I'll assume that you are talking about very low light scenes with exposure times of typically say 5 seconds plus. If not, please advise.
If that is a Vibration Correction (VC) lens the VC feature should be turned OFF when on a tripod. The VC feature tries to "correct" vibration that ideally is not there and makes extra vibration!
Do not touch the camera during exposure. Allow vibrations to settle as much as possible and if you have a prerelease timer try using it allow vibrations to settle more.
I find that very good tripods on a rock solid surface need litle delay but I find that with only OK gear that 2 seconds is not enough delay and if it matters I allow 10 seconds pre-shot delay for the vibrations to settle.
A release cable or remote electronic release can help.
In extreme cases where you want some "settling time" but cannot use a timer for some reason you can hold something as flat black as possible in front of the lens - and preferably move it around somewhat to ensure it as evenly "not there visually" as possible. This obviously decreases the exposure time but can be allowed for.*
Set focus to manual and find a bright enough object (star etc) and focus on that as well as you can.
On longer exposures some cameras allow light to enter via the eyepiece and cause image degradation. Depending on ambient light it may pay to use an eyepiece cover. In informal hurried situations if there is rear lighting I stand between camera and light source and may hold a shield between camera and light or drape somewthing over the eye piece.
You are probably not using small apertures eg f/16, f/22 BUT if you did then diffusion may cause loss of sharpness.
That lens should be OK at f/2.8 but slightly stopped down may be better.
Because night shots may take 10 or 20 or 30 or more seconds, and dark pixel compensation may take as long again afterwards, and if you add 10 seconds timer pre-delay ... you can take a long time to get a few shots. To assit with composing and getting the feel for the effect of light levels of parts of scenes which may be hard to know about in advance, I try taking high ISO maximum aperture shots first, then when I have seen the likely results, change over to lower ISO and smaller apertures. This can greatly speed up proceedings.
You should tell us as much as you can about your setup and situation:
- The more we know the better we can help.
- What is the full lens spec (it may be one of several).
- You should give settings - exposure time, aperture, ISO.
- How did you ensure focus was OK?
- What are you photographing?
- More ... ?
A very occasionally useful "trick":
On rare occasions where an unexpected light source suddenly threatens to enter the image area I have slapped something as light blocking as possible across the lens to terminate the exposure - this MAY produce a usable shot depending on circumstance. This would not be deemed good practice by most and should be avoided where possible :-). Some cameras will terminate a long exposure shot AND give a saved image if you turn them off mid image - some may not. Know what yours does. If it saves the image thus far you can block the lens and then turn the camera off and long again. Desperate - but worth knowing.