Photo Close-Ups

This file is part of Timex Sinclair Public Domain Library Tape 1002. Download the collection to get this file.
Date: 198x
Type: Program
Platform(s): TS 1000

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

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

Appears On

Tape 1002 of the Timex Sinclair Public Domain Library. Assembled by Tim Ward from many sources.

Image Gallery

Source Code

   5 REM PHOTO CLOSE-UPS
  10 PRINT "CLOSE UPS"
  20 PRINT "*********"
  30 PRINT "NORMALF/NUMBER: ";
  40 INPUT F
  50 PRINT F
  60 PRINT "LENS-FILM DISTANCE ";
  70 INPUT D
  80 IF F=0 THEN GOTO 70
  90 PRINT D
 100 PRINT "LENS FOCAL LENGTH: ";
 110 INPUT L
 120 PRINT L
 130 LET N=F*L/D
 140 PRINT "EFFECTIVE F/NUMBER ISF/";N
 150 FOR Z=1 TO 8
 160 PRINT 
 170 NEXT Z
 180 PRINT "FOR MORE,PRESS ANY KEY"
 190 IF INKEY$="" THEN GOTO 190
 200 CLS 
 210 GOTO 10
 250 SAVE "1005%7"
 300 RUN

Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.

Tags

Downloadable TS 1000 Photography