Question

I've been taking a few shots of the Milky Way using my 16-35mm lens @ f2.8 ISO2000 and 30 secs exposure. I'm getting okay shots, but nothing great - not really sharp and lacking in colour.
Is there a filter I need to use to block out light pollution from nereby cities or do I simply need to adjust my camera settings?

Marcus says

Light pollution is astrophotography's nemesis. The two cannot exist happily together. There is nothing you can do to remove the light pollution (unless you have access someone who works at the local power grid!) but there is a night filter you can use to help achieve more naturally balanced astrophotography shots. Basically it takes out the orange light which allows more detail to be captured by the camera. This enriches the delicate structure of the Milky Way and other nebula.

With regards to sharpness, there are several reasons why this may not be as good as it should be. The first potential culprit is that the focus isn't accurate. As you are using your lens at its widest aperture setting, there is no room for error when it comes to focusing. Due to the extreme low light levels, this needs to be done manually. However, with only tiny distant stars to focus on, it can be quite difficult to get this spot on. There is a tool which looks like a regular slot filter which makes focusing in the dark much easier. You simply place it in your filter holder and focus manually using the live view (at maximum magnification). The tool splits the stars when not focused and brings them together when they are focused (much like an old split level focusing display on a rangefinder camera). Once focus is achieved, the tool is then removed (being careful not to touch the focus ring on the lens).

The second potential culprit is camera movement. When taking very long exposures, the camera needs to remain totally static. Wind, soft ground, or vibrating ground will all prevent sharp images being achieved. By "vibrating ground" I mean a man made platform such as a jetty, balcony, elevated walkway etc, which will constantly be moving as either you or other walk on it. This may not be noticeable to you, but will be picked up by the camera. A decent tripod will be effective in the wind, but even the best tripod is only as stable as the ground it is on, so if this isn't solid then you will still have problems.

Thirdly, another potential problem with sharpness is that your exposure is too long. You say you are doing 30 seconds exposures which is pushing the limits of the 500 rule. Due to the rotation of the earth, the stars move slowly across the sky. The longer your focal length, the faster they move across the frame. There is a rule of 500 which basically means that you should divide 500 by your focal length to get the maximum possible exposure time before stars start to appear as streaks rather than points of light. If you are using 16mm, then 500/16 = 31, so 30 seconds is right on the limit. However, if you are using 24mm, then 500/24 = 20, which means at 30" you won't get pin sharp stars due to motion blur.

Lastly you need to consider the limitations of your lens. All zoom lenses have 2 variables (aperture and focal length) and they will work better at some settings and poorer at others. Generally speaking the poor settings where the lens will be working at its weakest performance will be at the extreme of the variables - ie, 16mm, 35mm, f/2.8, and f22. What you are probably doing is using your lens at two of these weakest settings - 16mm, f/2.8. The lens is never going to give the best results when used like this, especially around the edges of the frame. However, there is a problem with solving this issue because if you go to f/4 then you will need to increase your ISO or your exposure time which in turn will have a negative impact on sharpness. So basically you are faced with a catch 22 situation. The only real way improve things is to get a fast prime lens with an aperture of f/1.4 or f/1.8, as prime lenses only have one variable (aperture). You can then use it at f/2.8 (away from its weakest setting) and you should get much better sharpness and definition in your astro shots.

Also, if you are including some foreground in your astro shots, then it is possible this is closer than infinity (at 16mm this will be around 15 metres or less). If so, then at f/2.8 it isn't possible to get both stars and your foreground to appear sharp. The way to solve this issue is to ensure any foreground is past the infinity threshold (that's another tutorial in itself!).

If you are interested, we now sell a night filter kit, consisting of the focusing tool and the night filter described above. It's not particularly cheap, so it is only worth buying it if you take a lot of astro images and want to get serious about it.

I hope this helps.

Marcus