With a pinhole camera, the exposure time ranges from a few seconds in full sun to several minutes in shade or indoors: start from a rough figure based on the light, make a test, then lengthen the time if the image comes out too pale and shorten it if it comes out too dark.
You do not really press a shutter release, you wait, because light takes time to pass through the pinhole and imprint itself on the paper. This time is counted in seconds, sometimes in minutes, and it is what decides the photo.
The exposure time is how long light enters the box, and with a pinhole camera it is always much longer than with a conventional camera.
Why pinhole camera exposure time is so long
Two reasons add up.
The first lies in the pinhole. With no lens to concentrate the light, the pinhole camera simply filters it through a tiny opening, so very little light gets in at a time. That is the very principle of the process, explained in the practical guide to pinhole photography, where this hole acts as an extremely stopped-down aperture.
The second lies in the paper. The black and white photo paper used in pinhole photography has low sensitivity, around 3 to 6 ISO, whereas an ordinary analogue film is far more responsive. Paper therefore needs much more light to react.
With little light coming in and a medium that needs a lot of it, the exposure inevitably gets longer.
How to calculate pinhole camera exposure time
There is no single figure, because the time depends on the light at that moment, your pinhole and your paper. You estimate, you test, then you adjust.
The exposure chart as a starting point
The Sténoflex kit comes with an exposure chart that gives a starting point for each light condition: 20 s in sun, 30 s in hazy sun and 40 s under cloud, with the paper in the kit. You start from this guideline, make the exposure, then note the result.
It is a basis, not a rule, because no two days of full sun are alike. The chart points the way, but your eye decides.
The exposure test to fine-tune the time
The most reliable method is still the test. Take a first photo with the estimated time, then develop it: if it is too pale, the exposure was too short, and if it is too dark, it was too long.
In workshops, this is the step that brings the fastest progress, because the first photo rarely serves as a masterpiece, but rather as a measurement.
Noting each exposure to adjust the next
Note each exposure, recording the light, the duration and the result. After a few photos, you can read a scene at a glance and know which range to go for, and it is this memory that gradually replaces the chart.
Exposure times by light
Here are a few rough figures. They remain starting points and never certainties, since your paper and your pinhole shift everything.
| Light | Rough figure |
|---|---|
| Full sun | a few seconds to a few tens of seconds |
| Overcast sky | tens of seconds to a minute or two |
| Shade, late afternoon | several minutes |
| Indoors near a window | many minutes |
Remember the logic more than the numbers: the less light there is, the longer the exposure stretches, and it stretches quickly.
Rough figures for a pinhole camera loaded with paper. With the Sténoflex, start instead from the guidelines on its exposure guide card: 20 s in sun, 30 s in hazy sun, 40 s under cloud.
The Schwarzschild effect on long exposures
In low light, the exposure does not lengthen steadily: it runs away.
Halving the light does not double the time, because in low light the paper becomes even less responsive and the exposure needs far more than a simple calculation would predict. Taken to the extreme, this logic leads to solargraphy, which pushes the exposure to several months. Photographers call this drop-off the Schwarzschild effect, or reciprocity failure. The pinhole calculator builds in this correction.
The answer is still the same: test, take notes, and lengthen the time decisively rather than by a little, because an exposure that is too long is better than a ghost photo.
Calculating exposure time without a light meter
- Without a light meter, start from the sunny f/16 rule: in full sun, at f/16, the correct exposure is roughly the reciprocal of the medium's sensitivity, in seconds.
- Next, calculate the f-number of your pinhole camera by dividing the distance between the pinhole and the paper by the pinhole diameter, which is usually between 0.2 and 0.8 mm. For the Sténoflex, 47 mm divided by 0.26 mm gives about f/180.
- Finally, multiply the reference time by the square of the ratio between this f-number and 16.
The result gives a starting point for full sun, to be lengthened under an overcast sky or in shade.
From exposure time to the finished image
Once the exposure has worked, you still need to bring out what the light has laid down: that is the stage of developing the photo, where the image appears in the developer.
If your first tries come out too pale or too dark, that is the very material of the adjustment. The Sténoflex is designed for exactly this, with its exposure chart and its ten sheets to experiment with. When a photo really fails, you can often work out the cause by reading back through your notes.
With a pinhole camera, the exposure time is the central skill, the one you learn to feel, photo after photo.
Éric Marais — Photographer, author of two books on pinhole photography (published by Pearson) and creator of the Sténoflex. He has run introductory workshops in pinhole photography and analogue developing from the very beginning.
