Every flash photo needs the right amount of flash: too little and the subject is dark, too much and it is washed out. There are two ways to set flash power. TTL lets the camera measure and decide automatically. Manual lets you set the power yourself. Each has clear strengths. In this lesson you will learn how both work, how to adjust TTL with flash exposure compensation, and how to choose the right mode for each situation.
TTL and manual, defined
| TTL (through the lens) | Manual | |
|---|---|---|
| Who sets the power | Camera and flash, automatically | You |
| How | Fires a small pre-flash, measures it through the lens, then sets power | You choose a power fraction, such as 1/4 or 1/16 |
| Changes from shot to shot | Yes, it recalculates every time | No, it stays the same until you change it |
| Best when | Distance or scene changes constantly | Subject and light stay in the same place |
| Adjusted with | Flash exposure compensation | Power setting, distance, aperture, ISO |
Each brand has its own name for TTL, such as E-TTL (Canon) and i-TTL (Nikon), but the idea is the same.
How TTL works
When you press the shutter, a TTL flash fires a very quick, low-power pre-flash before the main exposure. The camera measures how much of that light comes back through the lens, calculates the power needed, and then fires the main flash at that power. It all happens so fast that most people see only one flash.
Because TTL uses the camera's metering, it can be fooled just like normal metering, which you learned about in camera metering. A person in white clothing may come out too dark, and a person in black may come out too bright.
Flash exposure compensation
Flash exposure compensation (FEC) tells TTL to make the flash stronger or weaker than it calculated, in steps of a stop, just like exposure compensation does for ambient light in exposure compensation. It changes only the flash part of the photo, not the existing light.
| Situation | Typical adjustment |
|---|---|
| Flash looks too strong | −1/3 to −1 FEC |
| Subject in white or light clothing looks dark | +1/3 to +1 FEC |
| Subject in dark clothing looks too bright | −1/3 to −1 FEC |
| Using flash as gentle fill outdoors | Often about −1 to −2 FEC |
Remember:These amounts are common starting points, not fixed rules. Take a test shot and adjust.
How manual flash power works
In manual mode you choose a fraction of the flash's full power. Each step halves the light, which is one stop:
| Power | Light compared with full power | Stops below full |
|---|---|---|
| 1/1 (full) | 100% | 0 |
| 1/2 | 50% | 1 |
| 1/4 | 25% | 2 |
| 1/8 | 12.5% | 3 |
| 1/16 | About 6% | 4 |
| 1/32 | About 3% | 5 |
Many flashes also allow third-stop steps between these. Lower power usually means a shorter flash duration and faster recycling, which helps freeze motion and shoot quickly.
With manual flash, the flash brightness on the subject depends on four things: flash power, distance from flash to subject, aperture and ISO. Shutter speed does not change it, as long as you stay at or below sync speed, as explained in flash exposure and ambient light.
Guide number
Flash makers describe power with a guide number (GN). In simple terms, guide number = distance × f-number, for a stated ISO (usually ISO 100) and zoom setting. For example, a flash with GN 40 (in metres) can correctly light a subject 10 metres away at f/4, or 5 metres away at f/8, at full power. Guide numbers are useful for comparing flashes, but different brands measure them at different zoom settings, so compare carefully.
Choosing TTL or manual
| Situation | Better choice | Why |
|---|---|---|
| Events where people move and distances change | TTL | Power adjusts automatically |
| Studio portraits and product photos | Manual | Identical, repeatable results |
| Off-camera flash in a fixed position | Manual | Distance stays the same |
| Learning flash | Both | TTL for quick results, manual to understand cause and effect |
Tip:A useful trick: take a test shot in TTL, then switch to manual at a similar power setting to lock in consistent results. Many flashes show the power TTL used.
Common mistakes
- Using exposure compensation instead of flash compensation: the wrong one changes the background, not the flash.
- Trusting TTL with very light or dark subjects: adjust with FEC.
- Changing distance in manual mode without adjusting power: the subject becomes too bright or too dark.
- Changing shutter speed to fix flash brightness: within sync speed, it changes only the ambient light.
How professionals choose
Many event and wedding photographers use TTL with flash exposure compensation for moving people and changing rooms, because speed matters more than perfect consistency. Studio, product and portrait photographers mostly use manual flash for repeatable results, often with a handheld flash meter or test shots. Experienced photographers understand both, so they can switch instantly when a situation changes.
Practical examples
A bride in a white dress
- Situation
- In TTL, the bride's dress and face come out too dark.
- What to do
- You add +1 flash exposure compensation.
- Why it works
- The bright white dress fooled TTL into reducing flash power.
- Result
- Bright white dress with natural skin tones.
Product shots on a table
- Situation
- You photograph 30 items with a flash on a stand.
- What to do
- You set the flash to manual at 1/8 power and keep its distance fixed.
- Why it works
- Manual power and a fixed distance give identical lighting on every item.
- Result
- Consistent brightness across all 30 photos.
Key points
- TTL measures a pre-flash through the lens and sets flash power automatically.
- Flash exposure compensation adjusts only the flash part of the exposure.
- Manual flash power uses fractions; each halving is one stop.
- Flash brightness depends on power, distance, aperture and ISO, not shutter speed (within sync).
- Use TTL when distances change; use manual for consistency.
Frequently asked questions
What does TTL flash mean?
What is flash exposure compensation?
Is manual flash better than TTL?
What is a guide number?
Conclusion
TTL sets flash power automatically and is adjusted with flash exposure compensation; manual gives consistent, repeatable power that you control. Learn both, and choose based on whether your subject's distance and scene keep changing. Next, you will learn how flash and existing light combine in one exposure.
Finished reading? Track your progress through Flash and studio lighting.