Apertures for nightshots

[westy79]

I read somewhere that a large aperture does not affect dof so much when shooting a night scene at say 18mm. This confused me a little as i want to take some city shots but i thought for max sharpness and detail a mid f. number would be required. I understand that a wide open aperture lets in lots more light but surely the dof would be shallow. Plus on a tripod having f.11 say wont be an issue because the camera will be stable regardless of exposure time.

I also read a photography magazine where a top landscape photographer used a fast lens of 2.8 aperture. Again why is this the case when you want the whole shot in focus, thats what i keep hearing anyway.

Hope you guys can clear this up, you usually do :)

[herc182]

You would use a wide aperture if you are taking portraits or shots of people in low light conditions (so you can hand hold a shot).

for landscapes, a higher f number would be more appropriate since, as you rightly point out, the DOF would be shallow. On a tripod it wouldnt matter.

Something worth bearing in mind is that for night shots, the higher the f number (i.e the smaller the hole), the more "starry" lights will look. It get progressively more pronounced with a higher f number.

[K1W1]

I read somewhere that a large aperture does not affect dof so much when shooting a night scene at say 18mm.

Think this through.
DOF is the relationship between the aperture (the actual opening that lets light in) and the focal length of the lens.
Where is that statement does it mention ambient light?

The reason a large aperture appears to not have such an effect on DOF at night is that most of the image will be dark so how will you be able to see whether or not it's in focus?

[TheWengler]

The aperture affects the DOF the same. The wide focal length will give you more DOF. People use wide apertures for pictures of stars sometimes to help avoid star trails. Plus large DOF isn't used for every landscape, just most of them.

The SS doesn't matter too much once you pass a certain point, but when you're considering the difference in DOF between f/8 and f/11 vs standing outside waiting 10 min vs 20 min for an exposure, I'll usually take the f/8 so I can move on to the next shot. If your SS is below a minute or two I'd just use base ISO and the same aperture I would if it were daytime.

Of course with city lights you have the star effect to consider as herc mentioned.

Here are a couple recent shots of a harbor
Pillar Point Harbor 1 (http://www.flickr.com/photos/lswenger/3863557503/) - This one has a longer exposure and a wider aperture because it was taken later in the evening and there's no moonlight.
Pillar Point Harbor 2 (http://www.flickr.com/photos/lswenger/3877485450/) - I was able to stop down more in this one due to the extra light from the moon.

With my camera, the time I have to wait is doubled with long exposures. The extra time is for noise reduction I think. I assume all cameras work this way, but I've never tried it with any other cameras. So 5 minute exposure = 10 minutes of waiting.

[herc182]

With my camera, the time I have to wait is doubled with long exposures. The extra time is for noise reduction I think. I assume all cameras work this way, but I've never tried it with any other cameras. So 5 minute exposure = 10 minutes of waiting.

I think I have switched this off. Apply noise reduction afterwards...or I end up freezing my tits off waiting for the camera to process :)

[K1W1]

With my camera, the time I have to wait is doubled with long exposures. The extra time is for noise reduction I think. I assume all cameras work this way, but I've never tried it with any other cameras. So 5 minute exposure = 10 minutes of waiting.

With Nikons it's the same. If you have long exposure NR turned on you effectively double the processing time for each shot. The question pops up from time to time when people ask why they can only get half the stated fps rate from their cameras.
On Nikons I think that they recommend that the long exposure NR is only turned on for exposures of 1 minute or longer.

[Dread Pirate Roberts]

I would turn NR back on Herc as it's different from PP NR. (I have it set on for >10 sec exposures)

The cam takes a second shot with the shutter closed and subtracts the values from the first shot, essentially removing any hotter pixels.

I'm sure you knew it as your landscapes are leagues ahead of mine but I had to type anyway.

[achuang]

I'd keep the long exposure NR on for anything over say 30 seconds as it's for reducing hot pixels and amp noise (the purple noise around the edges which is worse on some cameras). I've tried a long exposure without NR on an exposure of a couple minutes on my D70s and there was plenty of purple.