How to use the Lens Calculator #
The calculator works with four numbers: subject width (how wide a slice of the scene you want to capture), sensor width (how physically wide the camera's sensor is), distance (from camera to subject) and focal length (the lens's focal length). Know three of them, and the tool can always work out the fourth — just leave exactly one field empty.
- Pick your camera from the camera icon next to the sensor field, or type the sensor width in by hand if your camera isn't on the list.
- Fill in two of the remaining three fields — subject width, distance and focal length — and leave the one you want to find empty.
- Press Calculate. The result appears below the button and is saved to the history, so you can compare several calculations.
Tip: The sensor field doesn't have to hold a plain number. Pick a camera from the list and the field shows the camera's name, while the calculator still works with the correct millimeters behind the scenes. Start typing in the field yourself, and it automatically switches back to a plain number field.
Example: finding the right focal length for a close-up
You're shooting with a Sony FS7 (Super 35 sensor, 23.6mm wide) and need a tight close-up where just the face and shoulders fill the frame — a shot about 0.5 meters wide. Space constraints mean the camera can only stand 2.5 meters from the actor.
So you know subject width (0.5m), sensor width (23.6mm, from the camera pick) and distance (2.5m) — and leave focal length empty. The calculator says:
Focal length = Sensor × Distance ÷ Subject = 23.6 × 2.5 ÷ 0.5 ≈ 118mm
You'd want a lens around 100–135mm, depending on what's in your kit.
The formula works just as well the other way. Say you only have a fixed 85mm lens and want to know how far from the actor you need to stand to get the same framing — leave distance empty instead:
Distance = Subject × Focal length ÷ Sensor = 0.5 × 85 ÷ 23.6 ≈ 1.8m
How the distance formula works
Picture the camera's optical center as the tip of two mirrored triangles: one out in the real world (with subject width as its base and distance as its height), and one inside the camera (with sensor width as its base and focal length as its height). Because light travels in straight lines through the optical center, the two triangles are similar — and in similar triangles, the ratio of base to height is the same for both:
Subject width ÷ Distance = Sensor width ÷ Focal length
That's exactly the equation the calculator solves for whichever field you're missing. The reason it's specifically sensor width and subject width that go into the formula is that they're the two bases of those two triangles — they define how wide a slice of reality gets compressed onto how wide a piece of sensor. Distance and focal length are the matching heights that tie the two slices together.
Sensor width determines the focal length
Because focal length and sensor width sit on opposite sides of the ratio, focal length is directly proportional to sensor width when you want to keep the same framing at the same distance. Switch from a Super 35 camera (23.6mm) to a Full Frame camera (36mm), and the focal length has to grow to match, to get the same shot:
Full Frame: 36 × 2.5 ÷ 0.5 ≈ 180mm
Super 35: 23.6 × 2.5 ÷ 0.5 ≈ 118mm
The ratio between the two focal lengths (180 ÷ 118 ≈ 1.5) is the same as the ratio between the sensor widths (36 ÷ 23.6 ≈ 1.5) — that's the so-called crop factor you may know from Super 35 vs. Full Frame discussions. A wider sensor "captures" more of the image at the same focal length, so it needs a longer lens to zoom in to the same framing as a smaller sensor.