slaved flashes

[Ray Heath]

G'day all,

I have a puzzling problem in regard to synching an external flash to the built in flash on my Canon 400D DSLR.

When I set up a remote flash with a simple slave device, the in built flash will fire the remote unit but the camera will not record the light produced by the remote unit.

However, when I mount a small flash unit on the camera's hot shoe that flash will also fire the remote but now light from the remote is recorded.

This makes no sense to me.

My flash units are all simple, non-dedicated types and in fact this problem also occurs with my Elinchrom studio lights.

This problem of not recording the remote flash even occurs when I set a very long shutter speed.

Any ideas?

 

[Grinsha]

Your camera probably fires the onboard flash multiple times.

 

[Ray Heath]

Your camera probably fires the onboard flash multiple times.

Hi Grinsha,

Thnx for the reply, but no, that is not the problem.

 

[Grinsha]

What are you using for a slave?

 

[Worker 11811]

I'd say it has to be shutter sync. Either the exposure has already completed before the flash fires or the flash is firing before the exposure even begins.

If the camera's built-in flash is operating, the camera will automatically sync the shutter to the flash. Depending on exposure conditions, the shutter speed could be anywhere from 1/200 or slower.

If the camera is operating on a flash mounted on the shoe it might not synchronize.

User manual page 71:

The flash exposure will be set automatically to match the aperture that was set. The shutter speed will set automatically between 30 sec. - 1/ 200 sec. to suit the scene’s brightness. In low light, the main subject is exposed with the automatic flash, and the background is exposed with a slow shutter speed set automatically. Both the subject and background look properly exposed (automatic slow-speed flash sync).

Page 152 also states:

Non-Canon Speedlites:

The camera can synchronize with compact, non-Canon flash units at 1/ 200 sec. or slower. Be sure to test the flash unit beforehand to make sure it synchronizes properly with the camera.

If the camera is used with a flash unit or flash accessory dedicated to another camera brand, the camera may not operate properly and malfunction may result.

Also check page 104 for instructions to manually set shutter sync to 1/200 sec.

 

[themichael]

Are you set to use rear curtain sync? If you are then the slaves will fire after the shutter has closed.

 

[Worker 11811]

This camera is a digital SLR. It doesn't really have a shutter, per se. When talking about a digital camera, we usually use the word "shutter" out of convenience instead of a term like "exposure period" or similar. (I think you already know this. I'm just restating so we are all on the same frequency, so to speak.)

Anyhow, what is happening with this camera is that the exposure begins by firing the flash. While the "shutter" is still open, the camera takes a "normal" exposure to fill in the background as well as it knows how. The reason why the slaved flash isn't appearing is because, by the time the flash fires, the exposure is already done. (Or it hasn't begun yet.) In any case, the flash just isn't synchronizing to the camera.

The problem occurs because Canon uses some proprietary system to fire the flash in sync with the camera which may or may not be compatible with equipment from other manufacturers. This is specifically stated in the manual. The quote from page 152 says so:

If the camera is used with a flash unit or flash accessory dedicated to another camera brand, the camera may not operate properly and malfunction may result.

As stated elsewhere in the manual, it might be possible to solve the problem by changing camera settings or putting it in a different mode. Try using it in shutter priority mode instead of aperture priority mode. If that doesn't work, try full-manual mode.

If those methods don't work, check your manual for a method to change the shutter sync mode. On pages 101 through 106 there is a section which details how to change some of the camera's "custom functions." Specifically, on page 104, there is a section entitled, "Fn-3 Flash sync. speed in Av (Aperture-priority AE) mode" which can put the camera into a fixed 1/200 sec. shutter speed mode. Secondarily, there is another section on page 106, entitled, "Fn-9 Shutter curtain sync." which will allow you to change the shutter sync to "1st curtain" or "2nd curtain" sync. (i.e.: The flash fires immediately after the shot is exposed or it fires immediately before the shot is exposed.)

If you don't have a manual for this camera you can download it from Canon: www.canonusa.com...

Proprietary systems which are incompatible with other commonly available systems really peeve me off!

 

[pschwart]

This camera is a digital SLR. It doesn't really have a shutter, per se. When talking about a digital camera, we usually use the word "shutter" out of convenience instead of a term like "exposure period" or similar.

Just FYI, according to Canon the 400D has an electronically-controlled focal-plane shutter

http://www.canon.co.uk/For_Home/Product_Finder/Cameras/Digital_SLR/eos_400d/index.asp?specs=1

 

[Ray Heath]

Thnx guys,
All great responses, but all issues i've considered.

