Recently I have problems with my AFD lens and had it repaired (they took two goes). It seems to be fine on most day to day shots, with the noticeable light dropoff on the corners of the images at F2.8.
I took it on a trip under a new moon to the clearest of southern skies and took great delight at showing my boy the wonders of the universe, and took this image.
Sun 30-Sep-12 09:55 PM | edited Sun 30-Sep-12 09:56 PM by TomCurious
Yes, coma gets less as you stop down and should be pretty much gone by f/8. Of course I understand your reason for not using f/8 in this scenario. Good lenses for astrophotography are expensive if you want the stars rendered as points and not trails, as you need a lens that is fast (to allow a short exposure time) and coma free at wide apertures.
The image was taken camped on the side of the Tanami track in the desert in the mid NW of Australia. The track is a 1200km dirt road, with no towns along the way. The moon was new and the sky black. We'd been lying on the ground watching the ISS and other satellites go overhead when I thought of this idea. I set it up on a tripod, climbed to the top of a dirt mound with him and fired the wireless remote. He was happy to stand still for thirty seconds as well.
It was a really special thing to stand there under such a black sky with the milky way streaking across it filling a little boys head with awe and wonder about his place in the universe. This image makes me feel so very lucky.
I think this would be a wonderful stock editorial photo. A father and son under the stars... great stuff! If you ignore the pixel peeping, this photo has alot going for it, and I bet it would reproduce easy for print. Would make a great book cover for a novel or page art.
I think you did a great job with what is basically impossible to improve on much. And you could throw a lot of money at expensive optics without necessarily solving the problem. I've seen some reports that the 24/1.4, for example, has bad corner coma too (at least wide open). Most lenses need to be stopped down to f/4 or so for perfect stars even if they are much faster. It's more or less the nature of the beast.
Your image does not show classic coma, which would look like a comet (wide tail at one end, pointed at the other), with the head pointed directly at the optical center and the tail away from center. You have "two tails" that are about 90 degrees to the center. (edit: forgot to add that this appears to be astigmatism, which is a symmetrical aberrational)
You also have about 6 pixels of star motion which probably confuses the issue of the aberration by stretching it approximately in the vertical orientation per the movement of the stars.
You could not solve the problem with a motor drive and longer exposure stopped down because then you and the terrestrial scene would be blurred. That's why it is more or less impossible to solve.
I think you got a great image and as mentioned previously, a terrestrial/Milky Way shot should not be pixel peeped
And that is all before considering the personal side of the image, experiencing that with your son. Enjoy the wonderful memento!