Focal Length, Angle of View, and Equivalent F-number Calculator
Enter the source lens setup on the left and view converted results on the right. Check the nominal focal length needed for the target format and the F-number that gives comparable depth of field.
Source settings
Enter the image format, lens, and converter in use
Three required calculation inputs
Additional optical settings (optional)
Show calculated values and angles of view for the source setup
Target format and results
The right side always shows converted results
Target format
Main results
This F-number is a depth-of-field conversion for the same angle of view, shooting distance, and final output conditions; it is not an exposure value.
Show angle-of-view and 35mm-equivalent details
How this tool works
Image format and angle of view
A lens projects an image circle: the area where it can form a usable image. Differences between formats are mainly about which part of that image the sensor or film records.
With the same lens from the same position, a smaller format captures a smaller central crop of the image circle, so the field of view becomes narrower. This is similar to cropping an image afterward.
Changing the image format does not change the lens's actual focal length or F-number.
F-number and exposure
The F-number is an optical value of the lens determined by the ratio between focal length and entrance pupil diameter.
Changing the image format does not change focal length or F-number, and ideally the exposure per unit area at the same F-number and shutter speed does not change.
Therefore, “F2.8 on APS-C is equivalent to F4.2 on 35mm format for exposure” is not what this calculator means. For exposure, F2.8 is F2.8 on both formats.
Depth-of-field equivalent F-number
To capture the same framing on different formats, you need lenses with different focal lengths. As a result, even at the same nominal F-number, depth of field and defocus blur are not the same.
The equivalent depth-of-field F-number is an estimate when angle of view, camera position, subject distance, final image size, viewing distance, and circle-of-confusion scaling are matched.
This converted F-number does not indicate exposure or lens brightness.
About converters
Reducers and teleconverters act on the optical system itself. This tool first calculates effective focal length and theoretical F-number after the converter, then converts angle of view and depth of field between formats.
Meaning of calculated values
- Theoretical F-number after converter (exposure basis) is the nominal F-number multiplied by the converter factor.
- Depth-of-field equivalent F-number assumes the same angle of view, shooting distance, final output size, and viewing conditions. It is not an exposure value.
- 35mm equivalents use the common diagonal basis, even if long-side or short-side matching is selected above.
- When aspect ratios differ, diagonal, long-side, and short-side angles of view cannot all match at the same time.
- Listed dimensions combine nominal, active-area, and representative values. Editable dimensions are available only for representative or custom formats.