Enter the dimensions of your eyepiece and camera specs, and we will pop out the front frame size for a viewfinder that matches your camera's field of view!

sports_viewfinder_calc.py

import numpy as np

def sports_viewfinder_calc(
    eye_to_rear_mm,
    tunnel_length_mm,
    horizontal_fov_deg,
    vertical_fov_deg,
    aspect_ratio=None  # e.g. (3, 2) for 2:3; None = full camera FoV
):
    """
    Calculates rear and front rectangle sizes for a sports-style viewfinder.

    Parameters:
    - eye_to_rear_mm: Distance from eye to rear frame (E)
    - tunnel_length_mm: Distance from rear frame to front frame (T)
    - horizontal_fov_deg: Camera horizontal field of view
    - vertical_fov_deg: Camera vertical field of view
    - aspect_ratio: Optional (width, height) tuple for cropping (e.g., (3,2))

    Returns:
    - rear_width_mm, rear_height_mm, front_width_mm, front_height_mm
    """

    h_fov_rad = np.radians(horizontal_fov_deg)
    v_fov_rad = np.radians(vertical_fov_deg)

    # Full-FoV rear size
    rear_width_mm = 2 * eye_to_rear_mm * np.tan(h_fov_rad / 2)
    rear_height_mm = 2 * eye_to_rear_mm * np.tan(v_fov_rad / 2)

    # Aspect ratio cropping
    if aspect_ratio:
        target_aspect = aspect_ratio[0] / aspect_ratio[1]
        current_aspect = rear_width_mm / rear_height_mm

        if target_aspect > current_aspect:
            # Crop height
            rear_height_mm = rear_width_mm / target_aspect
        else:
            # Crop width
            rear_width_mm = rear_height_mm * target_aspect

    # Scale factor from rear to front
    scale = (eye_to_rear_mm + tunnel_length_mm) / eye_to_rear_mm
    front_width_mm = rear_width_mm * scale
    front_height_mm = rear_height_mm * scale

    return rear_width_mm, rear_height_mm, front_width_mm, front_height_mm

print("\n*************** Welcome to the Sports Viewfinder Calculator! ***************")
print("""
Enter the dimensions of your eyepiece and camera specs, and we will pop out
the front frame size for a viewfinder that matches your camera's field of view!
""")

eye_to_rear_mm = float(input("Please enter a eye-to-camera distance                            (mm) : "))
tunnel_length_mm = float(input("Please enter the viewfinder tunnel length                        (mm) : "))
horizontal_fov_deg = float(input("Please enter the camera's horizontal field of view              (deg) : "))
vertical_fov_deg = float(input("Please enter the camera's vertical field of view                (deg) : "))
aspect_ratio_input = input("Aspect ratio if you want to crop the image (E.g. (3,2)). Blank if not : ")

if aspect_ratio_input.strip():
    try:
        w, h = aspect_ratio_input.split(":")
        aspect_ratio = (float(w), float(h))
    except:
        print("Invalid aspect ratio format — using full FoV.")
        aspect_ratio = None
else:
    aspect_ratio = None

print("\n********************************** Results *********************************")

rw, rh, fw, fh = sports_viewfinder_calc (
    eye_to_rear_mm,
    tunnel_length_mm,
    horizontal_fov_deg,
    vertical_fov_deg,
    aspect_ratio
)

print("Rear frame width           (mm) :", rw)
print("Read frame height          (mm) :", rh)
print("Front frame width          (mm) :", fw)
print("Front frame height         (mm) :", fh)

print("****************************************************************************")
print("\n Have a nice day :)")