**Metric used**

When talking about measurements for a pinhole it is easier to think in metric, as in millimeters. You already do this if you own a 35 mm camera in that yours lensâ€™ focal length is sized in millimeters. A normal lens being around 50-55 mm and a telephoto being anything greater than say 100 mm. A wide angle lens is from 18-35 mm and so on.

**Deciding on sizes**

In deciding on the ideal pinhole you can go from a known focal length and then determine the size of the ideal pinhole or you can go from the size of the pinhole and go for the ideal focal length.

There are many factors influencing the size of the pinhole and authors will disagree as to the intensity of the effects of any particular factor. One factor is the color of the light. Blue light has the highest resolution and red the lowest. This affects the size of the hole. For practical purposes, we compromise on green light (being in the middle of the visible spectrum).

Here are the metric (mm) formulas for green light (550 nm):

**focal length of lens = (size
of pinhole) X (size of pinhole) X 1090**

174.4mm = 0.4mm X 0.4mm X 1090

(green = 1090, blue = 1332 and red = 922, in case you use a colored filter or, ortho film)

Now the other way round:

**size of pinhole = squareroot
(focal length/1090)**

0.4mm = sqrt( 174.4/1090) = sqrt(0.1597) = 0.4

**Just for fun**

Let us assume you are going to use your living room for a giant pinhole camera and it is 15 feet from the window to the wall. How large a pinhole would we need?

25.4mm = 1 inch

15 feet = 15 x 12 inhces= 180 inches or 4572 mm

and into the equation and we get 2.05 mm

Remember: F# = focal length / diameter of lens opening.

4572 / 2.05 = F2230!

We can do tables of common
values starting with focal length:

TABLE OF FOCAL
LENGTH TO PINHOLE (mm)

This can get confusing in terms of
using a light meter that stops as F22, so to help out here is a normal **progression
of F numbers** going into the pinhole range:

*REMEMBER each jump in F# is a halving
of the amount of light available*

*One of the neatest things about pinhole
photography is the time dilation effect.*

*Over a one-hour exposure, people
and cars will disappear!*

Now a table for the other way, as most of the time we cannot make a pinhole to an exact size, but we can adjust how we put the camera together to get the optimum focal length for a pinhole that we have.

**TABLE OF PINHOLE
SIZE TO FOCAL LENGTH**

**Why do larger format pinhole cameras
look sharper?**

As the size of the negative gets bigger, so does the size of the ideal pinhole for the same angle of view.

Let's calculate the total lines of resolution for each format, from 35 mm to 8x10, assuming that the resolution is the same as the pinhole size (actually under ideal conditions it can be up to 10 fold higher, but the conditions would be the same for all formats)

Format |
Focal Length |
Ideal Pinhole |
Neg Diagonal |
Lines/negative |
f-stop |

35mm |
22 |
0.142 |
43 |
303 |
155 |

6x7 |
42 |
0.196 |
90 |
458 |
214 |

4x5 |
74 |
0.262 |
154 |
587 |
282 |

5x7 |
100 |
0.303 |
210 |
693 |
330 |

8x10 |
150 |
0.371 |
308 |
830 |
404 |

The change in negative diagonal from 35 mm to 8x10 is 7.16 fold, but the resolution is only 2.74 fold higher, so clearly there is a diminishing return for going to a larger size. Basically, use the largest size you are willing to carry. [and can afford the film for!] ALSO, remember the f-stop gets higher with the larger formats, so you run out of light/time at some point.

Personally, I find the 4x5 format to be near ideal. There are a large number of films available [more varied than 35 mm actually], access to roll film backs, etc. The negative is just large enough to contact print and still be visible. Enlargers are in the painful, but still possible range [8x10 enlargers are in the obscene range!]. Lastly, using a pinhole camera is already a SLOWER process, so carrying around a 4x5, though slower to use than 35, is not that much different for pinholes. [8x10 cameras need an assistant or pack animal to carry around]

Lastly, for the **truly
anal**, here are formulas for accounting for the angle of view, film format,
color of light and even the thickness of the metal used to make the pinhole.