I have an underused Pentax DLSR camera which I keep meaning to get better use out from. I have a 18-55mm, f3.8 kit lens and a nifty fifty f1.4 (or 1.7 I forget).

I’m hoping to get some photo’s of the night sky during winter bivys and also capture the change in autumnal trees.

It seems like 28mm f2.8 lenses are common, would I likely see a noticeable difference over the lenses I currently have?

Best thing for nighttime is keeping the camera steady. A cheap gorilla pod will do that for you as it’s adaptable and fairly easy carried.

The kit lens will work fine as it’s wide. The slightly smaller aperture will just mean slightly longer exposure. That’s all.

Other than that, cold nighttime temps kill the batteries quickly.

The shorter the focal length the longer you can expose without getting star trails so your 18-55 woudld be best. Otherwise you have to bump up the ISO higher.

50mm is a bit awkward at night as unless there’s a moon a lot of the aiming well for me has been assumptions so a wider angle was better. Having said all that I think I’m only a couple night shoots ahead of you experience wise my first result I’m happy with:

Sigma 10-20 at 10mm:

For star trails, remember to do many short (30 second) exposures rather than a single long exposure (very long exposures lead to massive noise in digital cameras). Then join them using an image stacker.

You’ll probably need an intervalometer (get a cheap one off ebay).

Also take a few frames at the start and end with the lens cap on.

There are many image stackers available (see here for one useful link), some can use exposures taken with the lens cap on (which contain only noise) to help remove noise from the other pictures.

Edit: for static pictures shutter speed of about 30 seconds will prevent much star movement. Anything over a minute will become streaky.

Can I extend this topic to asking for advise on photographing the Northern Lights (off to Iceland in a couple of weeks)
I also have the standard 18-55 kit lens (a Nikon in my case) and was contemplating getting a second hand 35 or 50mm lens to complement it

It really depends on how bright the lights are as to how good the kit lens will reproduce them. On good nights even a smartphone would be fine, but on many nights you’ll only notice a faint glow until you look at your long exposures and see all the colour. In these instances a kit lens will struggle to get as good results as you’ll need long exposures which give star trails at 18mm (assuming it’s an APSC camera). If you can get a nice 12-16mm prime (Samyang are great value) you’ll be set for landscapes with the lights, but if you’re hankering after a 35mm or 50mm for general use then you’ll still obviously be able to get some really nice frames of say the peaks of a mountain, trees or a bit of nice architecture with the lights overhead.