Focal Ratio Calculator

Summary

Enter aperture and focal length to instantly calculate focal ratio, image scale, and angular field of view for any telescope or camera lens.

How it works

1. Enter the aperture (objective or lens diameter) in millimeters.
2. Enter the focal length in millimeters.
3. Click Calculate to get the focal ratio and related optics summary.
4. Optionally enter your camera sensor width and height in millimeters.
5. Optionally enter the pixel size in micrometers to get image scale in arcseconds per pixel.
6. All results update instantly with the primary focal ratio displayed prominently.

Use cases

• Determine how "fast" a telescope is for deep-sky astrophotography.
• Compare two lenses or telescopes side-by-side before purchasing.
• Calculate image scale to match resolution to your seeing conditions.
• Find the angular field of view your camera sensor will capture.
• Check whether a focal reducer changes your f/ratio to a useful range.
• Evaluate whether a telescope is suitable for wide-field or planetary imaging.
• Plan exposure times — faster f/ratios require shorter exposures.
• Teach students the relationship between aperture, focal length, and light-gathering speed.

Frequently Asked Questions