In need of major flash explaination

[cowman345]

Ok, I've got my 420EX and i'm learning about controlling flash lighting. I've got a couple questions.

Here's a scenario. I'm shooting a portrait with a brightly lit background (sunlight) and my subject in the shade. If I set exposure manually to the background, is fill-flash auto-balanced to fill in the subject appropriately? (goal is perfectly exposed background and perfectly exposed subject).

Second scenario: same deal, but I want to darken the background. If I were using a manual flash head, I'd take a reading from the flash (making sure it's brighter than the sun in the background) and expose for that. The background should automatically come out darker. But without having precise control over the flash (E-TTL only, right?) how can I acheive this?

-dave-

[RichardtheSane]

My flash experience is limited, but it is all with the 550ex so the principles are the same on the 420.

Scenario one. Short answer - yes. If you expose for the background and focus on the foreground then both should be well exposed in ettl - just make sure the foreground is covered by the active focuspoint as ettl will expose for that. In my experience Av mode works best for this.

Scenario two
Havn't a clue I'm afraid, I thing we need a more experienced flasher to attempt this one ;)

[robertwgross]

First of all, it might be helpful if you identify what camera you are using. I'm always amazed when some of the photographers here purchase a really good camera with all sorts of really intelligent features, and then they switch off the automation and set things manually, either right or wrong.

Second, if you set exposure manually, then you are on your own. You must depend on an external meter, or you must read an internal meter and then decide the right thing to do, and hope you decide correctly.

If you use a Canon camera with E-TTL and flash unit set to E-TTL, then it reads the background and foreground and makes the right decisions for you. I've found it to be accurate at least 90-95% of the time, which is better than I can guess.

---Bob Gross---

[fredlord]

Okay, just a relatively quick point. I shot architectural interiors for years using professional strobes which entailed balancing outside light coming through windows but the principles are the same. When you're mixing ambient and strobe (flash) sources of light the general rule is this: the shutter speed is your control for the ambient light, the aperture is your control for the flash. With the SLR, you're limited in that there is an upper limit to flash synchronization because of the focal plane shutter. My view cameras did not have a problem with high shutter speed synchronization. Some of the high end Canon SLRs will correctly synchronize all the way to 1/500 second. My D60 says 1/200 is the upper limit. This is a general restriction and if the flash pulse is extremely short you can get away with higher shutter speeds. This takes some experimentation to get right but is very rewarding.

For example: If you want the ambient (sunlit) light areas darker you must use a shorter shutter speed without changing the aperture. This will not affect the flashlit areas. If you want the flashlit areas darker you must close down the aperture. This will affect the ambient lit areas so you must slow the shutter speed accordingly so as to make up for the smaller aperture. The flash is only affected by the aperture. The ambient exposure is affected by both the aperture and the shutter speed. This takes some getting used to so don't expect to understand it immediately.

Good luck and best regards,
Fred Lord

[daveh]

robertwgross wrote:
I'm always amazed when some of the photographers here purchase a really good camera with all sorts of really intelligent features, and then they switch off the automation and set things manually, either right or wrong.

I resemble that remark ;) But then through-out most of my time in photography a "good camera" was by definition simple minded. (i.e. Canon F1) I'm trying to get used to all this auto-stuff but most of my 10D shots have been in manual so far. In fact manual mode was one of the main reasons for upgrading from my last digicam.

Back to the original questions:

1) Should work.

2) If the background is well separated from the foreground (in the eyes of E-TTL) then you should just meter (in manual) for an underexposed background and let the flash fill the foreground.

So far I've mostly used my internal flash or 550 with studio flashes. If the E-TTL flash points at something that is also being lit by a studio flash, then I typically flashmeter, set the camera 1 stop smaller, then also set E-TTL to -1 stop and everything seems to work out. (Because E-TTL is doing it's metering before the studio flash fires.)

[cowman345]

Running a quick experiment at my desk at work with 10d, 420ex, and Sekonic L-358 Flashmaster.

I read the ambient light: iso 400, f2.8, 1/30sec.
Take a photo without flash at these settings, exposure is dead-on.

Take a second photo with E-TTL flash, manually set to SAME settings as previous photo. (theoretically, shouldn't need any flash at all since it's already perfectly exposed at these settings, is my understanding correct?) Took a light reading of the flash, and it's coming out reading a much smaller aperture (like 16 or something). This just doesn't make sense to me.

If exposure is manually set with a light meter, why would the camera add any fill flash at all (assuming it's own light meter is reading a similar result as the L-358)? Wouldn't adding flash to a manually set perfect exposure OVER-expose the photo?

-dave-

[rdenney]

cowman345 wrote:

If exposure is manually set with a light meter, why would the camera add any fill flash at all (assuming it's own light meter is reading a similar result as the L-358)? Wouldn't adding flash to a manually set perfect exposure OVER-expose the photo?

-dave-


I learned this one the hard way, too, with my Sekonic L-718.

The Canon EX flashes use a constant-output, low-power preflash to measure the scene before the mirror swings up. This preflash triggers your flash meter, so all you are reading is the preflash. You can use a flash meter with manual mode because it doesn't use a preflash. But in E-TTL, you can't use a flash meter.

Rick "been there, done that" Denney

[cowman345]

AHHHH I understand! Thanks for clearing that up... I was really banging my head against my desk trying to figure that out. Thought I had a bum meter or something.

-dave-