Canon 580ex FAQ
Stuff You
Might Miss
In The Manual

580ex Design and function

The 580exMkII is currently the flagship of the Canon hot shoe flash line but suffers from performance issues due to the redesign of the foot and move to camera menu control of flash functions: there are numerous reports of 580exmkII units switching at random into the legacy flash mode for film bodies Canon calls "TTL". TLL meters of the film in real-time during the exposure and on a digital camera an EX flash in TTL mode will fire at full power because it never gets a signal from the camera to end the flash. The redesign also eliminated the convenient OFF-MASTER-SLAVE switch on the base which is convenient when using a 580ex on a bracket as Master. The MkII requires a cumbersome trip to the menus to switch mode. So for the time being I'm sticking with the 580ex and would recommend buying a used 580ex over a new 580exMkII for use as a Master. When used as a Slave the MkII doesn't seem to have the same hot shoe related problems.

The 580ex has zooming flash head matches the area it illuminates to the field of view of the lens being used with the guide number (GN) ranging from 91.9 at the wide angle 24mm position, to 190.1 at 105mm. As with any flash dividing the guide number by 10 reveals the aperture needed for correct exposure at 10 feet at ISO100: f/9 at 24mm, and f/19 at 105mm. But since the 580ex also links seamlessly with the sophisticated metering of the Canon camera bodies we never need to do the math. The flash even senses the crop factor of the body sensor and adjusts the zoom scale to it. The 580ex also communicates white balance information and will automatically adjust WB when the more recent camera bodies (20D and later) are set to AWB or Flash.

The 580ex is the most versatile flash in the Canon arsenal. It can be used individually on the hot shoe or TTL extension cable, or in a multi-flash configuration as MASTER (to a 580ex, 550ex, 430ex and 420ex) in three separately controllable groups (A:B C) or as a SLAVE to a ST-E2, 580ex, or 550ex master. If you are planning on building a multi-flash Canon EX flash system I recommend buying only 580ex flashes and avoid purchasing the ST-E2 which an only control two groups. Read the overview for my rationale and cost comparisons.

Things the manual doesn't tell you or explains poorly.

My intention here is not to re-write the manual but just explain functions which may not be clear from the manual. Please read your 580ex manual to become familiar with its displays and controls.

WIRELESS CONTROL SWITCH: That is what the "OFF - MASTER - SLAVE" is called. It would be less confusing if Canon had labeled it "SOLO - MASTER - SLAVE". This switch was eliminated in the mkII re-design. On the MkII the Master/Slave is configured via menu on the flash.


If a single 580ex is used alone or to trigger non-Canon flashes via optical slaves the wireless switch should be in the OFF position. In ETTL mode the flash will emit a pre-flash for exposure metering. Metering is controlled by the camera body not the flash and method varies by body type and model (i.e., TTL for film, E-TTL and E-TTL-II for digital). In M mode there is no pre-flash and the user must manually input the amount of power needed via the control wheel. Note: second curtain can only be used when the flash is in the OFF position because it is incompatible with multi-flash metering.


The MASTER position is only used when the 580ex is connected to the hot shoe of the flash either directly or via the two-foot long extender. Changing to MASTER will also change the function of the control buttons and disable some flash functions such as second curtain and stroboscopic flash. The flash head will also automatically zoom to the widest 24mm position so its coded pre-flash signals can be seen by the slaves. The AF assist light (red lens on front of flash base) functions normally, projecting a grid pattern the camera AF points use.

While in MASTER mode the 580ex can control all functions on the slave flashes from its back panel in either ETTL or M modes. Note: changes made on the Master will not be seen on the display of the slaves until after the first shot in the new mode. For example, it is possible to switch from a 1:4 A:B ratio to A:B manual power without touching the slave, but that change will only be sent to the salve during the set-up flashes in the next shot. A 580ex MASTERcan be set (via the zoom button) to disable the main flash, a function typically used when there are two slave flashes on stands set to Group A and B respectively.

In the Canon EX system the MASTER defaults to Group A automatically


The SLAVE position is only used when the 580ex is off-camera and controlled by another Canon master flash (580ex/550ex) or ST-E2 near-IR controller. The control signals from the MASTER are received via the small sensor window above the word "Canon" on the front of the flash. That window must be pointed towards the MASTER with the head of the flash turned to light the subject.

Because the EX system assigns the Master to group A by default a single slave should be set to Group B via the ZOOM button. When two off-camera Canon EX flashes are used in an A:B C configuration the one used as key light on the subject should be set to Group B and the one used to illuminate the background to Group C.

In the Canon EX system Group C is metered separately from A:B and should be used, per Canon, for the background, not the subject.

Also note: ST-E2 can only control two flash groups groups ( A:B) Groups in ETTL mode. If a 580ex/550ex/430ex flash is used in M mode as a slave to the ST-E2 the power settings must be entered separately on each slave.

In SLAVE The AF assist light (red lens on front of flash base) becomes a second "ready" indicator. When it is flashing the slave is ready to fire. After firing the AF assist light will go out until the slave is recharged and ready for the next shot. There is no way to turn off the $@%& blinking light, but it can be covered with tape without affecting the operation of the flash. A 580ex used as SLAVE will automatically power off after 60 min. if not fired. That time can be changed to 10 min. via C.FN 04. C.FN 05 can be used to change the cancellation of the slave auto power off from one (default) to eight hours. See p.28 of the 580ex manual for a chart of the custom functions available.


