Last autumn, after a few years away from it, I got back into 35mm stills photography. I’ve been reading a lot of books about photography: the art of it, the science and the history too. I’ve even taken a darkroom course to learn how to process and print my own black and white photos.
Shooting stills in my spare time gives me more opportunities to develop my eye for composition, my exposure-judging skills and my appreciation of natural light. Beyond that, I’ve discovered interesting parallels between electronic and photochemical imaging which enhance my understanding of both.
For example, I used to think of changing the ISO on a digital camera as analogous to loading a different film stock into a traditional camera. However, I’ve come to realise it’s more like changing the development time – it’s an after-the-fact adjustment to an already-captured (latent) image. There’s more detail on this analogy in my ISO article at Red Shark News.
The importance of rating an entire roll of film at the same exposure index, as it must all be developed for the same length of time, also has resonance in the digital world. Maintaining a consistency of exposure (or the same LUT) throughout a scene or sequence is important in digital filmmaking because it makes the dailies more watchable and reduces the amount of micro-correction which the colourist has to do down the line.
Anyway, this is all a roundabout way of explaining why I decided to make a pinhole attachment for my SLR this week. It’s partly curiosity, partly to increase my understanding of image-making from first principles.
The pinhole camera is the simplest image-making device possible. Because light rays travel in straight lines, when they pass through a very small hole they emerge from the opposite side in exactly the same arrangement, only upside-down, and thus form an image on a flat surface on the other side. Make that flat surface a sheet of film or a digital sensor and you can capture this image.
How to make a pinhole attachment
I used Experimental Filmmaking: Break the Machine by Kathryn Ramey as my guide, but it’s really pretty straightforward.
You will need:
- an extra body cap for your camera,
- a drill,
- a small piece of smooth, non-crumpled black wrap, or kitchen foil painted black,
- gaffer tape (of course), and
- a needle or pin.
- Drill a hole in the centre of the body cap. The size of the hole is unimportant.
- Use the pin or needle to pierce a hole in the black wrap, at least a couple of centimetres from the edge.
- Cut out a rough circle of the black wrap, with the pinhole in the middle. This circle needs to fit on the inside of the body cap, with the pinhole in the centre of the drilled hole.
- Use the gaffer tape to fix the black wrap tightly to the inside of the body cap.
- Fit the body cap to your camera.
The smaller the pinhole is, the sharper the image will be, but the darker too. The first pinhole I made was about 0.1-0.2mm in diameter, but when I fitted it to my camera and looked through the viewfinder I could hardly make anything out at all. So I made a second one, this time pushing the pin properly through the black wrap, rather than just pricking it with the tip. (Minds out of the gutter, please.) The new hole was about 0.7mm but still produced an incredibly dark image in the viewfinder.
Exposing a pinhole image
If you’re using a digital camera, you can of course judge your exposure off the live-view screen. Things are a little more complicated if, like me, you’re shooting on film.
In theory the TTL (through the lens) light meter should give me just as reliable a reading as it would with a lens. The problem is that, even with the shutter set to 1 second, and ISO 400 Fujifilm Super X-tra loaded, the meter tells me I’m underexposed. Admittedly the weather has been overcast since I made the pinhole yesterday, so I may get a useful reading when the sun decides to come out again.
Failing that, I can use my handheld incident-light meter to determine the exposure…. once I’ve worked out what the f-stop of my pinhole is.
As I described in my article on aperture settings, the definition of an f-stop is: the ratio of the focal length to the aperture diameter. We’re all used to using lenses that have a clearly defined and marked focal length, but what is the focal length in a pinhole system?
The definition of focal length is the distance between the point where the light rays focus (i.e. converge to a point) and the image plane. So the focal length of a pinhole camera is very simply the distance from the pinhole itself to the film or digital sensor. Since my pinhole is more or less level with the top of the lens mount, the focal length is going to be approximately equal to the camera’s flange focal distance (defined as the distance between the lens mount and the image plane). According to Wikipedia, the flange focal distance for a Pentax K-mount camera is 45.46mm.
So the f-stop of my 0.7mm pinhole is f/64, because 45.64 ÷ 0.7 ≈ 64. Conveniently, f/64 is the highest stop my light meter will handle.
The website Mr Pinhole has a calculator to help you figure this sort of stuff out, and it even tells you the optimal pinhole diameter for your focal length. Apparently this is 0.284mm in my case, so my images are likely to be quite soft.
Anyway, when the sun comes out I’ll take some pictures and let you know how I get on!