Re: Light fall-off furmula

Thanks Eric for both your suggestion and your best wishes for my shoot.

Actually, I'll be working from the 23rd floor of the Radio-Canada tower,
here in Montreal, (aiming towards the Jacques-Cartier bridge for those
familiar with the city). The white sheet idea is unlikely to be prudent
from that floor. -:)) But I will try it on some other site as an
additional tool in my bag of tricks.

Best wishes for your move to the country. Perhaps there should be an OOT
discussion on people moving to the country and their experience. I've got a
country house and sometimes I feel like moving there as a permanent place,
out of the hassle of the city. But then, there's so much in the
city... -:))

I'll keep the list posted on my shoot,

Guy

----- Original Message -----
From: "eric reuben nelson" <nelsone@tznet.com>
To: <pinhole-discussion@spitbite.org>
Sent: Wednesday, July 27, 2005 10:22 AM
Subject: Re: [pinhole-discussion] Light fall-off furmula

> Guy,
>
> Here is a suggestion from an intuitive non math person.
>
> Would it be possible to place a standard target out side the camera while
> you measure the light? For example, if it is possible to hang a large
> sheet of white foam core in front of the pinhole you might be able to have
> a consistent white target subject image to meter on your film plane.
>
> This of course is ridiculous if you are using an upper room in a tall
> building. I wish you the best weather for your exposure! And a large
> developing trough and a coordinated friend for processing!
>
> eric nelson
> marshfield wisconsin usa
>
>
> On Jul 25, 2005, at 11:14 AM, Guy Glorieux wrote:
>
>> Hi friends,
>>
>> I'm looking for a formula to calculate the light fall-off from center to
>> edges.
>>
>> I'll be working on a giant pinhole this week-end (8 feet x 9 feet) and I
>> need to anticipate the amount of dodging required at the center of the
>> print in order to get a fairly even exposure across the paper negative.
>>
>> As a rule of thumb, I've been working on the basis of Eric Renner's
>> book - where he states that the maximum diameter is equal to 3.5 times
>> the focal length. However, I note that Larry Fratkin's calculator works
>> on the basis of just under twice the focal length.
>>
>> What I'm looking for is a formula that will calculculate the amount of
>> fall-off from the center of the image. Alternatively, what would be the
>> diameter of the circle with minus-1 stop exposure, minus-2 stop exposure,
>> minus-3 stop exposure times.
>>
>> Guillermo, how does one calculate the light fall-off with a pinhole
>> sieve?
>>
>> Failing all this, I guess I'll have to work with a light meter. But, I'm
>> not really satisfied with this option since the differences in reading
>> could result from either the scene or the light-off.
>>
>> Thanks for your comments,
>>
>> Guy _______________________________________________
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Received on Wed Jul 27 08:55:46 2005