ISO and "the Flash"

[DonSchap]

I was doing some flash work, last night, and was using Manual Settings on the camera to adjust the pictures.

Okay, BEFORE everyone rolls their eyes and says, "Oh my God, not MANUAL!"

Yes ... if you want control ... there is nothing like a little MANUAL camera persuasion. But, I think I'm fighting an "automatic" problem between the flash and the camera.

Scenario:

First (MASTER) 580EX atop the Canon EOS 20D
Second (SLAVE) 580EX on a tripod footed monopod, off to the right of the subject.

So, there I am with an 85mm f/1.8 lens ... got my aperture (f/2.8-3.2) set for the depth of field I am looking for ... speed set to sync (1/160 sec.) with the elex flashes.

Foosh! - Hmmm, looks a little too bright. ISO=400. I adjust down ...

Foosh! - Hmmm (<- Don's evaluation speak). ISO=200 Looks really about the same. I adjust down.

Foosh! - "Uh, this sucks." ISO=100 And the darn image is definitely not looking like two full stops down, in my estimation.

(I cannot even imagine fighting this with film, as the ISO is set for the entire roll. There is no between-shots adjustment, without removing it and slipping another roll in)

I believe the E-TTL components are "automagically" adjusting for my camera-only manual adjustments to the exposure. In other words, as I ramp down the ISO ... the flashes are cranking up their intensity. Anyone else noticing this kind of "automated" response when using Canon Strobes?. Should I set the master and slave flashes also to "M mode" (manual flash) to force them to "lock" at full power (or some lesser setting - that I manually input) and not have them "float"?

I also could throw the ST-E2 Speedlight Transmitter at the situation and ratio the two flashes to get what I want, but I would like a little more conclusive answer as to why the shot appears as though the ISO change is being ignored ... or worse yet, adjusted for. :eek:

I suppose I could meter at the subject and see what is happening with intensity, but because this is such a snappy and quick bunch of folks ... perhaps someone else could shed some intellligent light on this situation. I'm still learning the flash ropes, this week. :rolleyes:

[GaryS]

I've run into the exact same thing you are seeing Don. The ETTL flashes compensate for what they decide are bad choices on your part....

You have two choices. 1 - Use Flash exposure compensation to force the flashes to lower their power (not sure how this will affect a 580 set as slave).

2 - As you suggested, put the flashes in M mode, and set them yourself.

I love having the option of using flashes, but they make the process SO much more complicated.

[DonSchap]

Thanks for confirming my suspicions, Dusty.

I was considering bringing in the big guns - the twin 750W strobes, as they have knobs on the side that control intensity and go off without E-TTL information, but I wanted to get a handle on this "automagic" nonsense, because it really plays hell with the creative aspect of this effort.

I guess what I am saying is that it is YOU that needs to manage your light, not the darn camera. We're not all lighting invalids. LOL :eek:

[Spookonthe8ball]

Don, You may have convinced me to stay with my old slr flash. I was contemplating getting a new 580EX, but if it works better with manual settings, that is what I do anyway. I grew up on manual flash settings with my film slr's and it seems to work fine with the digital format. My 25 year old flash has auto settings for iso and such, but I have always used the manual setup on the camera. If I used auto settings I'd be starring at the camera like a cow looking at a new gate:)
Spook

[erichlund]

Don,

I'm not sure I can specifically answer your question, because I don't know if I have all the facts. I'm presuming you were changing the ISO on the camera, but since the flash units were in an automatic mode, I would expect they would simply compensate for the difference in sensitivity. In other words, when you told the camera to change ISO from 400 to 200, the flash was told also, so it simply used more power for the now less sensitive film.

Instead of changing the sensitivity, if you had set flash compensation, that should have fixed your problem directly. Unfortunately, I'm not familiar with the Canon flash system (my old A-1's flash was purely manual), so I don't know how to do this.

On my Nikon, I'd do it like this:
- Set the camera to manual and set exposure for the background. Note: If you want a fully exposed background, you probably want more than 1/160, that's just the max sync speed for your flash (I presume). So, set your depth of field with aperture and adjust your shutter to get the desired background exposure (center for fully exposed, left for a little darker).
- Turn the flashes on and set for TTL (ie. turn BL (balanced) off)
I don't have 2, and I don't play much with off camera flash, so I won't
go into how I'd set up commander mode, but that's not really relevant
here.
- Assuming flash as the main light source (not fill) set EV on each flash unit between 0 and -1. (Making sure the camera's flash EV is 0, they're cumulative with the flash unit EV on Nikon cameras)
- Take the shot. If not perfect, readjust the EV on the flashes and reshoot.

[noyjimi]

manual flash or fec, as the others said, schapman!

you could also move the light source away, hehe

[pagnamenta]

Don, I've noticed this too while playing around with slave flash. Best way to compensate for this in ETTL is use the FEC. I've switched to using manual flash because of the constant light I work in.

Switching your ISO won't get the exposure right. The system thinks you want to shoot at ISO 100, not that you are trying to under expose by two stops. I find that when using the flash in ETTL, I'm usually at -1/3 to -1 FEC, otherwise I get some blow outs.

[DonSchap]

Okay ... so I repositioned the flash to either side ... well, not quite ... more like 30 degrees off center of the subject, on the left ... and 70 degrees to right.

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I also used a large sheild as the backdrop, instead of a curtain ... so the subject got a reflective bounce from the rear, also.


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Canon EOS 20D w/ TAMRON AF18-200mm f/3.5-5.6 XR Di-II LD Aspherical (IF) lens
EXIF: 91mm - f/5.6 - 1/60 sec - ISO-400 - E-TTL w/ST-E2 controlling 2x 580EX (slave)


I suspect a lot of you guys don't go in for non-animated model posing, but the fact remains, lit properly, these diaramas can be really dramatic ... and non-complaining shots. "Oh Mr. Schap, can I move, now?" or my very favorite question, when I've spent an hour setting everything up and only got off three shots ... "Are we done, yet?"

Look, I know I'm pretty annoyed with people failing to understand the complexities of setting these shots up, but once you get that light looking great ... the shot is definitely worth it. Obviously, anyone can blow a shot. That takes nothing special. In fact, the above shot suffers from "blow out" on the subject's left arm. You can kind of get away with it ... due to the subject matter, but still ... reducing the light on the right, just a bit, would have resulted in a more life-like appearance. Also, touching up the flesh with "Film Grain" allows it to look a little more "porous" and not so reflective.

Well, here:

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Yeah, it's subtle stuff ... but it does improve the composition to my eye.


"Here, let me adjust your lens a little ... you'll see."

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[drama]

Very informative discussion, cant wait to try out the tips suggested
Thanks :)

[DonSchap]

I have to admit, it does get a bit complicated when you allow the camera to contol the shot ... not knowing how far the control will go or whether this flash will react will that flash ... or what have you. Creatively ... forget about it. The camera isn't built to do that ... people are.

If you want a creatively lit shot ... it's all MANUAL ... from flash to shutter. Trial and error and throw in a light meter, too ... just like the good ol' days ... before IS. Before ... the Canon Empire.