OK, anyone have ETTL problems with a Fong?

06/14/2008 10:48:45 AM · #1

I'm wondering if anyone else has found a similar problem, and whether I'm going about solving it the right way.

Basically I like sometimes to use the Gary Fong lightsphere as a diffuser in the horizontal position--like for candid portraits when there's no chance of bounce flash. Trouble is, when I switch the flash to horizontal, the metering goes haywire & it consistently underexposes the shot. The lens I'm using is long enough that there's no lens flare from the flash affecting metering, and the problem goes away so soon as the flash is cocked slightly off of horizontal. It also goes away if I press in the button on the side of the flash that one uses to change the angle of the flash. So it seems like the camera is anticipating something when the flash is in horizontal that the Fong hinders it from delivering.

I wondered if the camera was anticipating fill flash, and the diffuse light from the Fong messes this up--but it baffles me what's up.

Anyway, the solution I came up with was to change the ETTL II flash metering from evaluative to average (I think that the ETTL II ambient metering remains the same). Doing this solves the issue totally.

However, I don't always like average metering, and like the evaluative metering, so I don't know if this is going to cause issues for flash metering in the future (I wonder if it messes up the camera's anticipation of situaitons needing fill flash). At least it's a temporary solution.

Can anyone give any idea what's happening? It's rather frustrating, and I don't remember hearing about it in anything I've read on using diffusers with ETTL.

Thanks.

06/19/2008 10:26:40 AM · #2

Has no one come across problems with exposure when using a lightsphere?

(it's only evident with the flash in horizontal position, and at focal lengths greater than about 40mm)

gloda

Canon EOS-5D Mark II

06/19/2008 12:39:04 PM · #3

Two ideas:
Pointing the Lightsphere straight at you subject you are shooting through the thick diffuser material. Whilst GF demonstrates that technique in his tutorial video, it comes with a significant loss of light. Most of it is lost through the sides instead of hitting your subject, and about 1 stop of what hits the dome is lost because of the diffusion material. Which means that your flash will struggle to provide enough light to balance properly with ambient light - even full power may just not be enough because most of it is lost.
However you say that the problem goes away when the flash head is not locked in the standard positon. This is the only position in which the ETTL-II system can make use of the distance information provided by some lenses in order to determine how much light will be needed, not anticipating the loss of light from the Lightsphere. See here. Your 24-105 provides distance information, the 28-90 and the 50 1.4 do not. The Sigma, I don't know. So, try using the flash as you described on a static scene and try if using different lenses stopped down to the same aperture makes a difference to the way your camera meters for flash.
Do check out the link above, it offers an incredible amount of information on the ways Canon speedlites work.

dwterry

Canon EOS-1D Mark III

06/19/2008 01:16:56 PM · #4

Soft light is the result of a large light surface, not the result of a diffused light source.

If there's no chance for bounce flash ..... then what's the point of using the lightsphere? All it really does is send light in all directions so that it bounces off of everything. That's what makes the light soft bouncing off walls, ceilings, floors, etc. If there's no bounce, then all that the lightsphere has really done is throw away your light and increased your recycle times.

(I'll temper my comments just a little in that the lightsphere is a least "marginally larger" than the flash head without the lightsphere attached, which does, in fact, make the light hitting your subject just slightly softer ... but only marginally so ... it really needs something to bounce off of to make it truly a soft light source)

06/19/2008 01:57:50 PM · #5

Bingo gloda!!! Thanks a bunch. It was the lens (changing it removes the problem). However, I intend to use the 24-105, but I guess I'll just have to keep the metering on average.

Do you know what I might lose by putting it on average (flash metering average, not ambient metering average)?

dwterry, thanks for your comments. I had the impression that the front panel of the lightsphere only reduced light by 1 stop. I just checked it, and it looks more like 3 2/3 stops! However, this still gives me enough reach on iso200 for those difficult times when bounce flash is not a possibility (this is the only time I intend to use is--the light just seems that less bit harsh than with bare flash).

06/19/2008 02:14:53 PM · #6

Amazing. I've tried it again, and the problem only occurs when I step closer than about 12 feet. Like, I step forward and it underexposes, and then I step backwards further than 12 feet, and it's perfectly exposed.
Any suggestions of any other workarounds?

gloda

Canon EOS-5D Mark II

06/19/2008 02:50:18 PM · #7

That sounds as if ETTL-II only made use of the distance information within a certain range. Presumably distance data isn't reliable enough beyond a certain distance and the metering system then no longer takes the distance measurement into account. For details on how the different metering systems work the Photonotes article explains it better than I could.
Picking up on dwterry's advice - the Lightsphere really isn't a good way to increase the size of the flash because, as you noticed, a lot of light is lost. Its most useful when used indoors where the light can bounce off walls and ceilings. If you want an attachment you can use to soften the light thrown forward, check out //www.abetterbouncecard.com or Chuck Gardner's DIY diffuser.

On a sidenote, dwterry, soft light is actually a result of both a light source with a relatively large size (relative to its proximity to the lit subject, i.e. its effective size) and the diffusion which scatters the angles at which the light hits the subject.

Message edited by author 2008-06-19 14:51:06.

06/19/2008 03:03:00 PM · #8

Interesting thing is that it's an instant change under a range of 12 feet. And the change is applied in a constant manner (once I'm closer than 12 feet, it underexposes by around 1 1/3 stops nomatter whether I'm 10 feet away or 3 feet away). The distance data then seems to trigger a 1 1/3 reduction (there's no gradation in it at all).

I know what you mean about the lightsphere being an inefficient way of gaining a diffuser, but I can't think of a better way right now. Eg. following a wedding party outside a building at night. I can use bounce flash with the lighsphere when I have walls or ceilings, but step outside the door and I'm not going to lose time by fiddling with changing flash attachments (aside from popping in the lid). I do think the relative improvement it gives in skin reflectiveness is worth losing even close to 4 stops light (!). But if you can come up with a better solution, I'm all ears.

Message edited by author 2008-06-19 15:10:12.

dwterry

Canon EOS-1D Mark III

06/19/2008 03:18:18 PM · #9

Originally posted by gloda:

On a sidenote, dwterry, soft light is actually a result of both a light source with a relatively large size (relative to its proximity to the lit subject, i.e. its effective size) and the diffusion which scatters the angles at which the light hits the subject.

I agree totally with the first part. But less so on the diffusion part.

The diffusion only helps if the scattered light is actually hitting something else and bouncing towards your subject. Go outside at night with no walls or trees or anything to bounce the light, use a bare flash to light your subject and take a shot. Repeat the same exact setup but with some sort of diffusion material in front of the flash head (the same size as the flash so we're not changing anything) and shoot your subject. The subject will be harshly lit in both cases ... it'll just take more power to light the subject with the diffuser on.

Outside with nothing to bounce against, you're just throwing light away with a diffuser.

Edit to add:
I said "less so" (i.e. I'm not disagreeing completely). Diffusion changes the angles of the light hitting the subject. But a flash head is so small ... the angles from one side of the flash head to the other aren't that great to begin with. It really doesn't make that much difference. The lightsphere at least makes the source of light a little larger than the flash head. So you do gain something from it. Just not a lot.

Message edited by author 2008-06-19 15:24:12.

gloda

Canon EOS-5D Mark II