Quote:
Originally Posted by Atmos
Totally agree, it works much better at F/9 than F/4 but I still don't see how it helps with focus except by increasing the critical focus zone. Doesn't actually effect focus, just makes it easier to achieve by increasing the depth of field.
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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 ...
