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[OM] Re: shooting stars

Subject: [OM] Re: shooting stars
From: Moose <olymoose@xxxxxxxxxxxxxxx>
Date: Mon, 07 Nov 2005 15:48:17 -0800
Komtanoo Pinpimai wrote:

>I don't know much of the theorem, it seems to me that the more f the more 
>sharp in the long distance, so everytime I shoot view, I try to increase the f 
>as much as my hand can handle the shutter speed. The star is really has a 
>great distance, so I had an envision I should maximize the f for shooting 
>stars. Could you explain why it will be extreme ?
>  
>
The simple version:
With objects on the earth, there are two things going on.

1. Most lenses are sharper when stopped down  2-3 stops from wide open, 
stay sharp for a few more stops, then actually start to lose sharpness 
at the smallest apertures (largest f numbers.).

2.The larger the f number (smaller aperture/diaphram opening), the 
greater the range of distances,  depth of field (DOF), in which the 
subjects will be sharp.

When shooting stars with a camera, these factors change significance.

1. Any small difference in lens sharpness wide open is lost in the 
movement of the star across the film and, in most locations, by bluring 
caused by movement in the cells of air through which the picture is taken.

2. Everything you shoot is at photographic infinity, so deep DOF doesn/t 
mean anything.

3. Lens speed is very important to allow reasonable exposure times. So 
since there are no other reasons to stop the lens down, people mostly 
shoot wide open.

There are some lenses where shooting one stop below wide open will give 
better results. The best way to find out about yours is to try it out.

Moose


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