Practical difference between TTL and flash sensor

Question

On an speedlite 600EX-RT, what is the practical difference between E-TTL II and Ext. A? I know the technical one, but how does it change flash exposure if you measure through the flash sensor rather than through the lens?

Answer 1

There is some situation where the flash sensor is just off:

• With a non coupled lens stopped down (unless you can input the f-number into the flash...)

• When dealing with a small subject detached from the background: spot TTL metering will do the job, flash sensor will overexpose the subject trying to get enough light to the background.

• All the situation where the flash is not on the body grip (as example: I do macro with a cobra flash hold in one hand in this situation the flash sensor is not directed to the subject and is not at the same distance than the camera)

Answer 2

In theory, ETTL or iTTL for Nikon, should be more accurate in more situations, especially for closeups where the flash sensor is not pointed directly at the subject. However, it has some drawbacks. It is a much more complicated process which takes some time and requires a preflash test which is measured. The preflash and the calculation can take just long enough that some people begin the blink which is captured. In some situations, it will slow down fast multi shot sequences. Flash auto sensor is always extremely quick, is consistent in how it behaves, and does not cause blinking. So, neither is perfect, but the technology of ETTL, used with lenses that provide distance data, gets the nod for most situations.

Hope that helps.

Answer 3

There are actually quite a few differences aside from ttl vs. external sensor.

E-TTL II uses a metering pre-flash to put flash into the scene so it can be measured (through the lens) and then uses that resulting information to set the power on the flash, together with all the other exposure settings. Some current models can let you set ambient priority vs. flash priority on ambient/flash balance, distance information from the lens is used, and additional attempts at optimizing for area/subject can be performed.

Ext.A doesn't need a preflash. The flash uses the iso and aperture settings on the camera together the sensor input to time power cut off. There's no additional digital finessing going on and minimum camera body/flash communication. In fact, in Ext.M mode, there is no camera body/flash communication at all (you manually set the iso and aperture you're using). But you can use Ext. M off-camera or with a non-Canon body (I used to use Ext.M on my 580EXII on my Panasonic G3's hotshoe until Godox came out with the TT350-O). E-TTL II can't be used cross-brand.

The sensor itself needs to be clean to be accurate; something you won't see without inspection vs. dirt on your lens. And on Canon speedlights, the Ext. modes have only use a narrow coverage of the scene (I want to say 30º but I can't find a reference), acting a bit more like spot metering than full scene matrixed metering.