Quote:
Originally Posted by multiweb
Thanks a lot for all the info Greg. So if I need to go to F4 that means I need to close the lens let's say 15-20% from its full aperture for the same zoom setting right? At the moment it's fully open. So you don't even see the diaphragm hexagonal shape. When it's on the Pentax body it's driven electrically. On the QHY8 I have a bit of plastic wedged at the back to keep it open. I'll have to make a thinner one so it opens a bit less and try again.
The first pic I did I got from Crux past M8 down to M16/17 in one field. That was set to 18mm. Last night I did 35mm so it zooms from crux to just past the pointers down to Ara not even reaching NGC6188. So it's much tighter. Using 35mm vs. 18mm didn't do anything about the aberrations I have in the field though.  |
Yes that's right. No matter what focal length you set the lens to, you will almost certainly need to stop it down unless it is one of these exceptional lenses.
I remember on the Yahoo digital camera group there was one guy who spent a lot of time testing lenses and his findings were almost always you need to stop them down. The last part of the curvature of these lenses are where all their aberrations are. Not true of all lenses so a bit trial and error. A few quick test luminance images at various F stops will tell you.
But for instance last time I used lenses was Canon FD and Nikon 50mm on my STL11. The Canons needed to be stopped down to f5.6 as I recall but the Nikon did not require that.
I am about to test my Pentax 67 lenses but its already been tested by others so it should be a safe bet. The 300mm may not be great for LRGB
(chromatic aberration) but fine for NB. I am hoping the shorter ones will be OK for LRGB.
I have also seen great results from Zeiss 300mm lenses. Wolfgang Promper I think it was or perhaps Johannes Schedler.
Greg.