π Focal Length Calculator
Calculate focal length, magnification, and subject framing.
What focal length do I need to fill the frame with a subject?
Required focal length
How to Use This Calculator
Three tabs cover the most common questions about focal length. "Find Focal Length" tells you which lens to grab when you know how big your subject is and how far away you'll be standing. "Image Magnification" shows how large your subject will appear on the sensor at a given distance. "Subject Framing" tells you how wide a scene a particular lens will capture at a given distance.
On the "Find Focal Length" tab, enter the real-world size of your subject in mm (a standing person is about 1800mm), the shooting distance in mm, and select your sensor size.
Read the required focal length. The result also shows the nearest standard lens focal length so you know whether your 85mm or your 105mm is the better choice.
On "Image Magnification," enter your lens focal length and subject distance to see what fraction of life-size the image will be. Values above 0.5 are in close-up range; 1.0 is true macro.
On "Subject Framing," enter a focal length and distance to see how wide a subject fills the sensor. Useful for scouting: how far do you need to stand with a 50mm to capture a full-length wedding party?
The Thin Lens Formula
f is focal length, u is subject distance from the lens, and v is the image distance (where the sensor sits). For real photography, u is much larger than f, which simplifies the math to the ratio shown above. The required FL formula is the most practical one: divide your sensor size times the shooting distance by the subject size and you get the focal length that fills the frame.
Real-World Examples
When You Need This
Before a commercial shoot in a small studio, knowing the required focal length ahead of time tells you whether your 70-200mm zoom will even work or whether the room is too small. If you need 120mm to fill the frame with a product at 2 meters, but your studio is only 1.5 meters deep, you either need a shorter focal length or a different location. Better to figure that out the day before, not when the client is standing there.
Event photographers use the framing tab to plan wide group shots. Type in the width of your group (say 6 meters) and how far you can stand back (maybe 8 meters), and see which focal length fits everyone in. You can also use this to check whether a wide-angle lens will distort the edges of a large group by comparing the required focal length against what you'd expect from a "normal" field of view for your sensor.
Frequently Asked Questions
What focal length do I need to photograph a subject?
Focal length = (image size Γ subject distance) / subject size. Example: to fill a 36mm sensor with a 1.8m person at 5m distance: f = (36 Γ 5000) / 1800 β 100mm.
What is image magnification in photography?
Magnification = image size / subject size = focal length / (subject distance β focal length). At 1:1 macro, the image on the sensor is the same size as the subject.
Does focal length change depth of field?
Yes β longer focal lengths create shallower depth of field at a given f-stop. However, if you maintain the same framing (move back to compensate), the DOF difference is minimal.
What focal length is "natural" for portraits?
85β135mm on full frame is traditionally considered flattering for portraits as it minimizes perspective distortion of facial features. 50mm is considered "normal" (close to human eye perspective).
What is the difference between focal length and zoom?
Focal length is a physical property of the lens that determines field of view. Zoom refers to the ability to change focal length with one lens. "2Γ zoom" means the focal length doubles.