TTL stands for ‘through the lens’ and in photography it is referring to through the lens metering.
Metering is basically another way of saying how the camera measures the light in a scene.
So putting the two together, basically TTL metering is how your camera will measure the amount of light coming through the lens which can be used to determine the power of flash produced for what the camera considers to be a well balanced exposure.
Depending on the camera, you may have various modes for TTL metering.
For example, you might want to use spot metering where the camera will only look at 1-5% of the scene and try to expose for that portion of the scene.
In flash photography, TTL metering works by the camera communicating with the flash unit and passing on the metering information so that the flash can fire at the right power output according to what the camera thinks is correct.
When Does TTL Metering Happen?
TTL metering takes place just before any image is taken.
Basically the sensor will read the level of light that comes through the lens and if the camera is in auto mode this will help the camera to take a picture with a decent exposure.
What About TTL With Flash Units?
So you know that cameras use TTL metering to analyse the amount of light in a scene but how do flash units do it?
In flash units with TTL, they actually fire one or more ‘pre-flashes’ which are basically flashes that briefly light up the scene so the camera can use its TTL metering to let the flash know what is the ideal power output for the most balanced exposure.
So think of the pre-flash as a sort of test flash or test shot before the actual exposure is taken.
Essentially it is like an auto mode for the flash and does a similar thing to what a camera would do in an auto mode except now you have the addition of a flash involved.
Advantages Of TTL Flash
- You don’t have to mess about with flash settings to get a decent exposure.
- Good when lighting conditions are constantly changing.
- Can use it in any camera mode.
- Can use it with multiple TTL compatible flashes at once.
Disadvantages Of TTL Flash
- The point of TTL flash is to get a well balanced exposure, while this can be useful in a pinch, you can often end up with images that are somewhat flat or dull.
- Exposures can be inconsistent as the camera will recalculate the “right” flash again on every exposure.
- If you are using multiple cameras on an infrared triggering system the triggering can be unreliable. They’ll all need an unbroken line of sight
- Costly. TTL flashes can end up being a lot more expensive than manual flashes.
- Lack of control over the exposure of the image. You could end up ‘fighting’ with the flash system over what is the ‘best’ exposure.
When Should I Use TTL Flash?
Situations where TTL flash can be useful include:
- Events where you need to take quick images but where the lighting can change quite often e.g. weddings.
- If you are a beginner and just want to experiment and learn to use a flash.
- When you don’t have any time to take any test exposures and you can’t miss the shot.
When Should I NOT Use TTL Flash?
- When you want consistent exposures to do composites
- When you want full creative control
- When you’re on a budget and can only afford a manual flash
Can I Use TTL Flash With Every Flash?
No, TTL flashes will specifically be designated as compatible with TTL flash whereas some flashes will only be able to use manual flash features.
An easy way to tell if the flash is a manual flash or a TTL flash is by looking at the number of contacts on the hot shoe.
Manual flashes will tend to have just one contact as this is used just to communicate with the shutter of the camera as the flash is triggered when the shutter is triggered on the camera.
On the other hand TTL flash will have more contacts as they need to communicate more information than just the trigger being released.
Can I Use TTL Flash With Every Camera?
No, some cameras will be incompatible with TTL flash, but most modern cameras should be fine. Double check for your specific model just to be safe.
Is TTL Flash The Same As E-TTL, I-TTL And P-TTL?
E-TTL, I-TTL and P-TTL basically do the same thing but are different acronyms for different camera companies.
E-TTL is the Canon branded version of the technology.
I-TTL is the Nikon branded version of the technology.
P-TTL is the Pentax branded version of the technology.
All of the above work by using a pre-flash as explained earlier in the post. While the technology is not exactly the same across the various brands they work in a similar way.
If you want an bit more explanation on TTL flash check out the following video:
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