How camera apertures control light

One question, one controlled example

Aperture is an opening that limits light through an optical system. This guide follows a 50 mm ideal thin lens imaging a distant scene, on its optical axis. We change only the circular stop unless a separate shutter control is shown.

A deliberately open apparatus

The upper green plate is the aperture stop; the lower rectangle is the sensor. Posts and retaining rings hold the stop over the sensor. Glass is omitted so the opening stays visible. This is a constructed optical teaching holder, not a working production camera.

A stop and an iris are related, but different

An aperture stop is the part that limits the light cone. An iris is an adjustable diaphragm: its overlapping leaves move to change the opening. Many photographic lenses use an iris, while this teaching holder uses removable circular stop plates.

What the comparison shows

The supplementary front-view iris schematic shows overlapping leaves around a polygonal opening. It is not a physical build or an area measurement. All numerical hole areas in this guide refer to the actual circular plates above.

The entrance pupil is an apparent opening

Look through a real lens from the front: the front optics form an image of the aperture stop. That apparent opening is the entrance pupil. Its diameter can differ from the physical iris hole. Here we explicitly make them coincide in an ideal thin-lens model.

Why the number runs backward

At fixed focal length, N = f/D. Dividing 50 mm by 25 mm gives 2; dividing it by 12.5 mm gives 4. A smaller denominator therefore produces a larger f-number. Both lengths use the same units, so the ratio is dimensionless.

Use the exact numbers beneath the labels

The exact sequence is 2, 2√2, 4. The middle value is about 2.828, conventionally labeled f/2.8. This guide uses the exact value for calculations and geometry: the pupil diameters are 25 mm, about 17.678 mm and 12.5 mm.

State what stays fixed

For the same distant, on-axis scene, focal length, focus and ideal equal transmission, image irradiance is proportional to 1/N². Irradiance is the light power arriving per sensor area. Shutter duration and ISO stay fixed during this comparison.

Square the diameter change

A circular opening has area π(D/2)². The f/2 stop has area about 490.87 mm²; f/4 has about 122.72 mm². The second diameter is one half as large, so its area is (1/2)² = 1/4 as large.

Area and light rate agree within this model

The physical holes change with the selected preset; the drawing is not a scaled whole holder. With the stipulated ideal conditions, one quarter the pupil area corresponds to one quarter the image light rate. This is not a claim that every real lens transmits exactly the same fraction.

A concrete exposure comparison

At f/2 and 1/125 s, the relative optical exposure is 100%. Keeping f/2 but changing to 1/250 s halves the time and exposure. Keeping 1/125 s but changing to f/2.8 halves the light rate and exposure instead.

A numerical shutter control

The duration buttons calculate accumulation time; they do not animate a physical shutter or move the stop. Doubling duration can compensate a one-stop reduction in light rate in this ideal exposure calculation. Motion blur and image processing are outside this model.

Entrance pupil versus physical opening

The physical aperture hole and entrance pupil coincide only because our model stipulates an ideal thin lens at the stop plane. Real front lens groups may magnify the aperture image. Use entrance-pupil diameter in N=f/D rather than assuming any measured blade hole is D.

Vignetting is outside the readout

Away from the image center, lens geometry and obstructions can reduce illumination: vignetting. Our percentages describe the paraxial on-axis image under equal transmission and distant focus. They are not a complete brightness map across a real sensor.

Other optical effects are not simulated

Real lenses have transmission losses, aberrations and diffraction. At close focus, effective f-number can differ from the nominal distant-focus value. Aperture also affects depth of field, but this guide supplies no depth-of-field model or quantitative prediction.

80/20 = 4 and 80/10 = 8. Diameter halves, so circular area and modeled light rate become one quarter: two stops down.

Transfer the relationship, not the example numbers

The same ratio works for another focal length. Find N=f/D at each setting, then square the diameter ratio to compare circular pupil areas. Keep the model assumptions explicit before interpreting area as an image light-rate ratio.

Three linked quantities

N=f/D defines f-number through entrance-pupil diameter. Under our stated ideal conditions, light rate varies as 1/N². Optical exposure multiplies that rate by shutter duration. ISO is held fixed and does not supply more incident light.

Try one deliberate comparison

Choose f/2, then f/2.8: observe the genuinely smaller hole and half light rate. Return to the duration slide to compare the separate effect of halving shutter time. In a real lens, identify its iris and remember that front optics can change the apparent pupil.