Metering in bounce flash - Chuck?

[Doug_Kerr]

I often use a Canon Speedlite 580EX II flash unit on my Canon EOS 40D body in a "ceiling bounce" configuration. I am operating in ETTL (II) mode, with evaluative flash metering.

Very often, I find that to get the exposure result I expect, I will have to set a substantial positive flash exposure compensation (perhaps +2 stops).

I don't understand this. The flash unit sends out a "metering burst", which should allow the system, in setting the flash unit's luminous output for the main burst, to be aware of the "path loss" from the flash unit to the subject (as radio engineers would say), whatever that is and whatever it results from.

The problem doesn't seem to be that the flash unit can't deliver the needed output, as I get a green confirmation indicator after the shot, and in any case, if that were the problem setting a positive FEC would be to no avail.

What am I missing here?

Thanks for any help.

Best regards,

Doug

[ChuckWestfall]

Hi, Doug:

What ambient exposure mode are you using when attempting bounce flash?

You might want to experiment to see the difference between manual exposure (i.e., manual control of shutter speed and aperture) with E-TTL vs. program, aperture priority or shutter priority with E-TTL.

I anticipate you'll be happier with manual exposure control for indoor bounce flash work, especially if you underexpose the ambient light intentionally.

[Doug_Kerr]

Hi, Chuck,

Originally Posted by ChuckWestfall

Hi, Doug:

What ambient exposure mode are you using when attempting bounce flash?

Often P, often with a substantial negative EC (especially if there are bright ambient things, like windows open onto the daylight or bright light fixtures).

You might want to experiment to see the difference between manual exposure (i.e., manual control of shutter speed and aperture) with E-TTL vs. program, aperture priority or shutter priority with E-TTL.

Yes I'll do that. Still, I often like to make sure I get a reasonable ambient exposure on the background.

I anticipate you'll be happier with manual exposure control for indoor bounce flash work, especially if you underexpose the ambient light intentionally.

Thanks. I'll try to gravitate to that mode when practical.

Now what is it that can make the ETTL II system not give an "appropriate" flash component of exposure in a bounce situation?

Best regards,

Doug

[ChuckWestfall]

Hi, Doug:

Especially in aperture-priority and shutter-priority modes, and to a lesser extent in program mode, E-TTL and E-TTL II are designed to give priority to the ambient exposure level and use the flash as a very subtle (some would say too weak) fill. Some photographers like this effect, others don't.
On the other hand, in manual exposure when the ambient light is intentionally underexposed by the photographer, the camera bases its exposure primarily off the illumination reflected from the subject by the Speedlite. Consequently, little if any flash exposure compensation is required.

Let's take a look at 3 different indoor scenarios, all shot with bounce flash in E-TTL II:

1. Subject is posed in front of a window with bright daylight behind. If everything on the camera and flash is left to default settings, it's likely that the camera will attempt to expose the background accurately. The more the subject fills the frame, the more likely it becomes that the flash will produce an accurate exposure of the subject. Conversely, the less the subject fills the frame, the more likely it becomes that the flash exposure level will be too low unless flash exposure compensation is applied.

2. Subject is in a room that's completely illuminated by artificial light above (i.e., no additional window light), creating heavy shadows under the eyes and chin. In this case, bounce flash in aperture-priority mode will still attempt to expose the ambient light properly and the flash fill level will be very subtle, perhaps too subtle for some photographers. Bounce flash is not necessarily the best technique in this situation, since it does little to alleviate the shadows. A diffuser or bounce card is likely to provide better results by changing the direction of the light from overhead to direct.

3. Same environment as the second example, but this time the camera is set to Manual exposure mode while the Speedlite is set to E-TTL. The ambient light is intentionally underexposed by selecting an appropriate shutter speed/aperture/ISO combination. For the sake of discussion, let's say 1/60 and f/5.6 at ISO 400 with bounce flash, whereas an accurate exposure of the ambient light would require a shutter speed of 1/8 at the same aperture and ISO. In this case, the metering system detects that the ambient light is being underexposed, so it now bases its exposure level primarily on the illumination provided by the flash. In this case, no flash exposure compensation is necessary with most mid-toned subjects. If the subject is wearing a white dress or a black tuxedo, it may be necessary to apply some FEC to compensate.

The exposure settings I suggested above are similar if not identical to those the camera would set under the same conditions in Program mode, and the results would be similar.

Hope that helps!

[Jerry Skrocki]

Doug,

Have you tried using Flash Exposure Lock? I find that it gives more accurate results because the flash exposure information is retained in memory before the actual exposure. On the 40D it is set by pressing the (*) button.

[Doug_Kerr]

Hi, Chuck,

Thanks so much. That was really helpful.

I'm quite sure that the flash fill "balancing" strategy of the system is the main source of the effect I mentioned.

Actually, at one time I was quite aware of the fill flash situation (you helped me quite a bit with it then), but it seeks to have gotten entirely away from me!

Originally Posted by ChuckWestfall

2. . . .Bounce flash is not necessarily the best technique in this situation, since it does little to alleviate the shadows. A diffuser or bounce card is likely to provide better results by changing the direction of the light from overhead to direct.

In fact, although I didn't mention it, in that situation I usually use a "super bounce card" (a Flashbender from expoimaging) on the flash unit to give a fairly substantial forward component. That has been a big help to operation in this scenario.

Thanks again so much.

Best regards,

Doug

[jrsforums]

Originally Posted by Jerry Skrocki

Doug,

Have you tried using Flash Exposure Lock? I find that it gives more accurate results because the flash exposure information is retained in memory before the actual exposure. On the 40D it is set by pressing the (*) button.

FEL will change the flash metering from evaluative (or averaging) to spot metering.

This may provide better metering, particularly if the main subject is under the spot or if there are items in the camera view which are closer than the main subject.

John