01-15-2011, 12:23 PM
[quote name='popo' timestamp='1295087051' post='5526']
Even around infinity, telescopes need accurate focusing too. And if you need to get closer than it allows, you just use the standard macro trick of moving the camera away like with extension tubes.
[/quote]
That's a scary thought... to add that many extension tubes!
[quote name='popo' timestamp='1295087051' post='5526']
Just for fun, I did see how close my Nexstar 4SE could focus (1325mm f/13). I managed as close as 3m for a magnification of 0.6x <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' /> It could almost work as a long working distance macro if I had any way to hold it steady enough in the field...
Back on the original topic, I did also wonder about astrographs doubling as a terrestrial long focal length manual lens. The main concern I had was although most are highly colour aberration corrected for their normal use at infinity, I wonder if that also applies at significantly closer focus distances? Consider that photographic lenses of a similar specification will typically have far more optical elements.
[/quote]
Needless to say, they won't be optimised for the close distances but for the sorts of terrestrial distances one would use a 1300mm lens for shouldn't be that far from infinity, so any performance drops should be minimal. I'm of course, not referring to macro or close-ups... just things like long distance sceneries, etc.
However, I don't expect bokeh to be particularly smooth. Any comments from your experience?
GTW
Even around infinity, telescopes need accurate focusing too. And if you need to get closer than it allows, you just use the standard macro trick of moving the camera away like with extension tubes.
[/quote]
That's a scary thought... to add that many extension tubes!
[quote name='popo' timestamp='1295087051' post='5526']
Just for fun, I did see how close my Nexstar 4SE could focus (1325mm f/13). I managed as close as 3m for a magnification of 0.6x <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' /> It could almost work as a long working distance macro if I had any way to hold it steady enough in the field...Back on the original topic, I did also wonder about astrographs doubling as a terrestrial long focal length manual lens. The main concern I had was although most are highly colour aberration corrected for their normal use at infinity, I wonder if that also applies at significantly closer focus distances? Consider that photographic lenses of a similar specification will typically have far more optical elements.
[/quote]
Needless to say, they won't be optimised for the close distances but for the sorts of terrestrial distances one would use a 1300mm lens for shouldn't be that far from infinity, so any performance drops should be minimal. I'm of course, not referring to macro or close-ups... just things like long distance sceneries, etc.
However, I don't expect bokeh to be particularly smooth. Any comments from your experience?
GTW