Working Distance Calculator
Find the subject-to-lens distance at a given magnification and the room you'll have.
Results
- Subject-to-lens distance
- 20.0 cm
- Lens-to-sensor (image) distance
- 20.0 cm
- Total subject-to-sensor distance
- 40.0 cm
Distance from the subject to the lens's optical centre, which sits somewhere inside the barrel. True working distance, the clear gap in front of the filter thread, is shorter than this by roughly half the lens's physical length.
How far the lens's optical centre needs to be from the sensor, which is why macro lenses and extension tubes physically get longer at higher magnifications.
The full span from subject to sensor plane, useful for setting up a focus rail or copy stand. At 1:1 this bottoms out at four times the focal length.
How it works
At macro distances, how much space sits between your lens and the subject decides whether a shot is possible at all. Too little and you block your own light, cast a shadow across the frame, or send a skittish insect flying before you press the shutter.
Working distance is governed by focal length. At the same magnification, a 100mm macro lens sits roughly twice as far from the subject as a 50mm, which is exactly why longer macro lenses cost more and are worth it for live subjects.
One definition worth being precise about. The distances here are measured from the lens's principal planes, which sit somewhere inside the barrel rather than at the front element. The clear gap in front of your filter thread, which is what most photographers mean by working distance, is shorter than the subject-to-lens figure by an amount that depends on the individual lens design.
The formula
object distance = f × (1 + 1/m), image distance = f × (1 + m)
f is the focal length and m the magnification. At 1:1 both equal 2f, so the total subject-to-sensor distance bottoms out at 4f, its minimum for any magnification.
Frequently asked questions
- Why is total distance smallest at 1:1?
- The subject-to-sensor distance is a curve with its minimum at 1:1, where it equals four times the focal length. Move to either higher or lower magnification and the total grows, which is a genuinely counterintuitive property of the thin-lens equation.
- How do I get more working distance?
- Use a longer focal length. Working distance scales directly with focal length at a fixed magnification, so a 180mm macro doubles the room a 90mm gives you. Teleconverters help too; extension tubes make it worse.
- Why is the real gap smaller than the number shown?
- Because the calculation measures from the optical centre, not the front of the barrel. Subtract roughly half your lens's physical length for a rough idea of the actual clear space, or measure it once at a known magnification and reuse that offset.