Canon E-TTL questions

[dr_who]

I've read the EOS bible but just want to get a confirmation that I interpreted all that info correctly.

1) If I want to use flash to freeze motion, the best way is to set my camera to manual and ignore the exposure indicator, as that reading is actually for the background, right? The flash should be set to ETTL auto, when camera fires, flash is metered to expose foreground properly, right? And because I set the shutter speed fast, the background will be underexposed, but I'd be able to freeze motion?

2) So in av/tv mode, the EC will change the exposure level of background, while the FEC will change the exposure level of foreground? If FEC is unchanged, the foreground exposure will be exactly the same if I change EC? Looks like if I want to freeze motion all I had to do is in AV mode set -2 stop EC?

3) If I set off-camera flash unit to optical slave, when in this mode, does the flash unit's own meter automatically expose foreground correctly, seemn by the flash unit? And I would adjust the FEC on the flash unit itself to adjust foreground exposure and the EC on camera for the background?

[Curtis N]

As for your first two questions, I think you understand the concepts correctly. The flash does a good job of freezing motion if it's the main light source with little ambient contribution to the image.

For question #3: Your Sigma flash has no light sensor built in, and no way to meter itself. In optical slave mode, it's just a manual flash.

[DaveG]

I've read the EOS bible but just want to get a confirmation that I interpreted all that info correctly.

1) If I want to use flash to freeze motion, the best way is to set my camera to manual and ignore the exposure indicator, as that reading is actually for the background, right? The flash should be set to ETTL auto, when camera fires, flash is metered to expose foreground properly, right? And because I set the shutter speed fast, the background will be underexposed, but I'd be able to freeze motion?

2) So in av/tv mode, the EC will change the exposure level of background, while the FEC will change the exposure level of foreground? If FEC is unchanged, the foreground exposure will be exactly the same if I change EC? Looks like if I want to freeze motion all I had to do is in AV mode set -2 stop EC?

3) If I set off-camera flash unit to optical slave, when in this mode, does the flash unit's own meter automatically expose foreground correctly, seemn by the flash unit? And I would adjust the FEC on the flash unit itself to adjust foreground exposure and the EC on camera for the background?




1) If you want to use the flash to freeze motion, your flash exposure has to be many stops away from the ambient light exposure. For example if the ambient light is 1/30 @ f4 and you use that exposure + flash you will get a sharp image AND a blurred image superimposed over the first, assuming that the subject's are moving. Even if you shot at 1/125 @ f4 there would be enough ambient light to create the ghosting. Now if you shot in total darkness the only thing that's lighting the subject is the flash. Thus the thing that makes the "shutterspeed" is the duration of the flash pulse. On manual the flash pulse will be somewwhere around 1/700 of a second. On automatic it will be MUCH shorter - 1/10,000 of a second or even shorter. THAT will freeze action.

I use both these approaches at the church during a wedding. The first would be the bride and groom posed on the steps of the alter. I take an ambient meter reading, and again let's use 1/30 @ f4. I have my flash set on E-TTL, the camera is on a tripod, and I just shoot. I can use the camera on Manual and set the aperture and shutterspeed, or I can use Tv or Av. By using the ambient light AND the flash I get a natural looking image as well as I've filled in the subject's eyes so they don't look like a couple of racoons.

Do I get blur/ghosting here? No, because they are motionless and I have the camera on a tripod. I even warn them that the exposure is going to be slightly longer than the flash pulse so that they don't move right after the flash pop.

The second example is the bride coming down the aisle with her dad and I usually shoot at something like 1/125 @ f8. I use Manual for this and ignore the meter. The meter doesn't know that I'm going to use flash and it will just inform me that I'm many stops underexposed, which I know, want, and don't care about.

I also know that the room is not completely dark, but the difference between 1/30 @ f4 and 1/125 @ f8 is FIVE stops! With that light level and exposure setting there is so little light stiking the sensor that for all intents and purposes the room IS completely dark. So when I do use flash the only light hitting the subjects has a duration of (arguably) 1/10,000 of a second. Subsequently the bride and dad are frozen solid. I DO get flash fall off - which means the background is dark, and it doens't look particularly natural, but sharpness in this shot is much more important than something that's possibly more real, but blurred.

2) You could figure out how to do the second exposure using an automatic setting like Av or Tv + FEC, but I can't tell you how as it makes my head hurt. Seriously it's much more straightforward to use Manual on the camera and then make adjustments instead of playing the Av/Tv/FEC game.

3) I'm not quite sure how to discuss this. To begin with no off camera Canon EX flash can be set to E-TTL and be triggered by a light slave. To do that you'd have to use the Canon wireless system which is a topic for another day. But if I used a simple Automatic flash (say a Vivitar 283) in the camera's hotshoe, and another one lightslaved off the camera somewhere, I'd just go back to 1). What is the ambient light? What exposure are these two flashes putting out? Is that exposure far enough from ambient so that I get "flash only" lighting? As soon as I get "flash only" lighting I know that the flash pulse will be short enough to freeze motion.

[Curtis N]

On manual the flash pulse will be somewwhere around 1/700 of a second. On automatic it will be MUCH shorter - 1/10,000 of a second or even shorter. THAT will freeze action.That doesn't compute.

The duration of the flash is a factor of the flash unit and how much of its power is used. Automatic flash, or manual flash at a setting that creates the same illumination, would have the same duration.

I agree that manual mode on the camera is a better choice when you're trying to minimize the ambient contribution.

[DaveG]

That doesn't compute.

The duration of the flash is a factor of the flash unit and how much of its power is used. Automatic flash, or manual flash at a setting that creates the same illumination, would have the same duration.


Yeah except that isn't how it works.

On Automatic you (or the camera) select an aperture of say f8. On older Automatic flashes you'd then set a dial or somesuch to give you f8. On Canon E-TTL 2 cameras and flashes you'd just set the aperture to f8.

Then the flash fires and off goes the light towards the subject. It strikes the subject and that little eye in your automatic flash, or the sensor inside your camera, sees the light and it builds and builds over time - well micro seconds - until it reaches f8. Then the flash shuts itself off. If it didn't need much power the flash shuts itself off very quickly and that's were the 1/10,000 of a second flash duration (or MUCH shorter!) comes from. The same flash on full power Manual will have a pulse duration of around 1/700.

[Curtis N]

If it didn't need much power the flash shuts itself off very quickly and that's were the 1/10,000 of a second flash duration (or MUCH shorter!) comes from. The same flash on full power Manual will have a pulse duration of around 1/700.Now you're comparing a low powered automatic burst verses full power manual?

I agree that the duration of the flash burst is much shorter with a low-powered burst vs. full power. But at a given aperture and distance, the same amount of flash power is required, no matter how you meter it.

When a low power setting is selected manually, the flash output is reduced the same way - by reducing the duration of the flash.

[DaveG]

Now you're comparing a low powered automatic burst verses full power manual?

I agree that the duration of the flash burst is much shorter with a low-powered burst vs. full power. But at a given aperture and distance, the same amount of flash power is required, no matter how you meter it.

When a low power setting is selected manually, the flash output is reduced the same way - by reducing the duration of the flash.

Sorry, I misunderstood your first post. Yes if you use half power the pulse will be shorter, and 1/4 power will make it shorter and so forth, and those will approach the really short pulse durations of an automatic flash. My start point was to use the flash in an automatic rather than manual way, since automatic is so much more convenient.