Canon 580EXII question - weird effect using a diffuser and zooming

[Anorakus]

Here's one for the Canon flash experts..

I've got a Sto-fen style white diffuser attachment for my 580EXII. The flash exposes fine in wide-angle setups - however as soon as I zoom in to 70mm, the pics are coming out about a stop or more underexposed.

HOWEVER, if I pull out the wide panel before fitting the diffuser, all is fine - zoomed shots are perfectly exposed.

What's going on? Is the diffuser duplicating the effect of the wide panel - therefore pulling out the wide panel tells the flash to expect the light to be spread out.

I thought E-TTL was supposed to expose the shots properly, regardless of what you had fitted to the flash?

Shooting straight without the diffuser or wide panel in place exposes fine on zoom settings, so it has to be something to do with the diffuser attachment.


Many thanks,

J.

 

[onform]

Are you using it outdoors?

 

[HoppyUK]

How exactly are you using it? I'm thinking particularly about the angle of the flash head. If it is straight forward (which it shouldn't be with a Stofen) then you will get under exposure, because the camera doesn't know you've got the Stofen fitted and gets confused. Tilt the head up and all will be fine.

Any help?

 

[Anorakus]

I have been using it at 90 degrees, i.e. pointed straight at the subject - isn't that what you're supposed to do?

The diffuser I got is a Delamax, which is well-made and fits well but doesn't come with any instructions.

The underexposure happens when shooting indoors with low ambient light. I've used the diffuser outdoors for fill-in, and it works very well with accurate exposure.

A.

 

[HoppyUK]

I have been using it at 90 degrees, i.e. pointed straight at the subject - isn't that what you're supposed to do?

The diffuser I got is a Delamax, which is well-made and fits well but doesn't come with any instructions.

The underexposure happens when shooting indoors with low ambient light. I've used the diffuser outdoors for fill-in, and it works very well with accurate exposure.

A.

No, you should point it anywhere but directly at the subject! Angle it upwards at 45-90 degrees from horizontal. Almost all these diffuser attachments work by sending most of the light to a nearby bounce surface of some kind, usually a light ceiling, which then becomes a large, soft light source. However, this casts shadows under eyes and chins so a small amount of light is sent directly from the Stofen which fills them in and brightens faces, plus puts a sparkle in the eyes.

Using a Stofen outside is pointless - it just wastes light and gains you nothing. The reason why it has worked for you is that a bit of under exposure from the flash has gone unnoticed since there is plenty of ambient light around anyway, and even though most of the light is wasted the gun still has enough in hand. But it will soon run out of puff if you engage the HSS high speed sync mode, which is very power hungry, so best to take it off.

What is happening with your under exposure is Canon's E-TTL being too clever for its own good. When the flash is pointed straight forward, the camera knows how far the subject is through focusing data and since flash is very distance sensitive this data overrides every other consideration. Wrongly as it happens, because you've got a Stofen on there that the camera doesn't know about and that cuts the flash brightness by spreading the light everywhere and the camera thinks it knows best and fails to compensate.

However, when you move the flash head up, or sideways, the camera then knows you're being creative and all the distance data is disabled. When you angle the head upwards, you'll notice that the range indicator on the bottom of the gun's control panel switches off, confirming that it will now meter the flash using the pre-flash exposure information, and all will be well.

You can use the angle of the flash head to adjust the ratio of bounce to fill-in light. Try it at home - a pot plant against a plain wall is quite handy, as the leaves cast good shadows. Shoot from a normal distance for a people shot and take identical pictures with the Stofen pointing straight up and then at a couple of positions down to 45 degrees. See how things change. When in doubt, use the 60 degrees clicked position.

 

[Anorakus]

Hoppy, thanks for the detailed explanation.. beginning to understand it a bit better now

I've experimented using a Gary Fong diffuser as well. The results are highly variable, and seem to depend on how close you are to the ceiling. Shooting zoomed in always seems to underexpose the shot.

I have found a way of getting far better exposed results with either the stofen or (particularly) the gary fong attchment. The answer is simply to set the flash to wireless master mode.

This disables the zoom function on the flash, and (I'm guessing) forces it to calculate exposure purely on the pre-flash because the camera doesn't know where the light will be coming from - which is what I've wanted all along. It's noticeable that with the gary fong fitted and shooting a zoomed shot away from the ceiling, the flash fires at full power and takes ages to charge up again.. I don't know why it wasn't doing this in single-flash E-TTL mode. Just goes to show how much light it absorbs..

A.