It is a common misconception that the EX system uses Infrared control signals. Only the ST-E2 controller uses near infrared light for signaling. When in MASTER mode the 580ex (and the 550ex) utilize coded pre-flash pulses of visible light to signal the slaves. The coded signals, which can involve up to six pre-flashes of master and slaves occur so rapidly between the time shutter is pressed and the main flash fires that the entire sequence appears to be a single sustained flash.

The red plastic window on the front of the 580ex is the focus assist light. When the 580ex is in "off" or "master" mode the AF assist light will function normally, creating a grid pattern which helps the AF point sensors in the camera focus. When the 580ex is used in "slave" mode the red AF assist light will blink constantly as an indication the slave is operational and ready to fire. If will go out when the slave fires and begin blinking again when the slave is fully charged. Canon apparently designed it to function as a second "ready" light because the one on the back will not be visible when the front of the flash is pointed at the camera. There is no way to turn of the AF assist light on the 580ex in slave mode. If the incessant blinking is a problem the red window can be covered with black tape without affecting flash function.


The DOF preview button on the camera will pulse all the flashes so the modeling they create can be visualized, but must be used sparingly to avoid overheating. The normal DOF preview function can be restored by setting C.Fn 06 to 1. A simple way to "see the light" is to just stand behind the off camera stand and look at the face. What is seen will be highlighted, what is hidden will be in shadow.


Flashes which emit a single flash for the exposure have an Achilles Heel; they can only expose the film or sensor when it is exposed. To understand High Speed Sync (FP Flash) it necessary to understand how a DSLR shutter works.

The shutter has two independently moving "focal plane" curtains, which is what the "FP" in "FP flash" describes. The first curtain is normally closed, preventing light from reaching the sensor. When the shutter is pressed the first curtain lifts and exposes the sensor to light. The shutter speed set by the photographer (M or Tv modes) or determined by the camera metering system (Av, P, other auto modes) triggers the second curtain rise from bottom and end the exposure. At speeds above about 1/250th (the "x-sync" limit) the second curtain starts closing before the first in one fully opens and the two shutter curtains, one chasing the other, form a moving slit which exposes sections of the sensor progressively. A conventional "one-pop" flash will only work at speeds at or below the "x-sync" limit of the camera body.

The x-sync speed is a function of the distance the curtains must travel and how flash the curtains move. The 300D Rebel has the same sensor size as a 20D but the curtains move slower so the Rebel has an x-sync limit of 1/200th and the 20D 1/250th. The 5D has a faster curtains than the Rebel but the sensor is much larger so its x-sync winds up being the same as the Rebel. The pro grade EOS 1 bodies with full-frame sensors also have far more sophisticated shutters which allow x-sync speeds of 1/500th.

Practically speaking the x-sync limit of a camera does not matter much when using flash indoors. Indoors the light level is low an not much of a factor in the overall exposure. In fact because low ambient light levels will result in very long shutter speed when Av mode is used indoors the best camera mode for indoor flash is M.

Outdoors in an entirely different situation. Until the day the sun explodes a camera exposure will always be about f/16 @ 1/(ISO speed) sec. , or 1/125th at @f/16 @ ISO 100. Now comes the problem. At the 1/250th x-sync limit of a 20D an outdoor ISO 100 exposure in sunlight will be 1/250th @ f/11. That is a problem because f/11 will require a great deal of flash power to match and because f/11 has great depth of field (DOF) and may result in undesirable sharp distracting backgrounds.

Faced with this dilemma the Canon engineers created a clever solution. They altered the flash so it would pulse repeatedly at shutter speeds faster than the x-sync limit. As the slit between the curtains travels across the sensor it is constantly bathed in light from the fast. This allows the use of any shutter speed, which in turn allows the creative use of nearly any aperture. Now even is sunlight it is possible to shoot at f/2.8 @ 1/2000th to get a background with good "Bokeh" (a Japanese word used to describe a smooth eye pleasing blur pattern). The only catch is that the flash power needs to be spread over many pulses rather than one, so the overall range of the flash decreases. But that is not as bad as it seems because as the shutter gets faster the overall duration of the flash is shorter requiring less output and the aperture is wider which also reduces the need for flash power.

The 580ex, either as solo flash or Master will know whenever the camera shifts to speed faster than x-sync and automatically shift back and forth between "single-pop" and High Speed (FP) if the flash is just left in High Speed Sync (FP Flash) all the time when used outdoors. Indoors it really doesn't matter. If slaves are being used with the 580ex Master they too will be instructed to shift to High Speed (FP) mode when needed.

When shooting outdoors with Av or M mode flash leave High Speed (FP) on at all times.

If you want to get the most out of your 580ex

Do what I do: Use a flash bracket, hot shoe extension cord 2, and a reflector-diffuser like the one shown in my DIY Diffuser tutorial. Then a second 580ex so you can begin to control your lighting creatively. Learn how to use a pair of hot shoe flashes in E-TTL and full manual modes as taught in my dual flash tutorial, but first experiment with a window and reflector so you understand how light models a face and facial angle controls the appearance of its shape. Yes I've got a tutorial for window lighting too.

Here is the Table of Contents for my other tutorials specific to Canon Flash

Holistic Concepts for Lighting
and Digital Photography

This tutorial is copyrighted by © Charles E. Gardner. It may be reproduced for personal use, and referenced by link, but please to not copy and post it to your site.

You can contact me at: Chuck Gardner

For other tutorials see the Tutorial Table of Contents