Question on DOF and long exposure night shots

bgarland

I am learning to use my new Canon 50D. Under normal lighting conditions I understand how the DOF is reduced at the low aperture settings. What I am confused about is when you take along exposure night shot at F2.8 how can the foreground subject be in focus as well as the distant stars in the night sky?

I focused this shot on the tree in the foreground and used a 30 sec exposure at F2.8 and ISO setting of 800. I used the f2.8 setting to bring in the most light possible with my 17-55 lens. In normal daylight conditions, I would expect the background to be totally fuzzed. Why do the background stars seem to be in focus under these conditions?

Thanks for any education you can provide to help me get my head around this one. :scratch

Tim Kamppinen

They're not in focus, but they're so small in the frame that you can't really tell the difference.

jfriend

bgarland wrote:

I am learning to use my new Canon 50D. Under normal lighting conditions I understand how the DOF is reduced at the low aperture settings. What I am confused about is when you take along exposure night shot at F2.8 how can the foreground subject be in focus as well as the distant stars in the night sky?

I focused this shot on the tree in the foreground and used a 30 sec exposure at F2.8 and ISO setting of 800. I used the f2.8 setting to bring in the most light possible with my 17-55 lens. In normal daylight conditions, I would expect the background to be totally fuzzed. Why do the background stars seem to be in focus under these conditions?

Thanks for any education you can provide to help me get my head around this one.

Actually, since you shot this at 17mm, you get a surprising amount of depth of field at 17mm, even at an aperture of f/2.8.

If you focus at 20 feet, you get everything in focus from roughly 9.4 feet to infinity. So, that's probably what you are seeing here. You can see the calculation for yourself in this DOF calculator.

The length of exposure doesn't change DOF at all.

bgarland

Great DOF calculator. Thanks for the link. And the learning continues.