580ex Flash - Please explain Preflash

Hi, Received my Speedlite 580 flash today. Read the manual and took quite a few shots to get use to the flash. Can somebody please explain to me what I am supposed to see when a preflash is fired before the main flash is fired?

The flash instruction manual reads as follows on page 11, under sub-section Taking the picture: -

“Check that the subject is within the effective range displayed on the LCD panel.

A preflash is fired right before the shot is taken, then the main flash is fired.

If a standard flash exposure was obtained, the flash exposure confirmation lamp will light for about 3 second”

Does the above mean I should see two flashes from my flash whenever I take my shot, i.e. the first being the preflash and then the second being the main flash?

Or, does the above “preflash” refer to the red AF assist beam firing first?

As described above, the subject is within the range displayed and after taking the shot I also get the green flash exposure lamp to come on for about 3 second. All my setting on the camera and the flash are as listed in the instruction manual. My camera is the 350D and my lens is a Sigma 28 – 300 DG.

Please explain.

The two flashes are so close together that unless you're looking really hard, you probably won't notice. They both happen when the shutter button is fully depressed.

One thing you've probably noticed is that in certain modes the shutter speed seems to be very slow, even though you have your flash switched on. I find it easiest to switch to M mode (on the camera) so that it is not trying to guess whether I want to use the flash as subtle foreground fill or as a main light source. I also tend to wind up the FEC by about +1 stop and bounce off the ceiling, which allows me to use a higher shutter speed and/or lower ISO.

Singh said:

Does the above mean I should see two flashes from my flash whenever
I take my shot, i.e. the first being the preflash and then the
second being the main flash?

Click to expand...

no, they are too close together. you see the preflash if you trigger it in advance by FEL (the * button).

Singh said:

Or, does the above “preflash” refer to the red AF assist beam
firing first?

Click to expand...

no. the preflash is emitted out of the main flash bulb, it's just a weak flash pulse used to meter the scene via (E)TTL mechanisms.

--

Nekdo je moral Josefa K. o'crniti, zakaj ne da bi bil storil kaj slabega, so ga nekega jutra prijeli.

Jurc said:

no, they are too close together. you see the preflash if you
trigger it in advance by FEL (the * button).

Click to expand...

My previous camera was a Canon G2, and I used to have to use FEL to prevent "closed eye syndrome" when taking pics.

Is this also a problem with the XT, or are the preflash and flash so close together that subjects can't react to the preflash?
Flash: Speedlite 420ex.

I find about 5% of my human subjects can react fast enough to the preflash.

--
Ray Chen
http://www.arrayphoto.com

like Ray said, some do react. i know this for a fact, because I am the one who reacts. if FEL is not used when shooting me (hopefully with a DSLR and not in a Cheney fashion), i usually close my eyes and look ridiculous.

Verne said:

Jurc said:

no, they are too close together. you see the preflash if you
trigger it in advance by FEL (the * button).

Click to expand...

My previous camera was a Canon G2, and I used to have to use FEL to
prevent "closed eye syndrome" when taking pics.
Is this also a problem with the XT, or are the preflash and flash
so close together that subjects can't react to the preflash?
Flash: Speedlite 420ex.

Click to expand...

--

Nekdo je moral Josefa K. o'crniti, zakaj ne da bi bil storil kaj slabega, so ga nekega jutra prijeli.

Hi, SIngh,

Singh said:

Does the above mean I should see two flashes from my flash whenever
I take my shot, i.e. the first being the preflash and then the
second being the main flash?

Click to expand...

There will be two flashes very close together, and it is very hard to discern them as distinct.

The first one (the "metering preflash") is used to detetrmine what the effect of the flash will be on the "main subject" so that the outpt level of the "real" burst can be set to give proper exposure.

Singh said:

Or, does the above “preflash” refer to the red AF assist beam
firing first?

Click to expand...

No, nothing to do with that.

Singh said:

As described above, the subject is within the range displayed and
after taking the shot I also get the green flash exposure lamp to
come on for about 3 second. All my setting on the camera and the
flash are as listed in the instruction manual. My camera is the
350D and my lens is a Sigma 28 – 300 DG.

Click to expand...

If the metering system finds that the necessary illumination to attain proper expsure cannot be attained with the maximum available output of the flash unit, the confirmation lamp will not light. But in your case, the lamp indicates that the metering system was able to have the flash unit do the necessary job.

Best regards,

Doug

Visit The Pumpkin, a library of my technical articles on photography, optics, and other topics:

http://doug.kerr.home.att.net/pumpkin

'Make everything as simple as possible, but no simpler.'

Verne said:

Jurc said:

no, they are too close together. you see the preflash if you
trigger it in advance by FEL (the * button).

