Photo Close-Ups

This program calculates the effective f/number for close-up photography, accounting for the increased lens-to-film distance used in macro work. The user inputs the nominal f/number, the actual lens-to-film distance, and the lens focal length; the program then computes the effective f/number using the formula N = (f × L) / D. After displaying the result, it pauses with a busy-wait INKEY$ loop before looping back to accept new values. The result is printed inline with the label text on line 140, producing output like “EFFECTIVE F/NUMBER IS F/4.5”.

---

Program Analysis

Program Structure

The program is a simple single-screen calculator that loops indefinitely. Its flow is straightforward:

1. Display title (lines 10–20)
2. Collect three inputs: f/number F, lens-to-film distance D, focal length L (lines 30–120)
3. Compute and display the effective f/number (lines 130–140)
4. Wait for a keypress, then clear and repeat (lines 150–210)

The Calculation

The core formula at line 130 is:

LET N = F * L / D

This implements the standard macro photography exposure compensation formula: Effective f/number = Nominal f/number × (Lens-to-film distance / Focal length). Note the variable order — the program computes (F × L) / D, which is equivalent only if the user supplies the ratio D/L > 1 as expected in extension work. In standard macro notation the formula should be F × D / L; the division by D and multiplication by L here appear transposed compared to the conventional expression, which would produce incorrect results for typical inputs.

Input Validation

Line 80 checks IF F=0 THEN GOTO 70, but this is logically misplaced: a zero f/number is physically meaningless, yet the branch goes to line 70 (re-prompting for the distance D) rather than back to line 30 where F is entered. This is a bug — it would loop requesting D again while F remains zero, and the subsequent division by D at line 130 still risks a division-by-zero error if D is zero.

Key BASIC Idioms

• Busy-wait keypress loop: Line 190 uses IF INKEY$="" THEN GOTO 190 to spin until any key is pressed — a standard idiom for this platform.
• Echo inputs: Each INPUT is followed by a PRINT of the variable (lines 50, 90, 120) to redisplay the entered value cleanly on screen.
• Blank line padding: Lines 150–170 use a FOR loop to print eight blank lines, creating visual separation before the “press any key” prompt.

Notable Anomalies

Line	Issue
80	Jumps to line 70 (prompt for D) when F=0, but should jump to line 30 to re-enter F.
130	Formula F*L/D appears to invert the standard macro f/number compensation formula (should be F*D/L).
140	Label text runs directly into the variable: "EFFECTIVE F/NUMBER ISF/" — missing a space before “F”.
250–300	SAVE and RUN lines are above line 300 but beyond the main logic; they serve as utility lines outside the normal execution path.