580ex and DoF Preview

why does the 580EX on a 20D emit flashes when using the DoF Preview function? o_O

On another note, doesnt the 20D's built in flash emit flash when using CF 4-1 focusing method when its dark ?

The 580EX (as well as the 550EX and the 420EX) emits a burst of pulses (referred to a modeling light) when you press the DOF preview button so that you can see what the DOF of the scene looks like. If you have the flash on, the assumption is that it's already dark to begin with. When you close down the aperture of the lens for the DOF preview, it's going to look even darker in the viewfinder so the burst of flash light pulses helps you see the image through the viewfinder a lot better.

Re the burst o flash when you press the "*" button with the built-in flash up, what you're seeing there is the burst of flash pulses used for focus assist. You would get the same thing if C.Fn-4 was set to 0 and you pressed the shutter button half-way, assuming you haven't overridden it with custom function and you are not using AIServo.

The answer to both is "because that's the way it's designed".

I suppose that Canon could have put an extra control button on the camera that is named "activate external flash modeling light." However, that would cost a few cents extra.

Canon figures that if the scene is dark enough that you are trying to use an external flash, then you probably will not be using the depth of field preview button for its normal purpose, so if the camera senses that there is an external flash present, it changes the function of the button to "activate external flash modeling light."

---Bob Gross---

The 580 EX has a CFn to change this: CFn 10 sets the modelling light to the test button of the flash. You can also switch the ML off alltogether with CFn. 6. It's all in the manual...

Bob, I get the feeling you are explaining another feature or use of the action.

What is external flash modeling light? Whats its purpose?

René, what is the modelling light?

tim, the answer is both? both what?


Leo, I was hoping someone would say to brighten up the scene to better see the DoF preview.

The answer to both of your questions is "because that's the way Canon made them". Ours is not to question why...

Im not questioning why Canon chose to include these features. Im trying to comprehend what the features are used for to take advantage if them.

The first, I guess it's so you can see what you're trying to take a photo of. I used mine as a torch this past weekend.

Does a speedlite's AF beam light when CF4-1's turned on? I really have no idea. I've told the camera to never use the internal flash as AF assist so i'd never have noticed myself. Hopefully someone can figure out the Canon logic...

The modeling light is the stroboscopic series of flashes (70Hz for 1 second) you see when you press the DoF preview button It allows you to see the effect of the flash (with regard to lightfall and shadows) similar like a modelling light on a studio flash unit would. As for using it as a torch: Comes in quite handy sometimes , but be aware the 550 manual says: "To prevent overheating (...), do not fire the Modeling flash for more than 10 times in succession. After 10 times, allow the 550 to cool for at least 10 minutes"

I would explain it similarly. A modeling light on an external flash unit works similarly to the modeling light on a studio strobe. It is simply a temporary test light that will illuminate the subject from the exact same angle as the real light. If you were set up in a studio with lights A and B, you might want to fire off the modeling light to see that both A and B were really ready to go and aimed correctly. The light intensity will seem to be low or flickering, because you don't need the real full flash pop for test purposes.

---Bob Gross---

Wow thats wonderful!

So its a preview to the flash output and it's effect.

But wait, if its low or less ouput then when I actually fire (take the photo), then how does it allow me to preview the flash effect?

Shouldnt it resemble the DoF Preview function? I mean, the DoFP literally stops down to allow you to see DoF. So,shoulndt the 580EX fire at the set output when pressing DoF P button?

Three things:
1) If it fired at full output, it'd be over in a, you should pardon the expression, flash. By firing at a lower intensity, but repeatedly, it gives you something that looks like continuous lighting, so you can grasp what it's going to look like.
2) Your eyes are much more sensitive, and flexible, than the camera's sensor, so full output isn't necessary for you to visualize the effect.
3) DoF preview is actually letting you see the effect stopping down the lens will have on the zone of sharpness; there's no analog for lighting preview. You don't need to see the light at full intensity so long as each flash put s out proportionally what it would in taking the picture.
OK - 4 things:
4) If the flash fired at full power and at a high enough rate to appear "continuous" to your eyes/brain you'd go through batteries at an incredible rate and overheat the flash quickly, requiring a 10+ min., cool-down.

I can speak to the 550EX, and I suspect that the 580EX is similar.

When you fire a normal flash shot, you get the pre-flash, and then you get the main flash at full intensity and for an extremely short duration.

When you fire a modeling flash, the aperture will stop down, and then it will fire a series of flashes at 70 Hertz for one second.

The comparison between these two radically different types of lighting depends mostly on what is going on. The normal flash is intended to provide the correct exposure onto the sensor. The modeling flash is intended to put some light out for your eyeball to judge, and your eyeball doesn't work as fast as a shutter.

---Bob Gross---

thanks again everyone!

That solved my puzzle.

damn good job Canon and POTN community