Calculation exposure time with f/490

[trooper97]

I am making a pinhole camera with an aperture of f/490 and focal length of 15" but I'm having problems coming up with the proper exposure time with ISO 100 film. I don't really even know where to start. I would estimate it to be some where around 4 months. Any help or tips would greatly be appreciated.

JW Purdy

[venchka]

That is 2 stops slower than f/128. The scale on most Gossen meters are gradutaed up to f/128. I'll look at mine tonight. Fujifilm Acros has little reciprocity. you would only loose 1/2 to 1 stop with Acros. So. Meter and set the aperture scale to f/128. Read the time. Add 3 stops-double the time 3 times. Piece of cake.

[venchka]

Here is what the Gossen says:
ASA 100 = Sunny 16 / 1/125 sec. = EV 15
EV 15 = F/128 & 1/2 second.
+ 3 stops = EV 12 = f/490 and 4 seconds.
Add more time if you have less than bright sunshine or a film other than Acros.

[venchka]

Correction. + 2 stops over f/128.
If you use Fujifilm Neopan 100 Acros, the pinhole exposure is EV 13.
f/490 @ 2 seconds.
Any other film or reduction in light will require more exposure.
Tell us more about the camera. Show us the photos of the camera & photos from the camera.

[The Loft Studios]

I'm coming up with 3 point something-something seconds.....
So I would start at about 4 seconds and bracket from there.....

[trooper97]

Thanks for the help. This should be fun. I will be building my camera in a few more weeks and I will share photos of it and from it.

[venchka]

What format? Why such a long lens? Is it going to be 8x10??????????????????

[trooper97]

Originally Posted by venchka

What format? Why such a long lens? Is it going to be 8x10??????????????????

This will be medium format and I'm doing it just for fun. I want to see if I can pull off a 4 month or longer exposure.

[venchka]

Contact printing paper is about the only thing I can think of slow enough for Uber long exposures. Some neutral density gels over the hole would help. Did you get the idea from the photographer who makes 6 month + exposures?

[venchka]

Aiming a 15" (381mm) lens is going to be tricky.

[trooper97]

I'm going to build a mock-up out of oatmeal containers to get a feel for it first and give it a few test runs and then I will build the final camera. I figure I can mount the camera on my roof or in a tree and get a broad landscape for photograph and just pray for no rain.