Is wide aperture required when shooting faraway objects in low light conditions?

Question

Assuming the following:

• the subject being shot is lit rather poorly
• no flash because the subject is far away
• the subject being shot doesn't move
• the camera is on tripod and doesn't shake
• no other objects in the field of view move
• the subject being shot is far enough to always fit into the depth of field so no need to care of bokeh

it looks like I can vary aperture and shutter speed however I want as long as their combination provides enough exposure of the sensor to the light. So I can do either of the following:

• make aperture wider and make exposure time shorter or
• make aperture narrower and make exposure time longer

with the only restriction that the number of photons hitting the sensor remains the same.

Do I prefer the widest aperture possible or do I consider anything else? Does having poor lighting have any implications on this?

Answer 1

As you hinted, the number of photons per pixel per exposure depends linearly on exposure time and also linearly on lens area, which goes as the square of the F-stop.

How you optimize depends on what you're willing to trade. Long exposure means risk of blurring as well as increased electronics noise. Wide aperture means somewhat reduced depth of field.

Further, in digital cameras, the pseudo-ISO setting affects the analog electronic gain, which can affect the electronic noise component.

That's pretty much your trade space. Shoot various combinations and pick the result you like.

Answer 2

You're correct in saying that you can pick any trade-off of shutter speed and aperture size that you want, so long as you obtain the same exposure (or as you say, number of photons hitting the sensor). In practice, your choice of aperture will depend primarily on one thing: your desired depth of field.

You will obtain higher quality images by reducing the ISO (which is essentially a digital multiplicative factor; it increases noise as well as signal), assuming there is no motion blur and the shot is well focused.

If you don't care about the bokeh, or depth of field, and you also don't care about having to wait 30 seconds for every exposure, then you should shoot at the aperture which produces the sharpest images with your lens, and at the minimum ISO available. Data on lens sharpness at various apertures can be found at DxOMark.

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For instance, here is the sharpness data for my D7000 + Nikkor AF-S 50mm f/1.8 G:

This tells me that the sharpest images (taking into account everything in the frame in some sensible manner; edges are invariably less sharp) are produced at about f/4.5. This is the case for almost all lenses: they aren't too sharp at their maximum aperture. You usually should go for an f number somewhere in the middle of the lens' range for maximum sharpness. The reasons for this are a bit technical so I won't go into that.

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Bottom line:

• Does shutter speed matter to you? Do you want to be waiting 30 seconds for each exposure?
• Does depth of field matter to you? Do you want to isolate your subject using bokeh?
• Lower ISO will produce less noisy photos.

Consider these factors in combination with the lens sharpness. If you have a lens worth US $1000+ then lens sharpness is probably a non-issue. If you have a kit lens, then it will matter more. Especially if it's a zoom-- in that case, focal length affects it too.