Star trails - focus

[prl]

Hello,

Yesterday I decided to take advantage of the clear sky, so I tried to do some star trails from my backyard. I live in a heavily polluted area which makes things difficult.

So I setup everything and took some test photos. I went for f/2.8, ISO 200 and 15" exposures (220 in total). Could probably stopped down the aperture to f/4 but it was a hard balance between getting some stars on the shot and not too much light pollution. I usually would go for 30" exposures but the low aperture and heavy light pollution made that impossible.

This is the final result, I think it came out decent enough:



Now, what I wanted to ask: when I got to Lightroom I noticed that all my photos were unfocused. Not too much but the stars weren't clearly focused. After the facepalm I decided to take a shot at doing the trail anyway, did a little processing and went to StarStax. Now I actually think that the fact that it was out of focus might actually have helped. The stars got a little bit bigger and that might have helped with the trails.

What do you think?

Thanks!

[Don Quixote]

It looks cool so far Prl.
I have always wanted to try this.
Thank you for posting your progress.

[Ylem]

I like

[OzEclipse]

Hi PRL,

The image looks nice at this scale.

Focus is a relative term. Imagine you are out of focus so that stars are spread over a whopping 81 pixels (9x9). Your camera sensor is 24MP so 4000 x 6000 pixels. Before posting, you downsize your final image from 6000 pixels to 640 pixels, now your star images cover 1 pixel and your image is back in focus. However, when the light of faint stars is spread out won't penetrate through the light pollution. So that out of focus image will show a lot less stars than a focused image.

So IMO you are always better off focusing the image. Turn off the noise reduction when you capture. If you do 30s exposures, you don't want to have 30s gaps while the camera does noise subtraction. Starstax can fill the gaps but better to capture a continuous sequence.

kind regards
Joe

[prl]

Hey Joe, thanks for the feedback. I never about focus that way, that sure was helpful.

I turned off noise reduction in this photo, I struggled for a long time with those gaps between shots and was never able to get clear trails. That's one of those tricks I wish I new from the get go!

[KathyNS]

Focusing stars with a DSLR is tricky. (Disclosure: I have not yet succeeded!) The camera's autofocus will not see the stars and will be unable to find a focus. When manually focusing, autofocus lenses have a peculiarity: the "infinity stop" is not at infinity! In the old days, to focus at infinity, you would turn the focus ring until it stopped, and you could count on it being in focus. But autofocus lenses need to be able to go past infinity, so that they can check either side of the true focus position. So, turning the ring to the stop will take you past the infinity focus.

The trick is to figure out where the infinity focus position is. You could try focusing on a distant scene in daylight, then taping the focus ring in that position. If your camera has live view, you can enlarge the display and focus on a bright star by eye. Without live view, you might be able to take test shots, enlarge them on the display, and find focus that way.

[JayTee]

In addition to what everyone else has said, have you looked at the software
StarStax? It is free and specifically designed to take multiple short star images and combine them so that you get very long star trails. Take a look. https://markus-enzweiler.de/software/starstax/

Cheers,
JT

[OzEclipse]

Hi Prl

All lenses, be they modern autofocus or older manual lenses go past infinity. On older cameras, it is so that they can focus at infinity on infrared light which is past the white light infinity point. A lens designed for manual focus wiil have a smooth fine focusing thread with some resistance so that it holds it's position.

I teach a Nightscape Photography Workshop.

The way I teach my students to focus is to use live view mode. If you are using a zoom lens, set the lens to the desired focal length. Turn off all autofocusing on the camera and/or lens. Find and centre a bright star. Activate live view.

Point the camera and centre the star. Increase the live view zoom to maximum magnification value. 8x to 20x depending upon camera makes & model.

Adjust the focus manually so that the star image is as small as possible. Do not touch the focus or zoom focal length again and perform all framing without touching focus or zoom.

* Temperature changes can change focus. Monitor focus by checking review images at high magnification from time to time. Repeat focusing procedure if required.

Joe