---
title: "Photography"
id: 57107
type: "computer_media"
slug: "photography"
url: "http://localhost/computer_media/photography/"
markdown_url: "http://localhost/computer_media/photography.md"
published_at: "2024-10-03T23:35:03+00:00"
modified_at: "2026-03-30T21:19:57+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/56_Photo.png"
excerpt: "Enter your flash guide number and subject distance to instantly calculate the correct f-stop aperture — a handy darkroom companion built in BASIC."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
genre:
  - name: "Photography"
    slug: "photography"
    taxonomy: "genre"
    url: "http://localhost/type/photography/"
media_contents:
  - id: 56733
    title: "Timex Sinclair Public Domain Library Tape 1002"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1002/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/10/56_Photo.png"
media_type_tags: "Photography"
---

# Photography

{This program calculates the correct f-stop aperture setting for flash photography using the guide number method. It prompts the user to enter the film/flash guide number and the flash-to-subject distance in feet, then divides the guide number by the distance to produce the required f-stop (F = G/D). After displaying the result, pressing any key clears the screen and restarts the calculation loop, allowing repeated use without reloading. The program structure is minimal and linear, making it a practical pocket-reference tool for photographers.

***

## Program Analysis

### Program Structure

The program is a tight, single-loop utility. Lines 10–100 handle input, calculation, and output; line 110 waits for a keypress; lines 120–130 clear the screen and restart. The flow is entirely linear with no subroutines.

1. **Lines 10–20:** Title and decorative heading.
2. **Lines 30–80:** Input prompts and echo for guide number (`G`) and distance (`D`).
3. **Line 90:** Core calculation: `LET F=G/D`.
4. **Line 100:** Displays the recommended aperture.
5. **Line 110:** Busy-wait keypress loop using `INKEY$`.
6. **Lines 120–130:** Clear screen and loop back to start.

### The Flash Guide Number Formula

The guide number (GN) method is a standard photographic relationship: `f-stop = GN / distance`. The program implements this directly at line 90. No unit conversion is performed; the prompt at line 60 specifies feet, so the result is only correct when the guide number is rated for feet rather than metres.

### Key BASIC Idioms

- **Keypress wait (line 110):**`IF INKEY$="" THEN GOTO 110` is a standard busy-wait loop, holding the result on screen until the user presses any key before cycling back.
- **Input echo (lines 50, 80):** After each `INPUT`, the entered value is immediately `PRINT`ed back, confirming what was typed — a common pattern to provide visual feedback.
- **Restart via `GOTO 10` (line 130):** Rather than using `RUN` (which would re-initialise variables), `GOTO 10` re-enters the display/input sequence while preserving any variable state, though in this case all variables are re-assigned on each pass anyway.

### Notable Anomalies

- **Line numbering gap:** Lines jump from 130 to 150, skipping 140. This is cosmetically unusual but functionally harmless.
- **Line 200 (`RUN`):** A `RUN` statement stored as a program line is never reached during normal execution; it exists as a convenience for manually typed execution or as a remnant of editing.
- **No input validation:** If the user enters `0` for distance `D`, line 90 will produce a division-by-zero error. A guard such as `IF D=0 THEN GOTO 60` would make the program more robust.
- **Result not rounded:**`F` is printed as a raw floating-point value (e.g., `F/6.666...`), which may not map cleanly to standard f-stop markings (f/2, f/2.8, f/4, etc.). A rounding or lookup step would improve practical usability.

### Variable Summary

| Variable | Purpose |
| --- | --- |
| `G` | Flash guide number (user input) |
| `D` | Flash-to-subject distance in feet (user input) |
| `F` | Calculated f-stop aperture (`G/D`) |

## Source Code

```
   5 REM PHOTOGRAPHY
  10 PRINT "FLASH EXPOSURE"
  20 PRINT "**************"
  30 PRINT "FILM GUIDE NUMBER: ";
  40 INPUT G
  50 PRINT G
  60 PRINT "FLASH/SUBJECT DISTANCE: ";
  70 INPUT D
  80 PRINT D;" FEET"
  90 LET F=G/D
 100 PRINT "SHOOT AT F/";F
 110 IF INKEY$="" THEN GOTO 110
 120 CLS 
 130 GOTO 10
 150 SAVE "1005%6"
 200 RUN
```