Hi everyone,

Is there anyone out there who uses a K-5 with a 31mm lens? Have recently joined this group and am an absolute newbie to AP, any thoughts?

BTW, I also have a few longer lenses, up to 300mm, cheers

HI Pentaxian359, and welcome.

Could you be a bit more specific about that lens? In "your range" I use a Tamron Adaptall-2 24mm f/2.5, a Cosinon-W 35mm f/2.8 and a Pentax-K 35mm f/3.5. Image Quality wise, the Cosina is best, because it has the least coma in the corners.

Anyway, just get started: With your camera and lens on a tripod just take some test shots and see what you get at various apertures. Study your images at 100% viewing on your screen - and do try some post-processing of your images, (you can google your way to many tutorials about that).

For a start, with a 30ish lens, I suggest that you try 8 seconds exposure time at ISO 1600. Use 2s shutter delay on your K-5, as this ensures mirror lock-up and disables shake reduction before exposure starts.

I've used the 35mm macro for some wide field images before. The advantages of macro lenses are the usually very flat field wide open. A challenge with using a wideangle lens is making sure that you get proper focus.

So setting the lens to infinity, and maybe turning it back just slightly won't do?

It depends on the construction of the lens. The 35mm macro has a hard stop at infinity, so I've been lucky with setting it there. For other lenses, I have to preview the image to check for focus. You could also use a couple of strings to make diffraction spikes. When the spikes are sharp, clear, and centered on the stars, then the image is in focus.

Here's an example of what I mean to make spikes:

Thanks Stone, sorry I wasn't specific enough regarding the lens, it's a Pentax f/1.4 31mm. Have tried with differing ISO between 800-3600, and almost every conceivable exposure time. I do use the 2s shutter delay, that works a treat, however it's the post processing that has me stumped. I can't seem to get that deep purple space colour that makes me love AP so much.

That sounds a lot like a few factors in play: the darkness of your skies (what's the limiting magnitude?), the depth of your data stack (how many subs, total integration time), and setting the black point appropriately.

Do you have a sample you can share for discussion?