Click to expand...

My previous camera was a Canon G2, and I used to have to use FEL to
prevent "closed eye syndrome" when taking pics.
Is this also a problem with the XT, or are the preflash and flash
so close together that subjects can't react to the preflash?

Click to expand...

On a DSLR and a speedlite, the pre-flash and main flash are much closer together than on something like a G2. Thus you'll see less people with closed eyes.

However, some might be still fast enough, to have the eyes at least partially closed.

All, Thanks for explaining preflash to me.

Only if the instruction had been a bit more clear about the preflash, i.e. it's hardly noticeable, then I would not have been confused.

Is there any reason NOT to leave the high-speed sync on? It seems to me that if you're under the 1/200 native sync, everything'll be fine and dandy, and if you go above it, you just get a weaker flash that the flash unit might be able to compensate anyways for.

sometimes you want to prevent high-speed sync and the reduced range - like in situations when adjusting (opening) aperture is a better choice. disabling high-speed sync will 'force' you (better: inform you) to do so.

WonJohnSoup said:

Is there any reason NOT to leave the high-speed sync on? It seems
to me that if you're under the 1/200 native sync, everything'll be
fine and dandy, and if you go above it, you just get a weaker flash
that the flash unit might be able to compensate anyways for.

Click to expand...

--

Nekdo je moral Josefa K. o'crniti, zakaj ne da bi bil storil kaj slabega, so ga nekega jutra prijeli.

Hi Singh,

I got this link from another thread. I don't remember who posted it, but once I read it, I had a better understanding of how flash works.

http://photonotes.org/articles/eos-flash/

Hope this helps.

Thanks.
--
Peter -
Canon Digital Rebel XT
Canon 28-90 f/4-5.6
Canon 50mm f/1.8
Sigma 28-70 f/2.8-4
Sigma 70-300 f/4-5.6 DG Macro
Canon Speedlite 580 EX

Jurc said:

sometimes you want to prevent high-speed sync and the reduced range

• like in situations when adjusting (opening) aperture is a better

choice.

Click to expand...

Ahum...

When you're getting into high-speed range, then opening the aperture is certainly never the better choice. That will only put you deeper into high-speed...

In that situation, only closing the aperture will help, because that will lower the high shutter speed, back to below x-sync.

Because the alternative to high-speed flash is a more closed shutter, the disadvange of reduced range isn't so bad. Ofcourse high speed flash lowers the range... but so does closing the aperture!

High speed looses a bit more than the aperture, but it's not as much as a problem as most people make it to be. Remember that you're generally getting into this situation because you're using a wide open aperture. Meaning that you have more than enough power anyway.

Jurc said:

disabling high-speed sync will 'force' you (better: inform
you) to do so.

Click to expand...

The camera will not stop you from taking the shot. It will just overexpose the ambient. The only warning you get is a blinking shutter.

From my own experience, that blinking shutter isn't much of a warning...

The first couple of times I've shot with fill-flash in daylight I completely missed the blinking, and wondered why everthing got overexposed.

Anthony de Vries said:

When you're getting into high-speed range, then opening the
aperture is certainly never the better choice. That will only put
you deeper into high-speed...

Click to expand...

correct, missed that one completely.

Anthony de Vries said:

Anthony de Vries said:

disabling high-speed sync will 'force' you (better: inform
you) to do so.

Click to expand...

The camera will not stop you from taking the shot. It will just
overexpose the ambient. The only warning you get is a blinking
shutter.

Click to expand...

what i was talking about is that the camera will not let you go beyond 1/200 if FP is disabled, hence preventing you from going into FP territory and getting reduced range, regardless of how small the loss of range really is.

--

Nekdo je moral Josefa K. o'crniti, zakaj ne da bi bil storil kaj slabega, so ga nekega jutra prijeli.

Jurc said:

Anthony de Vries said:

When you're getting into high-speed range, then opening the
aperture is certainly never the better choice. That will only put
you deeper into high-speed...

Click to expand...

correct, missed that one completely.

Jurc said:

Jurc said:

disabling high-speed sync will 'force' you (better: inform
you) to do so.

Click to expand...

The camera will not stop you from taking the shot. It will just
overexpose the ambient. The only warning you get is a blinking
shutter.

Click to expand...

what i was talking about is that the camera will not let you go
beyond 1/200 if FP is disabled, hence preventing you from going
into FP territory and getting reduced range, regardless of how
small the loss of range really is.

Click to expand...

But it does so by overexposing the picture... It doesn't stop you from taking the picture.

And I really think that I'd rather have a chance of asking too much from the flash and get the fill-in flash a little underexposed, then have the certainty that the picture is completely overexposed.