When the Speedlite 580EX II was launched it came with a new external auto and external manual metering mode that uses a sensor on the flash to determine when the subject has had enough light and then the flash autonomously switches off it’s own output. The Speedlite 600EX-RT also has this and you can set a group of flashes to use this mode from a radio master when using group mode.
External flash metering?
Normally Speedlites are used in one of two modes;
- Manual mode which delivers fixed and repeatable amounts of light
- E-TTL mode where the camera determines the exposure for the scene and instructs each Speedlite how much light to deliver.
Way back in time there were flashes that measured the light reflected from the subject and automatically quenched their output when the subject had received enough light; this was often called thyristor mode. Mostly due to a semi-conductor device called a thyristor being used to do the switching. On such flashes you chose the aperture on the flash unit, and then set the same aperture on the camera for correct exposure.
Speedlite 580EX IIExternal metering on the Speedlite 600EX-RT and Speedlite 580EX II is similar to the thyristor flashes of old. Using a small metering sensor window on the front of the Speedlite, the flash decides when to stop delivering light. To do this the flash needs to know the aperture value and the ISO value set on the camera. With external auto mode on the 580EX II, the camera can send the values to a single Speedlite on the camera hotshoe or on a long off-camera cord. For flashes triggered with a ‘dumb’ trigger such as a PocketWizard Plus II then the photographer needs to set the flash to external manual and configure the ISO and aperture value. Check the back of the flash as a distance scale will show you the range of the flash based on the chosen aperture and ISO.
Set C.Fn 05 = 2 for external auto metering
Set C.Fn 05 = 3 for external manual metering
Using a Speedlite 600EX-RT with radio and group mode then the transmitter, either ST-E3-RT or another 600EX-RT can pass the values of ISO and aperture to a remote flash. This means that external auto mode is now much more useful.
The built-in metering sensor in the flash has a narrow angle of view, about the same as a 135mm lens on a full-frame camera. So it is quite important to correctly orientate the flash so that it’s metering sensor is aimed at the subject. Of course the radio flashes make this easy as they don’t need line of sight between master and slaves.
So how and when might you use this mode?
In all honesty I only once found a practical use for the external metered flash with the Speedlite 580EX II. If you were doing light painting, you could set the flash to external metered manual (Ext. M) mode, set the aperture and ISO on the flash and then walk round the scene firing the flash ensuring that even with changing distance from the subject the flash output would be even. The use of external auto was quite limited as the flash needed to be on the camera hotshoe to get the ISO and aperture from the camera.
The radio flash system brings the possibility to use individual flash groups in different modes. So I’ve got a new and more useful use for external metered auto (Ext. A) mode.
I’ll often light a background or add a gelled light to the background, but in reality it just needs to be consistent, it’s not anything E-TTL and in the past I’ve set such flashes in manual mode. Now I set external auto for the group I am using on the background and make sure they are orientated so that the built-in meter can see the reflection. A simple test to check the background is as I need, dialing in compensation for the group if needed is fine. The key is that if I change the aperture then the background lights also get the new aperture and will change their power output, to give me the same results. It’s one more way that I’m finding the radio controlled Speedlite system is changing my use of flash to make it faster and more consistent.
There’s several articles on external metering on the Canon USA digital learning centre (DLC)