Re: [APML] Long Exposures (was: Recent Images)

[ZenAstro <zenastro@home.com>]

>
>I have the notion that medium format (MF) also supports more resolution and
>larger dynamic range than 35mm film. You get 4 times the film area to absorb 4
>times as many photons per unit sky, which yields yet another 2 photographic
>stops. For example, I shoot MF at 875mm focal length, which has similar
>angular coverage to my previous TV-85 setup (35mm film at 480mm). Note: film
>noise comes primarily from film grain.


This doesn't sound right.

I don't think the dynamic range of the film increases because you spread an 
object out over a larger area of film if you use a longer focal length. The 
dynamic range of the film is a function of the film, not the image scale.


 >You get 4 times the film area to absorb 4
 >times as many photons per unit sky, which yields yet another 2 photographic
 >stops.  For example, I shoot MF at 875mm focal length, which has similar
angular coverage to my previous TV-85 setup (35mm film at 480mm).

I don't think so.  This is similar to the incorrect argument advanced by 
some people that extended objects get brighter as you get closer to them. 
They don't, because they also get bigger.

You say "per unit sky", which I take to mean per area unit of sky.  For 
instance, at the same focal length and aperture, one square degree of sky 
might cover, say, one square inch of film.  If you take that same one 
square degree of sky, and spread it out over 4 square inches, yes, you have 
four times the film area absorbing photons, and you collect 4 times the 
total number of photons, BUT, they that same number of photons are spread 
out over 4 times the area, so the "speed" is exactly the same.

Your Astro-Physics 130 EDF at f/6.7 is slower than your TeleVue 85 at 
f/5.6, no matter what size film you shoot in either one.

You are not seeing increased photographic speed with the examples you gave.

For a given film, for extended objects, photographic speed in different 
optical systems comes strictly from the focal ratio of the optical 
system.  It has nothing to do with the film size and is independent of the 
image scale.

C'mon Matt, I know you know this! <G>







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