I have a question for which unfortunately I could not find much direction after research and experimenting. It is about long exposures at night and light metering. I am trying to learn how to determine the exposure length without over exposing your image. Say, I want to take a night sky photo (star trails) which includes a partially illuminated building, how do I determine the correct exposure time (for star trails to appear) while keeping the building without over exposing it. I guess if I meter light reflected from the building, I would not make it to have star trails, yet if I meter on the sky (black), I would get an over-exposed building.

The same applies for photos were trails of car lights are seen with buildings perfectly exposed.

Where do you meter or what technique am I missing?

Thanks - Matt
p.s. Using a Nikon D5000

 

[Patrice]

If the building is somewhat bright you can try blending two images, one for the building and a second for the star trails. Mask out the building from the star photo and layer it onto the building photo.

Exposure information, explanations and tips for both separate images are plentiful on this and other sites.

Good luck, show us your results if you can. Would be interesting.

 

[Garbz]

I see the words night-time, exposure, and stars. This is a dead giveaway that you want to start blending multiple exposures together. An example of what you want but with an extra step is http://www.garbz.com/stars.jpg This is a 1.5h exposure made up of 70 individual frames. The several frames were stacked first without any movement correction and became the background image (the campsite). Then all 70 were stacked together with alignment correction applied to eliminate the star trails. The two resulting frames were then blended together to make the final photo.

When doing startrails always go for multiple exposures. It's better for your camera, eliminates dead pixels, and reduces noise. The stacked images can also be blended together using a variety of techniques. One of them is the "Max Value" algorithm. This means each output pixel is the brightest of all the frames recorded. Based on what you said you're worried about the trails in the picture and I assume that the stars themselves are visible at the correct exposure? If so take repeated frames for the required exposure length to get the picture looking as bright as you want and separate each exposure by a few seconds.

/EDIT: Actually just thought of a better example: http://farm2.static.flickr.com/1353/773160897_fae20200db_b.jpg This picture is a 2h exposure made of 180x 30 second on 10 second off frames. This was taken only 5km from a city centre. If max value stacking weren't use it would be an almost white incredibly blown out mess.