I had a chance to look at the image. I see the Hyades (Taurus) and Pleiades in the image which means the star direction would have been moving toward the lower right (your camera was pointed roughly toward the southwest).
I see the various optical aberrations in the image and they don’t match the direction of star travel... so they aren’t caused by a long exposure (also your exposure time appears to be very short).
I see lots of optical issues ... the stars look like they aren’t quite focused, but the de-focused stars are giving odd shapes and it looks like more than one issue — and I suspect de-centered optics as well. I say this because the stars in the center actually don’t look good to me... the stars a bit right of the center look a little better (all of them look a touch soft).
Normally when you manually focus for night photography, you would turn on live-view and crank the ISO to max and crank the shutter speed to 30 seconds ... because the camera supports “exposure simulation” when using live-view. This will brighten the stars to make them easier to focus. Find the brightest star you can find (but it should be a star... not a planet. Currently Sirius is the brightest star in the sky (it’s the brightest star apart from the Sun). To find it, follow the 3 stars of Orion’s “belt” toward the left (east) and the brightest star you come upon is Sirius. It doesn’t matter if you plan to image a different section of the sky... once anything is focused, everything is focused (unless there’s a problem with the lens).
Get the star roughly close to focus (you can adjust the focus ring to infinity to be close), then use the LCD zoom feature (the magnifying glass icon with the “+”) to zoom in to 10x ... then very carefully adjust focus to get that star to a pinpoint as best as possible.
A focusing aid such as a SharpStar 2 (Lonely Speck makes it) will help. It creates diffraction spikes on the stars which will converge at a common center point only when the focus is perfect... so it’s easy to see when you’ve nailed the focus.) Take a test shot of a few seconds with the focus aid on the lens because this will create much bigger diffraction spikes than you can see in Live-View mode ... and confirm that your focus is bang-on accurate.
Once focused, return the exposure to sane settings and point the camera to the part of the sky you want to image (being careful not to touch the focus ring).
MOST lenses will give some optical aberrations when shooting wide-open (even high quality lenses). You can greatly reduce them by stopping down a touch. But you should have pretty even focus across the entire image field with only a tiny degradation near the corners. If you are still getting de-focused stars... the lens has optical problems.
Tim Campbell
5D III, 5D IV, 60Da
