Urban Night Photography: What's the Best Aperature?

Tom, I have found that with photos such as your example, the primary subject plane is usually at infinity focus so DoF is usually not an issue unless one shoots with a ridiculously fast lens wide open. For this reason, I usually shoot wide open (2.8 or whatever is available) on a cropped sensor digital camera.

I have also found that the perception of sharpness is dominated by many factors beyond intrinsic lens sharpness at aperture, such as point-source lights and edge detail. And since my objective is usually to capture "what the eye sees", scenes such as this are perfect candidates for layering bracketed shots (HDR). This not only brings out shadow details without noise penalty, it also offers control over highlights to increase perceived sharpness.

If only one shot is possible such as when hand-held, I will expose for highlights only and live with the noise in shadow details. To me, (unrecoverable) overexposed highlights is far more offensive than noise (which can, to some extent, be rescued with noise reduction).

When finally assembled, applying just the right amount of software sharpening will give the image the "natural" look.

Here's one, hand held single shot exposed for highlights, aperture wide open. It's a bit oversharpened from a time (5 years ago) before I understood finesse. Software tweaked to bring out shadows at the expense of noise.

Hi Michael,
The low light I'm mostly thinking of is too low to hand hold. I see you still had some sunset light in the example you show. Around sunset is of course a great time to shoot but not always convenient for me to do.
You bring out a point about shooting wide open I didn't really consider though, and that is by reducing shooting time is any deficiency in the sharpness of a lens more than compensated for by the shorter exposure time where a camera could shake a little even on a tripod, say from wind.
With pinpoint sources of light like lit windows at a distance, even a miniscule shake might blur them a little.
The shot I show above was taken at f/11, 30 seconds. It was actually taken with the Canon 18-55 kit lens (at 18mm, 100ISO) so the widest I could open that would be around f/3.5 I think, or a little over 3 stops. I could have exposed around 3 seconds at that aperture. There was a little wind coming at the camera on this shot. When looked at full size it's pretty sharp but not particularly so. Full size (100%) crop below.
I'm new to photography so I have't done that much experimenting yet. I have some old Nikkor prime lenses I want to try out also. I have to get a manual focus screen installed on the Canon XTi I use though, because it's hard to catch focus with them; and with the adapter I use, there's no guarentee that they won't focus past infinity, which I think a 35mm I tried does.

Michael, handheld with or without IS or VR?

Tom, I think it'll be near impossible to get this type of shot with one exposure. I see a couple of problems: First, in the particular example shown, the subject cityscape occupies about 1/4 of full frame, which in a 10MP camera means all the details will need to cram into a 2.5MP space placing more demand on perceived sharpness. Second, the dynamic range is tremendous from point source highlights to near black.

The lack of detail (mostly) isn't due to aperture setting or lens sharpness, rather overexposed highlights causing excessive blooming (spillover from sensor saturation). Your camera is capable of better detail but the overexposure causes the tower (for example, in the 100% crop) to appear as a blob of light.

The way to deal with this is shoot several frames exposed from highlights to shadows and blended in post-processing. The exact number of shots and shutter speed spacing will depend on the dynamic range of a particular scene but the objective is to preserve highlights without saturation. Here's a link to a 3-shot blended example which you can see, much of the highlights are still blown, but where the blending preserves highlights, the apparent sharpness is substantially increased:
http://www.picturecorrect.com/wallpaper/new_york/new_york_1680x1050.htm

Another issue is, in-camera noise reduction should involve only dark-frame subtraction, but some cameras will apply internal processing to further reduce noise often at the detriment of detail. If you can, I would suggest turning in-camera noise reduction off and do a dark-frame subtraction manually.

By the way, I don't see any motion blur caused by the wind disturbing the camera, and the distances are too short for atmospheric disturbances to be a factor.

Frank, that picture was shot 5 years ago with an ancient Sony DSC-F505V from year 2000. There was no VR/IS at the time.

Tom, here's an example of manually blended exposure - not very well done but illustrates the point. The comment section describes how it was made.
http://photo.net/photodb/photo?photo_id=4839363&size=lg

As you indicate in your initial criterial, I think f 5.6 to f6.7 is totally reasonable for most night shots-reasonably sharp but shortens the exposure and reduces noise. An exception would be if you want car headlight/taillight trails.

Ya, I have been shooting with in-camera noise reduction off. I'm going to try that HDR.

Will shooting wide-open make coma worse?

Ultimately, you'd probably want to "test" your lens to see which settings work best anyways.