John Howard Loc: SW Florida and Blue Ridge Mountains of NC.

I understand why if you exposure is too long the stars turn to trails, and that the wider the angle lens, the sooner this happens. I was shooting 20 sec exposures and the stars are 5x as long as they are wide. I slowed down to 15 sec and the trails are gone, but now the stars look like donuts when enlarged - black holes in the middle. What causes this?

I was unable to upload images before, so I am now trying to do it as an edit. This is straight out of the camera, 15 sec 1250 iso and 3.2. I can also try to add a processed version and the one with trails if you need. ALSO, I am just practicing to get ready for a mountain trip. This is just a driveway shot last night with a nearby street light. Not ideal conditions.

I was able to upload three shots. second is the donut shot sooc, first the processed version, WB corrected for the street light and a slight crop. Last is the trail version soon. I should note that the two images were with different cameras and lenses - should that make a difference? Bot cameras FF, one prime 21mm, the other 14-24 between 15 and 19mm.

But what causes the donut holes?

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ggttc Loc: TN

With telescopes its generally a focus issue or collimation issue...

SharpShooter Loc: NorCal

John, on my phone it's hard to analize anything but you might have a bit of movement or you could be experiencing some form of Coma which will distort points of light though usually elongating them.
You might want to look into Coma. Have you tried several lenses to see if the results are different? Good luck. ;-)
SS

GTinSoCal Loc: Palmdale, CA

The "donut holes" are caused by out of focus stars.

Notice that it is not visible on the brightest objects?
The flare covers the "hole".

Note: the wider angle lens (shorter focal length) the longer you can keep the shutter open before the stars start to trail.

GT

John Howard Loc: SW Florida and Blue Ridge Mountains of NC.

I am actually not doing a good comparison as with different cameras and lenses, hard to know. I have a really good (and heavy) tripod, there was no wind to speak of tho maybe enough to move the palm fronds real slowly, and shot timed release, so on a 15 or20 sec exposure, shutter vibration should not have been an issue (imo). The batch of images from my driveway in FL, since I could not avoid the palm trees, maybe I focused a bit under infinity, but still, should have been well outside the HFP.

I will research Coma as you suggest.

John Howard Loc: SW Florida and Blue Ridge Mountains of NC.

Thanks. I think maybe I was a bit inside infinity focus due to trying to capture the palm trees. And yes, I had the lens / time limit backwards in my post.

Bultaco Loc: Aiken, SC

The holes are in the brighter stars in photo 1?

JimH123 Loc: Morgan Hill, CA

Absolutely a focus issue. I also see chromatic aberations (the purple color) in some of the stars in image 3. I don't see any coma distortion. Stars at the edges look the same as stars in the center.

Focus is something that is extremely important when doing stars. You have to use live view. And it helps to magnify. If the intent is to also have the trees in focus, I would take two shots and the 2nd would focus on the trees. And then the two images need to be combined. I like to do this with layers in which I would have the tree focused image on top, and erase the star background so the stars in the bottom image are exposed. For that kind of job, I like to use Perfect Photo Suite 9 which has tools for doing this very job with complicated masks. Sometimes, the mask is easier to make if the picture order is switched with the focused stars on top and the trees are masked out exposing the focused trees underneath. In looking at the pictures, the trees are much darker than the sky and perhaps this would mask easier.

John Howard Loc: SW Florida and Blue Ridge Mountains of NC.

Thanks JimH123. I can use LV and the focus to assist zoom and will try that. I am only using LR5 right now and I don't think it can do the layering to combine two shots. Always more to learn.

JimH123 Loc: Morgan Hill, CA

That's right. Lightroom does not do layers. I do perhaps 75% of my work in Lightroom, but I do have additional tools to use when needed.

Stage Light Loc: Northeast

A mirror lens would cause donuts.

John Howard Loc: SW Florida and Blue Ridge Mountains of NC.

Interesting you say that. The shots with small trails were with a Sony FF mirror less A7R and the shots with donuts were with a Nikon D810. The D810 was on timed release so not mirror up as you cannot do both.

Do you say this due to the mirror vibration, in which case I am better of in M-U mode and a remote release? Or is there another reason beside vibration?

Thanks.

JimH123 Loc: Morgan Hill, CA

Also a lens that is not perfectly focused.

GTinSoCal Loc: Palmdale, CA

No, the brightest objects filled the hole.

It is easy enough to test, if you have a camera with live view.
Zoom in as far as possible and defocus on stars, you will see the donuts appear.
Any light source will work, but the brighter the light source, the more out of focus it needs to be to show the donut shape :-)

now I want a donut! :-/

GT

John Howard Loc: SW Florida and Blue Ridge Mountains of NC.

Thanks and you deserve coffee with that donut.