What is TTL, or through the lens, flash?

When you’re starting out with flash, there’s a lot of ground to cover. Just working out which type of flash you want to use, let alone how to use it, is daunting. One of those questions is TTL or manual? In this article, we take a look at what TTL, or through the lens metering, actually is and how it works. We’ll also go over when TTL is the best option compared to when manual flash is preferable.

[Learn Photography: Aperture | Shutter Speed | ISO | Exposure Triangle | White Balance | Panning | Vinneting | TTL | More…]

Different types of flash

Before we go into deeper into TTL, we should talk about flashes. There are three types of flash:

1. Pop-up flash – the flash that’s built in to your camera.
2. Speedlight (or speedlite) – the flashes that attach to your camera via its hot shoe. They are sometimes called flashguns or hot shoe flashes.
3. Studio flashes – these are known as monolights or strobes.

You can’t call a monolight a speedlight, but both of them are flashes.

Using TTL flash is most commonly associated with on-camera speedlights. However, you can use TTL mode with off-camera flash if your flash is TTL-enabled, which includes monolights and strobes, not just speedlights.

What do we mean by TTL?

TTL stands for through the lens. It’s a sort of automatic mode for flash. Rather than you having to decide on your flash settings, TTL takes a meter reading through your lens before the shot is actually taken. It measures the ambient light and the light from a pre-flash to set the flash’s power. It’s a relatively hands-free approach to flash photography.

How does TTL work?

A TTL flash fires a ‘pre-flash’ before you take your actual shot to measure the amount of light in the scene and obtain a good exposure. Your camera then takes a light meter reading, combining the pre-flash with the ambient light. This light reading sets the flash power needed for the correct exposure. If it’s relatively dark, the flash output is commensurately intense. For a brighter scene, your flash unit will emit a less strong burst.

The pre-flash might be white light or it could be infrared. It also happens microseconds before the actual flash, so you don’t even notice it.

You can use TTL with any camera mode. If you adjust your aperture or ISO the flash output will adjust automatically to compensate for your exposure settings.

What’s the difference between e-TTL and TTL flash?

As well as TTL, you might hear the terms E-TTL, A-TTL, and I-TTL in use. E-TTL stands for evaluative through the lens flash and it’s Canon’s name for the function. Nikon uses I-TTL, with the ‘I’ meaning intelligent. A-TTL is ‘advanced through the lens’ flash.

Back when through the lens flash was first developed, it wasn’t as accurate as the newer versions. E-TTL, I-TTL, and A-TTL just reflect the advancements in the tech but the principle is the same.

What’s the difference between TTL and manual flash?

You make the decisions and select the power output and exposure settings when it comes to manual flash. You are in control and you can recreate your manual lighting patterns to use again and again. With auto flash, you don’t know what the settings are.

We measure manual flash in fractions with 1/1 being full power and 1/2 half power. The lowest power output can be anything from 1/64 to 1/250. By deciding on the flash power, you have control over the shadows in your photographs, but it takes practice to know what flash you need in every scenario.

When you use manual flash, you need to use manual exposure settings on your camera. Adjusting your aperture or shutter speed can have an impact on your flash, too.

If you keep your flash power the same but open up or stop down your aperture, you will increase or decrease the impact of your flash respectively.

While you usually cannot go beyond that 1/250 second when shooting with a flash–black bands appear across your image because the flash isn’t in sync with the shutter–you will need to adjust your shutter speed to ensure that your ambient light and flash are balanced. When your background is too dark compared to a well-lit subject, try decreasing your shutter speed.

In short, the difference between manual flash mode and TTL flash mode is control and the ability to recreate your settings accurately.

When should I use through the lens flash?

If manual flash gives you more control, what’s the point of through the lens flash, you might ask. Well, apart from being a great introduction to flash that can help to build your confidence and working knowledge, TTL flash is invaluable when you are working with moving subjects.

Manual mode means that you will need to adjust your flash settings depending on how far your subject is from your flash. When your subject is on the move, that is impossible. By letting auto mode make the calculations for you, you will get a good exposure with each shot.

How do I use TTL flash?

Using TTL flash metering is as easy as selecting the button on our flash unit. But, there are things that you can do to get the most out of it, too.

Flash exposure compensation

A little like you can use exposure compensation to make adjustments to your exposure when you use a semi-automatic exposure mode, you can make flash exposure adjustments with the flash exposure compensation function in TTL mode.

Flash exposure compensation is usually made in fractions of a stop. You can adjust it up or down to add or reduce the light in a scene. Look for the +/- button and use it with the flash button. If your shot is overexposed, dial in some negative flash compensation. Think your photo is a bit dark? Tell your speedlight to emit a more powerful flash.

Off-camera flash

You can use the through the lens metering function with off-camera flashes, but it does have its limitations. In particular, because the meter readings happen through your camera’s lens, if your off-camera flash is being directed into an area where your camera doesn’t pick them up, you can end up with an over- or under-exposed image. It’s best to have your flash relatively close to your camera, therefore.

Wrapping up

Manual flash might give you more control and better opportunities to reproduce your lighting setups, but there are times when TTL flash is more than good enough and on some occasions, the best option. It’s definitely worth getting to know it.

FAQs and troubleshooting