[jhacker]

I have the Pentax 10-17 mm zoom lens and I want to try some wide field shots of the sky, especially during meteor showers. However, focusing on the stars is a royal pain, especially with this lens because the stars aren't bright enough to see to manually focus. I'll end up shooting an exposure and blowing it up to see if I'm close, then adjusting. But I can't tell which direction to turn the wheel from the photo, just whether or not it's in focus. You can't just spin the lens around to the stop on the infinity side because the stars are out of focus. In focus is somewhere before you get around to the stop. Is there a secret? Someone said use live view, but the live view is so bright, you can't see the stars. What's the secret to focusing any lens on stars? I've been trying for years and haven't figured it out.

[fotograffiti]

Try fitting your camera with a legacy lens, one where "infinity" actually means infinity. Then, when you turn the focus ring to the infinity symbol, the stars will be in focus, without any doubts. Maybe someone who has more experience than I can guide you to an auto-focus lens that has a true infinity setting, but I don't know of any.

[VoiceOfReason]

If you know you're going to be doing that then go during the day with it, autofocus on something really far away, switch to manual focus, and put some tape over the focus and zoom ring so it doesn't move.

[colonel00]

Can you take the lens out during the day and focus to infinity

[rkappleby]

What I do when I know that I am going out I will go out through the day focus on a distant point to achieve infinity focus - then I take a piece of tape and tape the focus ring so it cannot move - remember to turn the autofocus off. I found with zooms the infinity point of focus can vary slightly depending on the zoom setting. On some primes I simply take them out through the day and I make note of the position of the infinity symbol when the lens is focused - If you are focusing at night you can sometimes fid a distant light to focus on - you don't necessarily need to focus on a star - just remember that when you achieve infinity focus SHUT THE AUTOFOCUS OFF Hope that helps

[Adam]

I have the Pentax 10-17 mm zoom lens and I want to try some wide field shots of the sky, especially during meteor showers. However, focusing on the stars is a royal pain, especially with this lens because the stars aren't bright enough to see to manually focus. I'll end up shooting an exposure and blowing it up to see if I'm close, then adjusting. But I can't tell which direction to turn the wheel from the photo, just whether or not it's in focus. You can't just spin the lens around to the stop on the infinity side because the stars are out of focus. In focus is somewhere before you get around to the stop. Is there a secret? Someone said use live view, but the live view is so bright, you can't see the stars. What's the secret to focusing any lens on stars? I've been trying for years and haven't figured it out.

If the moon is out, you can use try autofocusing on it, or using live view. Otherwise, trial-and-error is generally going to be necessary, especially with very wide lenses where starts will be too small to be discernible in live view or through the viewfinder.

[Digitalis]

Then, when you turn the focus ring to the infinity symbol, the stars will be in focus, without any doubts.

This couldn't be any further from the truth, just because a lens has a hard infinity stop doesn't mean it is guaranteed to be anywhere near accurate. Nearly every old manual focus lens I have ever owned has needed some adjustment to nail infinity focus. AF lenses, especially zoom lenses commonly treat infinity as a range instead of a single point of focus due to the inherent lower tolerances in construction, modern AF primes have tighter tolerances on this but they all need some degree of tweaking for precise infinity focusing.

If your camera has live view capability, push the exposure compensation up as far as it will go. This should amplify the light from stars that are bright enough to allow you to focus on them with reasonable accuracy.


Pentax K5IIs- SMCP-FA 31mm f/1.8 ASPH Limited.

---

Last edited by Digitalis; 11-20-2015 at 12:53 AM.

[pixelsaurus]

Focusing at Night: A Tutorial – Lonely Speck

Bahtinov Mask generator http://astrojargon.net/maskgen.aspx

[Stone G.]

I doubt that a Bahtinov mask will work on a lens as wide as the OP's 10-17mm.

For me, based upon experiences with the DA 18-135 @18mm, the best approach is to focus at some remote, terrestrial object while there is still enough light and then clamp the lens at that focus position.

Using liveview with a Pentax camera is a bit tricky because of the autogain of the liveview image. The closest you can get to WYSIWYG is in Video M-mode. However, one can lock autogain in all program modes other than M-mode using the AE-L button.

Thus, set focus mode to MF and program mode to for example A; lock the gain while pointing the lens at something bright (I use a small LED lamp), press AE-L and point at some (very) bright star using 8-10X liveview magnification. Star images are still very small, but the combination of minmum size and maximum brightness may work for you. Remember to switch back to M-mode once you have focused.

[fewayne]

I use a 14mm Rokinon, which is not quite as tough to focus as the 10-17 but is challenge enough. My go-to technique:

1. Turn screen brightness all the way down
2. Find the brightest star visible
3. Center as best as I can in the optical viewfinder
4. Turn on Live View and crank the magnification till the star appears
5. Adjust the view as I zoom in to keep the star centered
6. Fiddle with the focus, cursing occasionally
7. Take a test shot at an insanely high ISO ==> exposure wasting least time possible
8. View image at high mag, curse more, fiddle more

It occasionally works.

[Digitalis]

View image at high mag, curse more, fiddle more

extra points are given for creative use of invective.