Viewfinder
Calculate how a lens and sensor frame a scene, then compare two exposure settings without confusing light captured with ISO gain.
WORKING TOOLStart here
Use this when choosing a focal length or working out why two setups frame a subject differently. The calculator gives geometry and exposure estimates; it does not simulate a finished photograph.
By the end: Read scene width and angle of view, separate focal length from equivalent view, and explain which exposure change alters captured light.
What you need: Focal length in millimetres, sensor dimensions, subject distance in metres and two exposure settings. The initial values can be used for practice.
Before you begin: No optics knowledge needed. Use a camera with a zoom lens, or a phone with clearly labelled lens choices.
How to use this tool
- Enter the sensor format, lens focal length and subject distance in the workbench. Read the predicted scene width before changing anything.
- Change only focal length, then restore it and change distance. Compare what each change does; a camera position change also changes perspective in a real photograph.
- Compare exposure A/B, then export the study with its assumptions. Use the example below to check the crop-factor calculation.
WORKING LAB · FRAMING + EXPOSURE
Predict the change, then calculate it
No camera or photograph is required for this model. First isolate one variable. Then use your own camera to check whether the prediction explains the photograph.
1. Frame from a fixed position
Focal length sets the lens’s angle of view for a sensor size. Distance sets how wide a flat subject plane is within that angle. Moving the camera changes perspective; cropping a frame does not move the camera.
54.4° horizontal × 37.8° vertical
Approximate scene width at 3 m: 3.09 m.
Diagonal crop factor: 1.00×. Equivalent diagonal view on full frame: 35.0 mm. The lens remains 35 mm.
Worked example: on 36 mm sensor width, a 50 mm lens covers about 2.16 m at a 3 m subject plane. Doubling focal length to 100 mm halves that width to 1.08 m without changing your viewpoint.
Check yourself
At the same position, what happens to scene width if you change 50 mm to 100 mm? What happens if you then double subject distance?
Answer and reasoning
The scene width halves, then doubles back. The final subject size can match the original, but moving the camera changes near-to-far size relationships. A matched frame is not proof of matched perspective.
2. Estimate depth of field
Using 35 mm focused at 3 m: near limit 2.35 m, far limit 4.16 m; hyperfocal distance 10.65 m.
Assumed circle of confusion: sensor diagonal / 1,500 = 0.0288 mm. This geometric convention estimates acceptable blur for an assumed viewing condition; it does not predict pixel-level sharpness. Diffraction, focus breathing, pupil magnification and macro corrections are outside this thin-lens model.
3. Trade exposure settings
A stop is a factor of two. Doubling exposure time adds one stop of light; multiplying the f-number by √2 removes one stop. ISO changes the indicated image brightness without adding photons to the sensor. Compare B with A below.
Light reaching the sensor: 0.00 stops.
ISO contribution: 0.00 stops. Nominal brightness change: 0.00 stops.
Approximately the same nominal brightness. Motion blur, depth of field and noise need not match.
Worked example: f/4, 1/250 s, ISO 400 → f/5.6, 1/125 s, ISO 400 trades one stop of aperture for one stop of time. The model predicts equal brightness. The longer time can record more subject movement.
Check yourself
Keep f/4 and ISO 400. Change 1/250 s to 1/500 s. What ISO restores the nominal brightness, and does it restore the lost light?
Answer and reasoning
ISO 800 restores the nominal brightness. The sensor still received half as much light because the exposure time halved. Higher ISO does not undo that; expect a possible noise tradeoff.
Model limits: rectilinear geometry at ordinary distances; no lens distortion, focus breathing, pupil effects or macro correction. Crop equivalence uses diagonals, so different aspect ratios do not match both edges. Exposure uses ideal full-stop aperture values behind rounded labels and nominal ISO; transmission, processing, flash and real camera metering are not simulated. Depth of field is a calculated blur-threshold model, not a photographic preview.
References: Nikon: focal length · DX and FX formats · Aperture.
Predict the frame before you zoom
Focal length is a lens property measured in millimetres. Field of view is how much of the scene fits on a particular sensor. A longer focal length on the same sensor gives a narrower view. Crop factor compares a format with a reference; it does not physically change the lens.
Keep viewpoint separate from framing. Moving the camera changes the relative sizes of nearby and distant objects. Zooming from the same position changes framing. This distinction helps explain why matching a subject’s size by stepping backward produces a different-looking background.
Worked example
Original comparison plan: place a mug one metre away with a doorway several metres behind it. From one marked position, take a wide and a tighter frame. Then step back and zoom until the mug matches its first size. The last photograph changes the mug–doorway size relationship because the viewpoint moved. A 35 mm lens on a format with 1.5× crop factor has an approximately 52.5 mm full-frame-equivalent field of view; it remains a 35 mm lens.
Try it yourself
Write your answer before opening the explanation. Use the worked example if you get stuck.
Question 1
A 24 mm lens is used on a 1.5× crop format. Calculate its equivalent focal length. Which comparison isolates framing: zooming from a fixed position, or moving backward while zooming?
Show answer and reasoning
24 × 1.5 = 36 mm equivalent. Keep the camera fixed to isolate framing. Moving backward changes perspective as well, so it cannot establish what focal length alone caused.
Watch for: Phone zoom can switch lenses, crop digitally, or combine images. Record the actual mode before interpreting a comparison.
Summary + next step
Read scene width and angle of view, separate focal length from equivalent view, and explain which exposure change alters captured light.
Keep the result: Export the study and record the actual viewpoint if you later test it with a camera.
Sources and further reading
Read these references for the concepts used in the lesson.
Lesson updated 2026-09-14 · Guidance updated 2026-10-22
Coverage and further development
Framing, format, depth-of-field and exposure calculations, worked studies, checkpoints and export.