Quote:
Originally Posted by ZeroID
There you've got it ...
By reducing the angle of incidence to the sensor, ie narrower cone from the lens you reduce chromatic aberration for a starter, ie reduced halos. And don't tell me your lens is perfect and hasn't got some dispersive elements in it's multiglass make up. Why do you think Pro Photogs prefer primary lenses, minimal glass.
The focal point becomes slightly less critical and in astro imaging even the tiniest deviation from the critical point will introduce some halo or 'blobbing' . You may think you have achieved perfect focus but your camera knows better. It's the one reason I bought a Canon DSLR (or two) just for astro imaging with BYEOS. I'm a SONY man by preference but there is no software for SONY gear that allows image focusing at the PC.
And focussing then shifting the zoom just isn't going to cut it. All Lenses I have ever used have shown some small amount of focal shift when zooming.
Yep, astro-imaging is quite demanding ...  |
Sigh...
First off, I have never said this lens is great for astro work as demonstrated in the opening of the following two threads.
http://www.iceinspace.com.au/forum/s...d.php?t=146564
http://www.iceinspace.com.au/forum/s...d.php?t=146994
All I have asked is whether the apparent star bloating has been caused by focus or whether it is a consequence of a Bayer matrix on a large pixel camera because I have no real experience with OSC cameras and astro work (only used it for day time photography).
What you say is correct and I have already agreed with you in earlier posts. Increasing the critical focus zone makes it easier to focus but has zero effect if it is in focus. It may decrease star bloat because it makes blue focus less demanding but the blue halos I have are likely caused by it being out of focus anyway.
It is good to know that there is focus shift with zooming, I had wondered by haven't been able to get any conclusive test results.