The F/D Ratio Calculator is used to find the ratio between the focal length and the diameter of an optical system, such as a camera lens, telescope, or microscope. This value, commonly known as the f-number or f-stop in photography, helps define the brightness and sharpness of the image. A smaller F/D ratio means a wider aperture, allowing more light to enter, which is ideal for low-light conditions. On the other hand, a larger F/D ratio gives a narrower aperture, improving focus depth and reducing image brightness.
This calculator simplifies the process by computing this ratio based on two simple values: the focal length and the aperture diameter. Whether you're setting up a camera for astrophotography or adjusting a telescope, this tool helps you quickly evaluate optical performance.
formula of F/D Ratio Calculator
F/D Ratio = f / D
Where:
F/D Ratio = Focal Length to Diameter Ratio (dimensionless)
f = Focal Length (distance from the lens/mirror to the focal point)
D = Diameter of the entrance pupil or aperture (limits incoming light)
This ratio tells us how much light enters and how focused the image will be. In photography, it’s usually represented as f/2.8, f/5.6, etc.
Common F/D Ratio Reference Table
| F/D Ratio | f-number Notation | Common Use Case | Light Brightness | Depth of Field |
|---|---|---|---|---|
| 1.4 | f/1.4 | Low-light photography, fast lenses | Very Bright | Very Shallow |
| 2 | f/2 | Portraits, indoor scenes | Bright | Shallow |
| 4 | f/4 | General outdoor shooting | Moderate | Moderate |
| 8 | f/8 | Landscapes, daylight shooting | Less Bright | Deep |
| 11 | f/11 | Macro, detailed sharpness | Dim | Very Deep |
This table provides quick insights into how changing the F/D ratio impacts photography or optical behavior.
Example of F/D Ratio Calculator
Let’s say you have a lens with a focal length of 200 mm and an aperture diameter of 50 mm.
Apply the formula:
F/D Ratio = f / D = 200 / 50 = 4
This gives an f-number of f/4, which is suitable for general photography, offering a balance between brightness and depth of field.
Most Common FAQs
A smaller F/D ratio like f/1.8 or f/2.0 is better because it allows more light through the lens, which is helpful in dark environments.
Yes, up to a point. Higher F/D ratios (like f/8 or f/11) increase depth of field and sharpness, but too high may cause diffraction blur.
High F/D ratios in telescopes help achieve high magnification and detailed viewing of distant objects, especially in planetary observation.