Prise de vue · 2026-09-11

Flash on Film: X-Sync, Guide Numbers, and TTL — Getting It Right Without Burning a Roll

Flash is probably the single thing that intimidates people most when they come back to film. With digital you adjust on the fly, check the histogram, shoot again. With film, the flash fires, the emulsion responds, and you won't know what you got for days. That uncertainty pushes a lot of people to avoid flash altogether, or to throw it on auto and hope TTL sorts everything out.

Getting the basic mechanics straight, once and for all, means you can walk into a situation with solid settings already dialed in and not waste a roll finding out what went wrong.

X-Sync: The Curtain That Changes Everything

A 35mm SLR uses a focal-plane shutter. Two curtains, either fabric or metal blades, travel across the film plane: the first one opens, the second one closes. At a slow shutter speed (say 1/60s), the first curtain has enough time to clear the entire frame before the second one starts closing. For a brief moment, the whole film is exposed at once. At a fast speed (say 1/500s), the two curtains overlap and form a narrow slit that sweeps across the film, so the frame is never fully open at any single instant.

A flash fires in just a few milliseconds, often under 1/1000s. To illuminate the entire film surface, it has to fire during that one moment when the frame is completely open. That's the X-sync speed, sometimes written X-sync or flash sync. It's the fastest shutter speed at which flash will work normally.

Go faster than that limit and the second curtain starts closing before the first has fully cleared. The flash illuminates a slit, not the whole frame. You get a black band across the image — the classic sign of a sync error.

X-sync varies by camera. Mechanical SLRs from the 1970s often synced at 1/60s, sometimes 1/90s. Later cameras from the 1980s and 90s commonly reach 1/125s or 1/200s. Some leaf-shutter cameras (certain Rolleis, Hasselblads with integral-shutter lenses) can sync all the way to 1/500s because the shutter sits inside the lens and opens and closes as a single concentrated motion. On a standard 35mm SLR, the sync speed is usually marked in red on the shutter dial, or indicated with a lightning bolt symbol. Stay at that speed or below — never above.

At or below X-sync (left), the full frame is open when the flash fires. Above X-sync (right), the second curtain has already begun closing, leaving only a slit — causing the characteristic black band.

First-Curtain vs. Second-Curtain Sync

Some cameras let you choose between first-curtain sync (the flash fires as soon as the first curtain finishes its travel) and second-curtain sync (the flash fires just before the second curtain begins to close). With moving subjects at slow shutter speeds, this matters a lot. First-curtain sync at a slow speed will put the motion blur in front of a moving car, as if it were going backwards. Second-curtain sync puts the blur behind the subject, which reads as natural. For portraits and stationary subjects, there's no practical difference.

Guide Number: The Formula You Won't Forget

The guide number (GN) connects aperture to flash-to-subject distance through a simple formula: aperture = GN ÷ distance. It's expressed in meters at ISO 100.

GN = distance (m) × aperture (f/)

Or rearranged:

aperture = GN ÷ distance

A concrete example: a flash with GN 30 (at ISO 100), subject at 3 meters, needs f/10. At 5 meters, f/6. At 1.5 meters, f/20 — which most lenses can't do, so you either reduce flash power or step back a bit.

The inverse square law explains why light falls off so fast: doubling the distance cuts the intensity by four, not two. At 6 meters, that same GN 30 flash gives you roughly f/5, about two stops less than at 3 meters.

When ISO changes, the effective guide number changes with it. A flash rated GN 26 at ISO 100 becomes GN 52 at ISO 400. That either extends your working range or lets you stop down for more depth of field.

Qualitative aperture curve for GN 30 at ISO 100. The steep fall-off at short distances illustrates the inverse square law — the curve is illustrative, not a substitute for the formula.

Film TTL: What It Actually Does

Film TTL (Through The Lens) is not the same system as Canon's E-TTL or Nikon's i-TTL on digital cameras. The principle is similar enough — flash exposure is measured through the lens and the flash cuts off when enough light has reached the film — but the execution is different.

On a film body, the flash fires at full duration and a sensor inside the camera measures the light reflected back from the film surface itself (or from a reference surface near the film plane). When the sensor judges the exposure sufficient, it sends a cutoff signal to the flash. The whole thing happens fast, often under 1/1000s, and it's reactive: the system is responding to what's actually in front of the lens.

This works well for the majority of everyday situations: indoor portraits at reasonable distances, reasonably uniform scenes. It runs into trouble in edge cases — a very dark subject against a bright background, a room with heavily light-absorbing walls, or a bounce off a warm-colored ceiling. Film TTL also has no pre-flash measurement like digital E-TTL, which sends an invisible burst before the actual exposure to calculate the scene. Film TTL is purely reactive; digital TTL is partly predictive.

