night sky photos

hi

i was photographing stars with my wide angle lens

i would put a tree in the foreground with the sky in the background

basically the longer the exposure ( around 30 seconds) the starts would be brighter in picture. a shorter exposure the starts were barely noticable.

But with a longer exposure the tree would still be sharp but the stars would not. probably because the earth is moving.

is there any way around this?

luke0355@aol.com

You can use higher ISO and larger aperture to make sure the stars show, and possibly light the tree with a strobe or "paint" it with a flashlight.

Guy

They actually make a device to mount your camera on that slowly moves as the same rate as the stars. That way, there are no star trails. That would solve that part of your problem, but I suppose you'd have a new one of a tree that looked like it was slowly moving.


Kent in SD

You can also work with the rule of thumb that to freeze star movement, you need a shutter speed that's the inverse of your focal length. I.e., with a 20mm lens, any shutter speed slower than 1/20th of a second is going to start showing star movement and blurring. With a 500mm lens, star movement would be apparent at shutter speeds slower than 1/500th of a second. Once you pick your shutter speed, you would use a combo of aperture and ISO to determine brightness of the rest of the scene.

Luke, I'm assuming you're using a tripod here, so I have only one suggestion - try using ISO 400. I've found through my own experience that 400 is the most optimum setting for exposure, sharpness and low noise.

The length of an exposure you can use without trailing depends on the focal length of the lens and the position in the sky that you are shooting. With a normal lens, you can shoot stars at the celestial equator for about 15 to 20 seconds without noticable trailing. The circumpolar stars can be exposed for nearly 45 seconds before trailing occurs.

The mounting that someone mentioned is based on an "equatorial mounting" commonly used for telescopes. The base of the mounting head pivots and you adjust it to the latitude of your location. Then the whole untit is aligned to true north. Naturally, the longer the lens, the more painstaking one must be in the polar alignment. Amateur telescope making websites are loaded with plans on how to make such a mounting.

Rick

You can also work with the rule of thumb that to freeze star movement, you need a shutter speed that's the inverse of your focal length.

Hank, are you sure? I heard of a "rule of thumb", that slowest shutter speed before stars show trail is 600/focal leinght in seconds. I haven't photograph stars for some time, but if my memory is worth it's name (probably not), it worked.

Marko

You got me Marko. It's been a while since I've done it too, and I was dredging from memory. Shows the old thinker isn't as sharp as it should be these days! I went back and checked my notes, and you're right. Thanks for the head's up, because I was planning some night shots!