Aperture
Part of Photography
Aperture controls the size of the opening that admits light into a camera, balancing exposure speed against depth of focus.
Why This Matters
Every camera — from the simplest pinhole box to a sophisticated lens instrument — must control how much light reaches the sensitive plate. Too much light and the image is overexposed, washed white. Too little and nothing appears at all. Aperture is the primary tool for this control when lighting conditions cannot be changed.
Beyond controlling exposure, aperture determines how much of the scene appears sharp. A large opening produces a shallow zone of sharpness — useful for portraits where you want the background blurred. A small opening makes everything sharp from near to far — essential for photographing documents, machinery, or landscapes where every detail matters. Mastering aperture means understanding this trade-off and selecting the right opening for each situation before the shutter opens.
In a rebuilding civilization, most photographic work involves documents, machinery, botanical specimens, and land records — subjects that demand maximum sharpness throughout. Knowing how to select the correct aperture stop makes the difference between a photograph that serves as a permanent, readable record and one that is too blurry or too overexposed to be useful.
What Aperture Actually Is
The aperture is any physical restriction placed in the optical path of the camera that limits the diameter of the light beam entering from the lens or pinhole. In the simplest camera, the pinhole itself is the aperture — fixed in size. In a lens camera, a set of interchangeable stops or a rotating disc with different holes provides variable aperture.
The key number is the f-stop, or f-number. This is the ratio of the lens focal length to the aperture diameter. A lens with a 100 mm focal length and a 25 mm aperture opening has an f-stop of f/4. The same lens stopped down to a 12.5 mm opening is f/8.
The mathematical consequence is that each time you double the f-number (halve the aperture diameter), you cut the light by a factor of four — because area decreases as the square of diameter. Standard f-stop sequences are built around this: f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. Each step doubles the required exposure time.
Making Aperture Stops
You do not need precision instruments to make usable aperture stops. A set of five stops cut from thin tin or brass sheet will cover nearly every photographic situation.
Materials:
- Thin sheet metal (tin can lids, flattened brass strips)
- Metal file or fine saw
- Drill or punch
- Black paint or soot
Construction:
- Cut rectangular blanks about 30 x 50 mm from thin metal sheet
- File or sand the edges smooth
- Mark the center of each blank
- Drill or punch holes of the following diameters, one per blank:
| Stop | Hole Diameter | f-stop (100mm lens) | Exposure Factor |
|---|---|---|---|
| Full open | 25 mm | f/4 | 1x |
| Large | 18 mm | f/5.6 | 2x |
| Medium | 12 mm | f/8 | 4x |
| Small | 8 mm | f/11 | 8x |
| Minimum | 6 mm | f/16 | 16x |
- Paint or blacken both sides of each stop with soot or flat black paint
- File a notch or mark each stop with a scratch to identify it by touch in the dark
Mounting the stops: The stops slot into a narrow groove cut into the lens barrel just behind or in front of the lens. Make the groove exactly as wide as your metal blanks so they slide in snugly but can be swapped without tools.
Aperture and Depth of Focus
Depth of focus is the range of distances from the camera that appear acceptably sharp in the photograph. It is governed primarily by aperture size.
With a large aperture (small f-number), light from points just in front of or behind the focused distance hits the film as a visible blur circle. Only a narrow range of distances appears sharp.
With a small aperture (large f-number), those same blur circles become tiny enough that they are invisible in the final print. A much wider range of distances all appear sharp simultaneously.
Practical consequences:
For copying documents or photographing machine parts lying flat, any aperture works — everything is at the same distance.
For photographing a person outdoors where background detail would be distracting, use a large aperture to blur the background while keeping the face sharp.
For a group photograph where people stand at different distances from the camera, use a small aperture so everyone falls within the sharp zone.
For architectural photographs or land surveys, use the smallest aperture available to keep both near and far features sharp.
Aperture in Practice: The Zone System Without Meters
Without a light meter, aperture selection becomes an exercise in systematic judgment. A simple rule set covers most daylight situations.
The Sunny-16 approach:
In bright direct sunlight with a medium-speed gelatin-bromide plate:
- f/16 (minimum aperture): expose for approximately 1/ISO seconds — roughly 1-2 seconds for typical hand-coated plates
- f/11: double the time — 2-4 seconds
- f/8: double again — 4-8 seconds
- f/5.6: 8-16 seconds
- f/4 (full open): 16-30 seconds
Condition adjustments:
| Lighting Condition | Adjust Aperture Or Time By |
|---|---|
| Thin overcast (no shadows) | 2x time or one stop larger aperture |
| Heavy overcast | 4x time or two stops larger |
| Open shade (bright sky, no sun) | 4x time |
| Deep shade or indoors by window | 8-16x time |
| Interior with lamp at 1 meter | 100-1000x time |
Testing and calibration: Your first roll or set of plates with any new emulsion batch should be a series of test exposures. Take the same scene at three different aperture settings. After developing, evaluate the negatives. The one showing full shadow detail without blocked-up highlights is your baseline. Adjust all future exposures relative to this reference.
Aperture Trade-offs: A Decision Framework
Before each exposure, work through this sequence:
- What is the subject? Is everything at one distance, or does depth matter?
- How bright is the light? Can you achieve a workable exposure with the desired aperture?
- Is the subject moving? Moving subjects require shorter exposure times, which may force a larger aperture even if sharpness would prefer a smaller one.
- What is the purpose of the photograph? A portrait permits blur in background; a technical record requires overall sharpness.
Most practical photography for rebuilding purposes — copying documents, recording equipment, documenting injuries, surveying land — calls for the smallest aperture the light level permits. Use the largest aperture only when working in very dim conditions or when you specifically need to isolate a subject from its background.
Common Problems and Solutions
Image too dark (underexposed): Use a larger aperture (lower f-number) or increase exposure time.
Image too light and washed out (overexposed): Use a smaller aperture or reduce time.
Everything appears blurry: Two possible causes — focus is wrong (adjust lens-to-plate distance), or the aperture was too large (use a smaller stop and longer time).
Sharp center, blurry edges: The lens has optical aberrations that are reduced by stopping down. Use a smaller aperture.
Dim corners of image (vignetting): The aperture stop is too small relative to the lens barrel diameter, or the stop is positioned at the wrong distance from the lens. Reposition or open up slightly.
Diffraction blur (using smallest possible stop): Very tiny apertures produce soft images due to light wave diffraction. Do not go smaller than needed for the required depth of focus. For most lenses, f/22 or smaller gives worse sharpness, not better.