Lens Focal Length Calculator
Calculate the required focal length from subject distance, sensor size, and desired framing. Enter values for instant results with step-by-step formulas.
Formula
Focal Length = (Sensor Dimension x Distance) / Subject Size
The required focal length equals the sensor dimension (width or height in mm) multiplied by the distance to subject, divided by the subject dimension. Frame fill percentage adjusts the effective sensor dimension used in the calculation.
Worked Examples
Example 1: Full-Body Portrait at 5 Meters
Problem: You want to photograph a 1.8m tall person standing 5 meters away, filling 80% of the frame vertically on a full-frame camera (36x24mm sensor).
Solution: Desired image height = 24mm x 0.80 = 19.2mm\nSubject distance = 5,000mm\nSubject height = 1,800mm\nFocal length = (19.2 x 5,000) / 1,800 = 53.3mm\nNearest common lens = 50mm\nHorizontal FOV = 2 x atan(36 / (2 x 53.3)) = 37.3 degrees\nCrop factor = 1.00 (full frame)
Result: Required: 53.3mm | Nearest lens: 50mm | FOV: 37.3 degrees horizontal
Example 2: Filming a Car at 20 Meters on APS-C
Problem: Film a 4.5m long car from 20 meters away, filling 70% of the horizontal frame on an APS-C camera (23.5 x 15.6mm sensor).
Solution: Desired image width = 23.5mm x 0.70 = 16.45mm\nSubject distance = 20,000mm\nSubject width = 4,500mm\nFocal length = (16.45 x 20,000) / 4,500 = 73.1mm\nCrop factor = 1.53\nFull-frame equivalent = 73.1 x 1.53 = 111.8mm\nNearest common lens = 70-200mm zoom at ~73mm
Result: Required: 73.1mm (111.8mm FF equiv) | Use 70-200mm zoom at 73mm
Frequently Asked Questions
What is focal length and how does it affect my image?
Focal length is the distance in millimeters between the optical center of a lens and the camera sensor when the lens is focused at infinity. It determines two key properties: magnification and angle of view. Shorter focal lengths (wide-angle lenses like 14mm to 35mm) capture a wider field of view, making subjects appear smaller but including more of the scene. Longer focal lengths (telephoto lenses like 85mm to 600mm) narrow the field of view and magnify distant subjects. Focal length also affects perspective rendering: wide lenses exaggerate the apparent distance between near and far objects, while telephoto lenses compress perspective, making objects at different distances appear closer together. This is why portraits typically use 85mm to 135mm lenses to produce flattering facial proportions.
What is crop factor and how does it change effective focal length?
Crop factor, also called focal length multiplier, describes how a camera sensor size compares to a full-frame 35mm sensor (36mm x 24mm). A smaller sensor captures a narrower portion of the image circle projected by the lens, producing a tighter crop that mimics a longer focal length. APS-C sensors have a crop factor of 1.5x (Nikon, Sony) or 1.6x (Canon), so a 50mm lens produces framing equivalent to a 75mm or 80mm lens on full-frame. Micro Four Thirds sensors have a 2.0x crop factor, making a 25mm lens equivalent to 50mm. The actual focal length does not change, but the field of view and magnification match the equivalent full-frame focal length. This affects lens selection: a 35mm lens on APS-C gives normal perspective similar to 50mm on full-frame.
How do I choose the right focal length for portraits?
Portrait focal length selection depends on the type of portrait and desired background compression. For tight headshots, 85mm to 135mm on full-frame is ideal, providing natural facial proportions without geometric distortion. The longer the focal length, the more the background is compressed and blurred, which isolates the subject effectively. For environmental portraits showing the subject in context, 35mm to 50mm works well, though you must be careful not to get too close with wide lenses as they distort facial features. Half-body and three-quarter portraits work beautifully at 50mm to 85mm. Group portraits benefit from 35mm to 50mm to include everyone without requiring excessive distance from the group. Wide-angle lenses below 35mm should generally be avoided for portraits as they elongate noses and distort features near the frame edges.
What is the relationship between focal length and depth of field?
Longer focal lengths produce shallower depth of field at the same aperture and subject distance, which is why telephoto lenses are prized for their ability to blur backgrounds. However, this relationship is nuanced. If you maintain the same subject magnification (same framing) while changing focal length, you must also change your distance to the subject, which partially offsets the depth of field change. A 200mm lens at f/2.8 from 10 meters and a 100mm lens at f/2.8 from 5 meters producing the same framing will have nearly identical depth of field. The practical difference is that the 200mm lens compresses the background more, making the out-of-focus areas appear larger and smoother. For maximum background blur, use the longest focal length available at the widest aperture while maintaining your desired framing.
How does sensor size affect the focal length I need?
Sensor size directly determines the focal length required to achieve a specific field of view and framing. A full-frame camera with a 50mm lens captures the same field of view as an APS-C camera with a 33mm lens or a Micro Four Thirds camera with a 25mm lens. When planning a shoot, you must factor in your camera sensor size to select the appropriate lens. This is especially important when following focal length recommendations from tutorials or guides that may assume full-frame cameras. The sensor width and height determine how much of the lens image circle is captured. Larger sensors also provide shallower depth of field at equivalent fields of view, which is why medium format cameras produce such distinctive shallow-focus images even at moderate apertures.
What focal length should I use for landscape photography?
Landscape photography most commonly uses focal lengths between 14mm and 35mm on full-frame cameras, though compelling landscapes can be created at any focal length. Ultra-wide lenses (14mm to 20mm) are popular for expansive vistas with dramatic foreground elements, as they exaggerate the sense of depth and scale. Standard wide-angle (24mm to 35mm) provides a natural field of view that closely matches human perception while still including a broad scene. Telephoto landscapes at 70mm to 200mm are underutilized but create stunning images by isolating specific features, compressing layers of mountains or dunes, and eliminating distracting elements. Many professional landscape photographers carry a 16-35mm wide zoom and a 70-200mm telephoto to cover both approaches and switch between expansive and compressed perspectives.