The image intensity will fall off from center intensity approximately
according to the formula:
I = cos^2[atan(r/f)]
where:
I = intensity relative to center intensity.
r = distance in the film plane from center image.
f = distance from pinhole to center image.
The fall off in stops is:
s = log(I) / log(2)
= 3.322 * log(I)
This assumes flat film plane with pinhole plane parallel to film plane.
Center image is defined as the point on the film plane where the normal
passes through center pinhole.
> -----Original Message-----
> From: owner-photoforum@listserver.isc.rit.edu
> [mailto:owner-photoforum@listserver.isc.rit.edu]On Behalf Of Chris
> Sent: Friday, August 22, 2003 11:53 AM
> To: List for Photo/Imaging Educators - Professionals - Students
> Subject: RE: Minimizing pinhole image falloff
>
>
> At a guess I would say the fall off was proportional to Sin(Theta) where
> theta is the angle of the ray away from the normal. There is no focal
> length for a pinhole. The brilliance of the image is proportional to the
> area of the pinhole. So the brightness at angle theta from the
> normal will
> be proportional to A.Sin(Theta)/d^2 where d is the distance of the element
> from the pinhole.
>
> Don't quote me I'm a beginner!
>
> Chris
> Web Page
> http://www.chrisweb.pwp.blueyonder.co.uk/
>
> |> -----Original Message-----
> |> From: owner-photoforum@listserver.isc.rit.edu
> |> [mailto:owner-photoforum@listserver.isc.rit.edu]On Behalf Of Gregory
> |> Fraser
> |> Sent: 19 August 2003 16:19
> |> To: List for Photo/Imaging Educators - Professionals - Students
> |> Subject: Minimizing pinhole image falloff
> |>
> |>
> |> I went to a web site that had a calculator for the image circle
> |> diameter of pinhole setups. I calculated that a focal length of
> |> 3 inches would give me an image circle that would cover 4x5 inch
> |> film. I forget the pinhole diameter. Then I remembered how
> |> drastic the falloff is at the edges of pinhole images so I
> |> thought perhaps by increasing the focal length, I would have
> |> more of the brighter central part of the image and that would
> |> reduce the effects of falloff. 'But wait,' I yelled, 'if this
> |> were the case wouldn't Guy have been able to find a hotel room
> |> long enough to prevent the falloff he experienced in Montreal?
> |> Certainly someone as intimate with pinholes as Guy would know
> |> about that.'
> |>
> |> So, does the light falloff of a pinhole camera image follow an
> |> inverse square rule? Will it always be an issue no matter how
> |> big your shoebox, cigar tube or Quaker Oats box is?
> |>
> |> Greg Fraser
> |>
> |>
>
>
>
