f-numbers

If the arithmetic on this page hurts your brain… don't worry about it, just bring your camera to class and we'll guess an exposure for you. But, if you're OK with arithmetic, you can save a few minutes, guess a little closer, and understand a little more about how cameras work.

http://en.wikipedia.org/wiki/Pinhole_camera http://en.wikipedia.org/wiki/Camera_obscura http://en.wikipedia.org/wiki/Normal_lens http://en.wikipedia.org/wiki/Photography  f-numbers =Focal Length= In order to guestimate what your exposure time will be, we need to consider a few numbers. The first thing we want to know about is the **//focal length//** of your camera. In lens photography, this is about how the lens is ground and assembled and how far away it focuses the rays. If you buy an SLR lens, you might hear that it is a "200mm f2.8" - 200mm is the "focal length" of the lens and "f2.8" is the "f-number" which tells you how much light the lens lets in.

A "normal" lens gives you a photo that "feels" normal... not close up or wide angle. A normal lens is one where the focal length of the lens is about the same as the diagonal of the film. So, in classic 35mm photography, the image was 24mm x 36mm with a diagonal of 43mm. So, actually, a "normal" 35mm lens is 43mm "long," but for technical reasons a 50mm lens has most often been considered "normal" in 35mm photography.

With a pinhole camera, there is no lens ground to some particular distance, so the focal length of your pinhole camera is just whatever the distance from your pinhole to your paper is: if your paper is 4"x5" - with a diagonal of about 6.4", then a "normal focal length" camera would have the pinhole about 6-1/2 inches in front of the paper. If the pinhole is 9 inches away, you have a "telephoto" pinhole camera, if the pinhole is 3 inches in front of the paper, you have a wide-angle camera.

No specific pinhole to paper distance is Normal, Tele, or Wide, it depends on the size of your paper.

=f-number= We need to know how much light your pinhole is letting in, in order to calculate exposure time. The //**f-number**// of a lens or camera is just the distance from "lens" to film, divided by the size of the hole. So if it's 10" to the film and the hole is 1" in diameter, then 10"/1" ``=`` an "f10" lens.

You can have all kinds of f-numbers, but here's a sequence starting with a sorta small hole, and proceeding to very very small:


 * 16 22 32 45 64 90 128 180 256 360 512**

You'll notice that every other number in this sequence, is double. In terms of light, however, every number is twice as much or as little light. Bigger f-numbers mean less light. So f22 is half the light of f16. If you double f16, to f32, that's 1/4 the light.

And what is our f-number? It depends on the size of your pinhole and the distance to the camera. My guess - I'm no expert at guessing the diameters of tiny holes! - but my guess is that the pinholes peeps showed me in class ranged from .010" to .045"

So, if your camera is only 2" deep, and you have a larger .045" pinhole, then your f-number is:
 * 2" / .045" ``=`` f44

On the other extreme, if your camera is 6" deep and you have a tiny .010" pinhole, then your f-number is:
 * 6" / .010" ``=`` f600

=Your Exposure= The four factors in exposure are:
 * Sensitivity of film, using an "ISO" number
 * Amount of light coming in, using an f-number
 * Amount of time of your exposure, in seconds
 * Amount of light in the scene, using a light meter, or a good guess

There's a famous rule of thumb in photography, that in //**Bright Sunlight**// (sun high in the sky, no clouds or haze) your exposure is f16 at 1/ISO. So, if you have ISO 100 film, or a digital camera with an ISO 100 chip, then in Bright Sunlight, your exposure is f16 at 1/100 second. You can change the combination around by doubling and halving: So you could also use twice the light and half the time: or you could use half the light and twice the time:
 * f11 @ 1/200 second
 * f22 @ 1/50 second.

Photographic paper is much slower than photographic film or digital sensors. I think our Arista II paper has an ISO of about 3. So in Bright Sunlight, our exposure is:
 * f16 @ 1/3 second

And the exposure combinations for Bright Sunlight and our ISO 3 paper are:
 * f16 @1/3'
 * f22 @ 2/3'
 * f32 @ 4/3'
 * f45 @ 8/3' or about 3 seconds
 * f64 @ 16/3' - 5 seconds
 * f90 @ 32/3' - 11 seconds
 * f128 @ 64/3' - 21 seconds
 * f180 @ 128/3' - 43 seconds
 * f256 @ 256/3' - 85 seconds
 * f360 @ 512/3' - 171 seconds - or about 3 minutes
 * f512 @ 1024/3' - 341 seconds - or 5"41' - or about 6 minutes

For our 2 extreme cameras in bright sunlight, one will require an exposure time of 3 seconds, and the other about 9 minutes.
 * If it's hazy or late in the day, exposure times will double, or double twice...
 * Indoors it will be hours…

=Your 1st Exposure= In class we'll guess what size pinhole you have and measure the distance to your paper. If you have a calculator, or if your phone has a calculator, bring it. We just need to do one division, eg, your 4" focal length divided by your .030" pinhole ``=`` f133

Then we'll look at our chart... and see that in bright sunlight, //this// camera wants about a 23 second exposure.
 * at 6:30pm it will be longer... maybe 90 seconds

Then you'll take your picture, process it, and... //drum roll please...// **look at it!** And depending on whether it seems lite or dark, we'll pick a new, improved exposure time for your camera about an hour before sunset... maybe 60 seconds... maybe 120 seconds...

If all this arithmetic hurts your brain, don't worry about it, we can sort it out in class. But, if you're ok with it, you can probably zero in on your exposure a little more easily.