Infinity focus question with night photography

3 months 2 weeks ago #675920 by Adam Cuffin
I'm trying my luck at night photography this weekend.  So, if I'm shooting with my 18-200mm and let's say I'm at 24mm and focus on a light that is 150m away.  Would that be OK for infinity focus for night  time photography?  

Thank you for your help.

Adam


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3 months 2 weeks ago #675959 by Rick Larin
If I'm understanding your question, no, only items at or around 150m will be in focus.  If you want the stars to be in focus you'll need to adjust closer to them.  Usually for me, things like stars and the moon are at infinity, but pulled back just a tad.  


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3 months 2 weeks ago #676032 by Ozzie_Traveller
G'day adam

Optics and the 'laws' of camera optics have been with us for many years. The industry standard for infinity is "1000 times the focal length of the lens"

Therefore (mathematically at least) infinity for your 18-200 lens is 18 metres to 200 metres depending upon where the zoom is lodged

The setting for infinity for most lenses can easily be achieved via
a) towards dusk, focus on something you class as 'infinity', flick from AF to MF, and then run some duct / masking tape across the focus ring to prevent any movement later on, or
b) after dark, focus on a bright light - street light - building in the distance, then also use the above tape to prevent the focus ring from moving

Your stars imagery will then be done using MF mode and whatever shutter speeds are relevant to your end result

Hope this helps
Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/


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3 months 1 week ago #676980 by effron
You have live view in that D7100, that's the best way to nail focus.

Why so serious?
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3 months 1 week ago #677050 by Ozzie_Traveller

effron wrote: You have live view in that D7100, that's the best way to nail focus.


Agreed - but sometimes, the stars are just too feint to get a real-good look at for focus purposes, -and- for all of us, as we age, our eyes lose the low-light / dark-light ability to see it all (it's a rods & cones thing)

Phil


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