Flash Photography: Manual vs TTL

In this episode, I show you the two main modes one can use with a speedlight (cobra flash) and their pros and cons for each: The Manual vs TTL. Before we dive into this seriously, one should not confuse the manual mode of one’s camera and the manual mode of one’s flash. Whether one uses the Manual or TTL mode of the flash ones can shoot with the camera set to one of the Priority modes (Av &Tv or A & S) or fully manual.

The Manual mode

As its name implies it is the mode where you set your flash manually. The Flash Power value is expressed in fraction such as 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1 (1 being the maximum power output of the flash). While all Speedlight have the Manual mode not all flash give you the same power output. Some my not go as low as 1/128 and the minimum might be 1/32.

The Flash Guide Number

It is worth knowing that Flashes are measured in Guide Number. This is how one can compare one Flash model over another one. So the higher the GN the more power the Flash unit can emit and the further away the subject can be from the Flash and still be well exposed. How is that number calculated? The GN number is the product of a distance and an Aperture value.  While we will go in greater details in future episode this is what this maths mean: A Flash with a GN of 50 means that if your subject is 25 feet, your Aperture is F2 and your Flash power is on full power, your subject will be correctly exposed:  50=(25 feet x F2).

The Advantage of the Manual Mode

The main advantage of using the flash in Manual mode is the consistency it offers. Once you have set the Power value then no matter what you do with the camera it will remain the same. The main rule to remember in Flash photography is that as long as the distance between the light source and the subject remains the same, the camera can move around the exposure will remain the same. This can be illustrated like this (Note that neither the camera nor the flash settings change throughout the experiment): 1st setup: my light source is 2 squares away from my subject & my camera is 3 square away from the subject:
Light setup as seen in the video
Light setup as seen in the video

2nd setup: we only move the camera further away from the subject. The exposure is identical as the 1st setup. Even if you change the focal length.

Light setup as seen in the video with camera away from the subject
Light setup as seen in the video with camera away from the subject
3rd setup: the light was moved but its distance to the subject remains the same. The camera also moved sideways. The exposure is identical as the 1st & 2nd setup.
Light setup as seen in the video with camera away from the subject in an angle
Light setup as seen in the video with camera away from the subject in an angle

The Inconvenient:

When set in Manual mode one still needs to figure out what the Power output needs to be to obtain a correct exposure. To some it can be a time consuming task however there is a tool that will significantly speedup this task and can get you up and running with a perfect correct exposure from the first shot: The Light/Flash Meter. I will cover this in the next episode.

If you do not have a Light/Flash Meter, you can still use the histogram at the back of your camera. Although this is more for guidance and will lead you to shoot a few test shots to get it right.

The TTL mode

TTL stands for Through The Lens metering and since an image is worth a thousand words (unless i try to draw it) here is how it works:

How the TTL system works
How the TTL system works
  1. The Camera triggers the Speedlight.
  2. The Speedlight burst a quick flash.
  3. The Camera meters the lit up subject and calculate what the correct power output of the Speedlight should be
  4. The Camera informs the Speedlight
  5. The Speedlight burst a final and correct flash to correctly expose the subject
  6. The camera takes the shot.

The Advantage

You do not need to figure out what the flash power output needs to be to achieve a correct exposure with the flash. TTL is basically the equivalent of the metering of a scene without flash where the camera identifies what the  exposure settings (Aperture, Shutter) are to get a correctly exposed photograph. Just like with standard exposure (non flash) one can use the exposure compensation for flash aka the Flash Exposure Compensation.

This automaticity is ideal when you want to capture a scene rapidly from the very first shot without having to do any guessing work.

The Inconvenience

Inconsistency of Light

While the above advantage may lead to believe that nowadays, you can simply let the camera defines the flash output and it is 100% reliable, think again. Given TTL relies on the camera to meter the light, this means that as you move the camera around the metering performed by the camera will return different result and therefore the flash output will not be consistent. The easy way to think about it is to imagine zooming in your subject to fill the frame and the next shot you zoom out and take the subject within the environment. In the second shot, the subject does not represent the same ratio in the frame and therefore the exposure will differ just like it does when you capture a normal scene (without flash) and you include or not a bright sky. This fools the meter and you then need to compensation under or over to get a correctly exposed photograph.

Metering Mode and TTL

In case you thought you could simply change your metering mode from matrix to spot and that would guaranty that your exposure will be calculated according to your subject alone, think again. This will depends on your camera and the type of TTL it support (i.e. Canon release ETTL II which acts differently and takes in consideration the distances, the position of the flash and whether it is bounced or not.). So where am I going with this? Well i cannot guaranty that using the Spot metering mode of you camera will make your flash behave better in TTL. I know i have tested all combination on my 5DIII and the result are still rather unpredictable. But for those who have been shooting TTL for a while and with the same gear will be better at making an educated guess on the Flash Exposure Compensation.

1- A shot done in TTL with 0 Flash Exposure Compensation. The result is overexposed
Overexposed result with TTL
Overexposed result with TTL
2- Same shot in TTL after applying the FEC by 1 2/3 stop. The shot is now correctly exposed
Correctly exposed with TTL once added some Flash exposure compensation
Correctly exposed with TTL once added some Flash exposure compensation
3- I move closer to the subject with the same TTL and FEC. The shot is suddenly under exposed
An other shot with the same setting (TTL + FEC) but result is underexposed
An other shot with the same setting (TTL + FEC) but result is underexposed
4- I keep the same setting TTL FEC and the same position but i change my Focal length from 105mm to 24mm. The subject is now correctly exposed
An other shot with the same setting (TTL + FEC) but result is correct once i changed my focal length
An other shot with the same setting (TTL + FEC) but result is correct once i changed my focal length
Not all TTL

Not all Speedlight (cobra flash) or flash triggers are TTL mode compatible. TTL requires an extra layer of compatibility between the flash unit and the camera which ends up increasing the price of the gear.

My conclusion

If there is one thing i have learnt over the years is there is rarely a straight answer in Photography and it always depends on some factors. Do i ever use TTL? I do when i am covering an event such as a wedding where the flash is there to provide some extra light and i do not have time to measure with the Light/Flash meter or do multiple test shots.

When it comes to portrait photography using Flash I always use the Flash Manual mode. This enables me to set up my light(s) the way i want to, move them around (while at equidistance to my subject) or move myself and the exposure is correct throughout. In my opinion, the Manual Mode is the mode that provides you with the most control over your light(s).

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