Re: [APML] Exposure Question

[Stuart Heggie <stuart.j.heggie@sympatico.ca>]

Brian, I am amazed you can shoot at f/4 on E200 for only 7 minutes and 
see anything on the film/slide. I shoot 40 min on Supra 400 at f/4 and 
my math says you should go 80 min on E200. That said, for every full 
stop you go down, you double the exposure time and for every factor of 2 
you change film speed you change the exposure by the same factor (in the 
opposite direction e.g. half the film speed requires double the exposure 
time to be even).

All this of course ignores reciprocity failure which is a function of 
the specific film (each layer in the film having its own curve), the 
outside temp, the sky fog limit and probably the optics of the 
lens/scope. Some folks subscribe to the argument that a 16mm at 2.8 is 
different than say a 50mm at 2.8 but I think this is suspicious (mind 
you the source of this view is pretty expert so I am "agnostic" in this 
matter).

Short answer: if you shoot 7 min now on E200 at f/4 you should shoot 14 
min on E200 at f/5.6 but that is ignoring the paragraphs above. I cannot 
imagine that exposures that short are going to get you much.

Stuart

Brian Larmay wrote:

>Hi all,
>
>Ive been slipping on everything concerning astrophoto since spring and will
>finally be getting out tonight for some long overdue practice.
>My question is concerning exposure time...I always stop my lens to f/4 with
>exposure times of 7 mins, but this time I want to stop down to f/5.6 ...
>what would the correct exposure time be then?
>Whats the formula?
>Will this sharpen my images noticably vs @ f/4?
>Will this reduce Vignette and help me to create mosaics alot easier?
>
>All exposures are made using Kodak E200 tonight.
>
>Ty,
>Brin
>
>My Photography
>Homepage: www.astrobri.com
>
>Geographic Longitude= 88W
>Geographic Latitude= 43N
>Corrected Magnetic Latitude= 53
>
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