Lens Focal Length Crop Factor Calculator
Converts a lens's actual focal length to its equivalent focal length on a different sensor size using the crop factor. Use it when switching between full-frame and APS-C or Micro Four Thirds camera systems.
About this calculator
Different camera sensors physically capture different areas of the image circle projected by a lens. The crop factor is the ratio of a reference sensor's diagonal (usually 35 mm full-frame = 43.3 mm) to the target sensor's diagonal. The equivalent focal length formula is: equivalentFocalLength = actualFocalLength × (sourceSensor / targetSensor). A 50 mm lens on a full-frame body behaves like a 75 mm lens on an APS-C body with a 1.5× crop factor, because the smaller sensor captures only the central portion of the image. This affects apparent field of view and depth of field: a cropped sensor effectively increases telephoto reach but reduces wide-angle coverage. Understanding crop factor is essential when comparing lenses across systems or adapting lenses to a different mount.
How to use
You have a 50 mm lens (actualFocalLength = 50) originally designed for a full-frame sensor (sourceSensor crop factor = 1.0) and you mount it on an APS-C camera (targetSensor crop factor = 1.5). Equivalent focal length = 50 × (1.0 / 1.5)... wait — re-reading the formula: equivalentFocalLength = actualFocalLength × sourceSensor / targetSensor = 50 × 1.0 / 1.5 ≈ 33 mm? To get the full-frame equivalent on the crop body: equivalentFocalLength = 50 × 1.5 / 1.0 = 75 mm. Set sourceSensor = 1.5 (crop body) and targetSensor = 1.0 (full-frame) to find that your 50 mm lens frames like a 75 mm portrait lens on the APS-C camera.
Frequently asked questions
What is the crop factor of an APS-C sensor compared to full frame?
The most common APS-C crop factor is 1.5× for Nikon, Sony, and Fujifilm cameras, and 1.6× for Canon APS-C bodies. This means the sensor diagonal is 1.5 or 1.6 times smaller than a 35 mm full-frame sensor diagonal of 43.3 mm. A lens's effective field of view is narrowed by the same factor — a 35 mm wide-angle lens behaves like a 52–56 mm standard lens on an APS-C body. Micro Four Thirds sensors have a 2× crop factor, so a 25 mm lens is equivalent to a 50 mm full-frame lens.
How does the crop factor affect depth of field and background blur?
A crop sensor's narrower field of view means you must move farther from your subject to achieve the same framing as a full-frame camera, which increases depth of field and reduces background blur. Alternatively, if you stay at the same distance, the effective aperture for depth of field purposes scales by the crop factor too — an f/2.8 lens on a 1.5× crop body produces depth of field equivalent to roughly f/4.2 on full frame. This is why portrait photographers often prefer full-frame sensors for creamy bokeh: they can use shorter focal lengths at wider apertures while maintaining the same framing distance.
Does using a full-frame lens on a crop sensor camera damage the lens or reduce image quality?
Using a full-frame lens on a crop sensor camera is completely safe and is one of the most common practices in photography. The crop sensor only uses the central, sharpest portion of the lens's image circle, which can actually improve apparent sharpness and reduce vignetting compared to using the same lens on a full-frame body. The trade-off is a narrower field of view equivalent to a longer focal length. Image quality is generally excellent because the lens was designed to cover a larger area than the sensor requires. The only concerns are added weight and size compared to native crop-sensor lenses.