How do I calculate the best pinhole size?

Classic guidance balances diffraction and geometric blur: diameter scales with the square root of focal length. This calculator applies practical maker formulas and shows how blur changes if you drill slightly off-optimal.

Does pinhole size matter for solargraphs?

Yes. Too large a hole softens the trail; too small increases exposure time and diffraction. Match hole size to your can depth and paper distance before committing to a multi-week exposure.

What focal length should I use?

Focal length is roughly the distance from the pinhole to the paper. Deeper cans mean longer focal lengths, narrower fields, and different optimal hole sizes — adjust the calculator to your measured depth.

Tools

Free Pinhole Optics Calculator

Compute optimal pinhole diameter, effective f-number, angular field of view, and geometric blur for your can or box camera.

ContainerPaper

Focal length (f)

Pinhole diameter (∂)

Wall thickness (w)

⚠ Preset dimensions are nominal — measure your container with calipers before building.

Open full simulator

Optics

Slant f (pinhole→base): 85.0 mm; Blur reference on flat geometry
Preview scale (2R): 65.8 mm; Cylindrical paper at bore
Effective f: 85.0 mm; f − wall/2
f-number: f/177
Optimal pinhole (slant): 0.411 mm; 1.9·√(λf_eff)
Optimal pinhole (2R): 0.361 mm; For paper preview scale
Trail width (preview): 0.61 mm; max(sun disk, blur) at 2R
Sun disk (2R preview): 0.61 mm; 2R·0.53°
Capture width: ±77°; Azimuth (cylindrical)
Capture height: ±44°; Altitude at bore centre
Vignetting limit: 78°; θ_vig = atan(∂/w)

Pinhole size is close to optimal for the cylindrical preview scale.

3D cutaway

f = 85 mm

∂ = 0.48 mm

w = 0.10 mm

blur ≈ 0.61 mm

θ_vig = 78°

±44° alt

Ø 66 × 157 mm · paper 178×127 mm

optimal ∂ = 0.361 mm

fbluroptimal ∂Drag · pinch zoom

Frequently asked questions

How do I calculate the best pinhole size?: Classic guidance balances diffraction and geometric blur: diameter scales with the square root of focal length. This calculator applies practical maker formulas and shows how blur changes if you drill slightly off-optimal.
Does pinhole size matter for solargraphs?: Yes. Too large a hole softens the trail; too small increases exposure time and diffraction. Match hole size to your can depth and paper distance before committing to a multi-week exposure.
What focal length should I use?: Focal length is roughly the distance from the pinhole to the paper. Deeper cans mean longer focal lengths, narrower fields, and different optimal hole sizes — adjust the calculator to your measured depth.

Use with the Solar Pinhole Camera simulator to validate sun trail framing after you lock in hole size and focal length.