The real difference from manual mode is repeatability. TTL adapts to the scene, which is convenient when you're moving around or changing angles, but that same adaptation can vary frame to frame if your subject's reflectance changes. Manual mode is fixed: once you've found the right output, every frame shot under the same conditions will be the same. For a repetitive portrait session or an event in a consistent space, manual is often preferable precisely because it's predictable.

There's also a compatibility question. Film TTL uses proprietary protocols. Nikon's TTL system from the 1980s and 90s (used on the F3, FA, FE2, F801 and others) is not the same as Canon A-TTL or Minolta TTL. A Nikon SB-28 running in TTL only works properly on a compatible Nikon body. On anything else it may fire (the center pin triggers regardless) but without any TTL control.

Choosing a Flash for a Vintage Camera

This is where things get a little complicated, and where the wrong choice can actually damage a camera body.

For straightforward compatibility, the safest options are system-matched flashes (Nikon SB-24, SB-25, SB-28 for F-mount Nikons; Canon 430EZ, 540EZ for EOS film bodies), or more recent Metz, Sunpak, or Godox units with low sync voltages. These newer flashes typically offer a full TTL mode for matched bodies and a proper stepped manual mode.

On a film compact, none of this is really an issue. Most compacts have a built-in flash calibrated to the film inside, and the few models with a hot shoe (Canon Sure Shot, Nikon L35AF...) generally work with any low-voltage flash.

Three Real-World Situations

Indoor Portraits

The classic case. Subject at 2–3 meters (roughly 6–10 feet), ambient light is dim. In TTL on a compatible body, the default settings usually get you something workable. In manual, you do the math: GN 28, subject at 2.5m, ISO 400 → effective GN ≈ 56 → aperture ≈ f/22, which is too closed down for most lenses. So you drop to 1/4 or 1/8 power and recalculate. In practice there's a little trial and error at first, but once you've found the settings that work in a given situation, you write them down and go back to them every time.

Bouncing off the ceiling softens the light completely and changes the whole feel of the shot. But bounce costs you power — easily two stops depending on ceiling height and color. Color negative film has enough latitude to absorb that; slide film requires tighter calculation.

Fill Flash Outdoors

Fill flash is the most underused and often most effective flash technique there is. Bright sun puts harsh shadows under eyes and chins in portraits. A fill burst at -1 or -2 stops relative to the ambient exposure lifts those shadows without the result looking anything like a studio shot. The constraint: you're locked to X-sync. If the ambient exposure calls for 1/500s at f/8, you can't just add flash. You need to stop down to f/11 or f/16 to bring the shutter speed down to 1/125s (or whatever your X-sync is), or accept a little ambient overexposure.

Manual fill flash: figure out the correct ambient exposure first, set the shutter at or below X-sync, choose the matching aperture, then calculate flash output to give 1–2 stops less than the ambient exposure. A slightly underexposed flash almost always looks more flattering than matching it to ambient.

Events and Parties

Here, range and recycle time matter as much as accuracy. Backgrounds are often dark, subjects vary in distance, and you don't have time to recalculate between frames. TTL earns its keep in this situation, as long as the body handles it well. If you're shooting manual, a simple approach: pick a fixed working distance (say 2.5m / 8 feet), calculate the right aperture for that distance, and stick with it by adjusting your feet rather than your settings. It's not flexible, but it's consistent and removes surprises.

Slow sync is an option on some bodies: you use a longer shutter speed (1/15s, 1/8s) to let the ambient background record while the flash freezes the foreground subject. The effect can be striking, but it needs either a steady hand or a tripod, and the subject has to hold still during the exposure.

Going Further

If you're shooting Nikon F-mount, the SB-24 and SB-25 remain excellent used buys: Nikon TTL, stepped manual mode, solid guide numbers (GN 38 and 42 respectively at ISO 100), and low sync voltage. You can often find them in good shape for under $50 Canadian.

For quick guide number calculations in the field, some manufacturers and independent sites publish ready-made exposure tables (distance × aperture by ISO). la-photo-argentique.com has a handy one for manual mode, the kind of thing worth printing and keeping in your bag.

For a deeper dive into fill flash technique and slow sync, the original SB-24 and SB-25 instruction manuals (available as PDFs at various archive sites) are remarkably well written and illustrated. Worth reading through even if you're not using one of those flashes.

Flash on Film: X-Sync, Guide Numbers, and TTL — Getting It Right Without Burning a Roll

X-Sync: The Curtain That Changes Everything

First-Curtain vs. Second-Curtain Sync

Guide Number: The Formula You Won't Forget

Film TTL: What It Actually Does

Choosing a Flash for a Vintage Camera

Three Real-World Situations

Indoor Portraits

Fill Flash Outdoors

Events and Parties

Going Further

Sources