It took me a while to cull through my archives looking for something that would fit your needs.
Originally posted by awscreo ....do you have non-AT exposures?
Ok, you are in luck, in that I forgot I actually took these and I normally just save everything, deleting very little. I have 2 that would probably interest you... I can email these if you wish?
- #2568 - 2.5 sec, f2.8, ISO25600, 15mm - no AT - landscape orientation
- #2569 - 13 sec, f2.8, ISO1600, 15mm - no AT - landscape orientation
I remember wanting to try 13 seconds without tracking because at that exposure time, the stars should be bounded within a 4 pixel box based on inputs of - crop factor 1, 36.4MP, 15mm and Pixel Tolerance of 4
Originally posted by awscreo I've found it adds trailing with UWA lenses (at least it did with my rokinon in past) in corners and edges, and I'm basically looking for a 100% crop of a corner or an edge of such photo. Trying to compare to other 15-30 copies, see if the amount of coma I see is normal and comparable to others.
PS: Any iso works for me, so whatever you have in your disposal basically
From what I have read, the trailing in the corners is due to the wide field of view at UWA - 15mm, in that the star's apparent rate of motion is greater at the lens' edge and corners than it is at the center. That has come from a couple of sources (one being Ian Norman @
Lonely Speck - How to Photograph the Milky Way – Astrophotography Tutorials, Tools, Gear + Inspiration), however I have not found any specific astronomy/optics reference literature to validate this.
The only logical analysis I can come up with is that with a UWA lens, the light at the lens' edge travels in a slightly longer path through the lens than it travels at the center (and there is a difference in direction of entry into the lens). Thus, when you are taking a long exposure of moving objects (stars) this slight difference will actually be noticeable and appears as trails.
On this particular aspect, I have lots of tracked examples at 15mm, f2.8. I find no real trailing on the left side, and a slight amount on the right side. I have concluded that I may have a slightly off center element, but will having Pentax only make it worse? I have been stitching, in that when combined you only wind up using the center section - thus it winds up being a moot point.