A practical manual flash guide for photographers who want to calculate flash power, aperture, and off-camera flash distance before the first frame.
A guide number calculator tells you how much flash power you need for a given aperture, ISO, distance, zoom position, and filter loss. FlashFlash handles that calculation instantly in both directions: you can start from the aperture you want to shoot at, or from the flash power level you want to use.
If you want a deeper explanation first, read Guide Number Explained. If you want the fastest practical workflow, continue below and use the examples in this guide.
FlashFlash is available on iPhone and iPad and is built for photographers who work with manual flash on camera or off camera.
Every flash has a Guide Number (GN) — a single number printed in your flash manual that describes how powerful it is at full power, ISO 100, and a specific zoom position. The Guide Number method lets you calculate exactly how much flash power you need for any combination of aperture, ISO, distance, zoom head position, and filter.
FlashFlash solves that calculation instantly, in both directions:
You choose the aperture. FlashFlash tells you what power fraction to set on your flash.
You choose the power level. FlashFlash tells you what aperture to shoot at.
Both tabs share the same flash settings — Guide Number, zoom, ISO, distance, and filter — so switching between them is instant and seamless.
The app has two tabs, a live scene visualization, and a sticky result area that is always visible at the bottom of the screen.
| Area | What it shows |
|---|---|
| Tabs (bottom on iOS, top on Android) | Switch between Power Calculator and Aperture Recommender |
| Input sections | Flash, Camera, and Scene settings. Tap any value to change it. |
| Scene visualization | Live diagram showing flash, subject, light cone, and distance ruler |
| Result area (always visible) | The answer — power setting or aperture — updating in a simplified text-first presentation |
The blog breaks the same method into practical walkthroughs, including how to set flash power without a light meter.
The Power tab answers: "I'm shooting at f/5.6 — what power do I set on my flash?"
| Field | Section | What to enter |
|---|---|---|
| Flash Model | Flash | Pick your flash from the built-in library to auto-fill the Guide Number. Leave on Custom to enter GN manually. |
| Guide Number | Flash | Your flash's rated GN at ISO 100 and full power, in the unit shown (metres or feet). Usually given at 105 mm zoom — check your manual. |
| Flash Zoom | Flash | The zoom position of the flash head. Zooming in concentrates the beam; zooming out spreads it. |
| ISO | Camera | Your camera's ISO. Higher ISO = more sensitive sensor = less flash power needed. |
| Aperture | Camera | Your lens aperture. Wider aperture (smaller f-number, e.g. f/2.8) = more light in = less flash needed. |
| Distance | Scene | Flash-to-subject distance — not camera-to-subject. See Distance below. |
| Filter Compensation | Scene | Stops of light absorbed by any modifier in the light path — gel, diffuser, softbox, ND filter. Each stop doubles the power needed. |
| Large number (e.g. 1/4) | The standard power fraction to dial in on your flash. |
The Aperture tab answers: "I want to shoot at 1/4 power — what aperture do I need?" Useful when you are battery-constrained, working with a strobe at a fixed power, or planning a shot before you arrive on location.
The Aperture tab has the same fields as the Power tab, with two changes:
Everything else (Guide Number, zoom, ISO, distance, filter) works identically.
| Large f-stop (e.g. f/12.1) | The exact aperture recommendation shown directly in the result area. |
| State | What it means | What to do |
|---|---|---|
| f/5.6 Normal | The recommended aperture is within f/1.0–f/32. | Set this aperture and shoot. |
| f/1.0 Too open | Your flash is very powerful for this situation — it needs an aperture wider than f/1.0. | Lower the power, increase the distance, or add a filter/modifier. |
| f/40.2 Too weak | The required aperture would be narrower than f/32. | Move the flash closer, raise ISO, or increase power. |
The dark panel above the result area shows a live diagram updating as you change any input.
| Element | Power tab | Aperture tab |
|---|---|---|
| Light cone brightness | Shows how hard the flash is working (power fraction) | Shows the selected power level (input) |
| Light cone spread | Fixed — aperture is an input, not an output | Varies with recommended aperture — wider cone = wider aperture |
| Distance ruler | Flash-to-subject distance in your current unit | |
| Terracotta badge | Current aperture (e.g. f/5.6) | Selected power level (e.g. 1/4) |
| Sage badge | Current ISO | |
The GN is always quoted at a specific zoom — usually 105 mm — and for ISO 100 at full power. Check your flash's manual for the correct value.
Units: Tap the M / FT toggle to switch between metres and feet. Both the GN and Distance switch at the same time. If you have a preset selected, the GN automatically swaps to the correct value for the chosen unit.
My Flash: Tap ★ Save as My Flash to save the current Guide Number. It will appear at the top of the Flash Model list every time you open the app.
| Setting | GN multiplier | When to use |
|---|---|---|
| 24 mm | ×0.50 | Widest spread — group shots, large spaces |
| 28 mm | ×0.57 | Wide |
| 35 mm | ×0.65 | Good default if you are unsure |
| 50 mm | ×0.75 | Standard focal length |
| 70 mm | ×0.87 | Short telephoto |
| 85 mm | ×0.93 | Portrait |
| 105 mm | ×1.00 | Rated position — use this if the GN is quoted without a zoom |
| 135 mm | ×1.07 | Telephoto |
| 200 mm | ×1.15 | Maximum concentration — long distance subjects |
Always enter flash-to-subject distance — not camera-to-subject.
Flash is on your hot shoe. Distance = how far your subject is from you.
Flash is on a stand. Distance = how far the flash is from your subject — not how far you are. These are almost never the same.
Use this for anything that absorbs light between the flash and the sensor:
Each stop of absorption doubles the required flash power. A 3-stop softbox means your flash has to work 8× harder to achieve the same exposure. Check your modifier's documentation for its stop rating.
The most common manual flash workflow.
Useful when you are battery-constrained, want a specific recycling speed, or are working with a strobe with limited power steps.
Checking if your flash is powerful enough before you arrive.
Softboxes, umbrellas, and gels all absorb light.
A Guide Number is a shorthand for how powerful a flash is at full power, ISO 100, and a specific zoom setting. It gives you the starting point for manual flash calculations. For a full explanation, see Guide Number Explained.
You combine Guide Number, aperture, ISO, flash-to-subject distance, zoom position, and any filter or modifier loss. FlashFlash does that in one step and returns the nearest real flash power setting.
Always enter flash-to-subject distance, not camera-to-subject distance. If the flash is on a stand three feet from the subject and you are eight feet away, the correct number is three feet.
Yes. That is one of the main reasons to use the Guide Number method in the first place. If you want the practical workflow, read How to Set Flash Power Without a Light Meter.
Zoom changes how concentrated the flash beam is, which changes effective output. Filters, gels, and diffusion absorb light, which means you need more power or a wider aperture to compensate.
The iOS version is the current storefront release. The Android build follows the same calculation model and interaction pattern, with a few platform-specific UI differences:
| Feature | iOS | Android |
|---|---|---|
| Tab bar | Bottom of screen, native iOS style | Top of screen, Material tab strip |
| Picking a value | Tap field → wheel picker slides up from the bottom | Tap field → scrollable list slides up from the bottom |
| Distance picker | Separate bottom sheet with a scroll wheel | Same bottom-sheet pattern |
| Unit toggle (M / FT) | Compact button pair inline with the Distance field | Same |
| Save as My Flash | ★ Star button below Guide Number | Same |