Back yard at night: 1.3 seconds, f/1.4, at ISO 100.
In your DSLR, the mirror flaps up and down every time you take a photo. This shakes the camera, ever so slightly, even when you are using a tripod. When you shoot at 1/500th of a second, you will never notice this. Also, if you shoot 10 second exposures, the slight vibration for a fraction of a second at the start will not likely show. But if you take photos with a shutter speed between, say, 1/15 sec and 1 sec, you may well notice this, since the “shake time” is significant with respect to the “shoot time”.
To avoid this, many cameras offer a “mirror lockup” function. This allows you to make the picture in two pushes: one to put the mirror in the “up” position; and the second, to actually release the shutter. You may want to try this if you want very, very sharp pictures. Here’s my street, just now:
It works like this: Activate it; then, when you press the shutter (or better, iuse the wire release) the mirror flaps up; now you need a second press to actually open the shutter.
Use this in combination with wire release and your photos will be steadier.
Tip: focus on infinity manually, and during the day, verify that the “infinity” marker on your lens actually means infinity, i.e. that it is well calibrated.