Mr Schwart is correct. The camera does have a "real" shutter, it can be seen when the mirror lock up function is used.

Slaves, my slaves are very simple devices that work with any flash unit. As far as I know they have no timing or delay issues.

Dedicated/non-dedicated, my original posting reads, "My flash units are all simple, non-dedicated types and in fact this problem also occurs with my Elinchrom studio lights."

Shutter speed/sync etc, my original post reads, "This problem of not recording the remote flash even occurs when I set a very long shutter speed."

So;
This problem does not occur with film cameras.

It does not occur when a flash unit (non-dedicated) is mounted on the hot shoe of the DSLR.

It is not a sync or 1st curtain/2nd curtain problem.

There must be something fundamenatlly different in a digital camera in regards to timing/synching when using inbuilt as opposed to hot shoe mounted flash.

Is it what I call a "digital fudge". You know, like how some newer model digital cameras have an ISO selection of "Hi" after the normally highest setting such as 1600. Why "Hi", it is 3200 or it's not. Why call it high?

Comments/suggestions?

 

[Worker 11811]

The digital shutter thing was my mistake. Where I work, I use a camera similar to this but a couple of rungs down the ladder. I don't believe my camera has a shutter. It doesn't have mirror lock-up or depth of field preview. Your camera has these things. I guess that's what happens when I skim the manuals too quickly.

I don't think there is anything else I can suggest that you haven't already eliminated. I suppose we can brainstorm the problem.

I know this camera uses its flash in a strange way. It opens the shutter (or begins the exposure) and fires the flash then makes additional exposure with the shutter still open so that the background is properly exposed.

What would happen if you took some test pictures with the camera pointed directly at the remote flash? (Turn the strobe slightly to the side so you don't take a direct hit in the face of the camera.)

Do you have anything that is phosphorescent which would glow for a brief period of time after the light hits it? What I'm going after here is to see if the slave flash is firing early or late. If the flash has already gone off before the exposure begins the phosphorescent object will still be glowing in the photo. If the flash is firing late, the phosphorescent object won't be glowing or at least not as bright.

Just trying to puzzle it out...

 

[SilverGlow]

This camera is a digital SLR. It doesn't really have a shutter, per se. When talking about a digital camera, we usually use the word "shutter" out of convenience instead of a term like "exposure period" or similar. (I think you already know this. I'm just restating so we are all on the same frequency, so to speak.)

Anyhow, what is happening with this camera is that the exposure begins by firing the flash. While the "shutter" is still open, the camera takes a "normal" exposure to fill in the background as well as it knows how. The reason why the slaved flash isn't appearing is because, by the time the flash fires, the exposure is already done. (Or it hasn't begun yet.) In any case, the flash just isn't synchronizing to the camera.

The problem occurs because Canon uses some proprietary system to fire the flash in sync with the camera which may or may not be compatible with equipment from other manufacturers. This is specifically stated in the manual. The quote from page 152 says so:



As stated elsewhere in the manual, it might be possible to solve the problem by changing camera settings or putting it in a different mode. Try using it in shutter priority mode instead of aperture priority mode. If that doesn't work, try full-manual mode.

If those methods don't work, check your manual for a method to change the shutter sync mode. On pages 101 through 106 there is a section which details how to change some of the camera's "custom functions." Specifically, on page 104, there is a section entitled, "Fn-3 Flash sync. speed in Av (Aperture-priority AE) mode" which can put the camera into a fixed 1/200 sec. shutter speed mode. Secondarily, there is another section on page 106, entitled, "Fn-9 Shutter curtain sync." which will allow you to change the shutter sync to "1st curtain" or "2nd curtain" sync. (i.e.: The flash fires immediately after the shot is exposed or it fires immediately before the shot is exposed.)

If you don't have a manual for this camera you can download it from Canon: www.canonusa.com...

Proprietary systems which are incompatible with other commonly available systems really peeve me off!

Worker! Stop with all the bogus BS....sorry bud, but you're spreading a lot of mistruths.

That 40D DSLR does IN FACT have a real genuine shutter, and of the exact same design as a film SLR. In fact ALL Canon DSLR's have a real film camera like shutter.

The issue seems to be that the EOS will first send out an initial flash to measure exposure, and this initial flash fires the slaves, so when the "real" flash happens next, the slaves ignore that "real" flash. During the pre-flash the picture is NOT taken. Yet.

Canon uses a pre-flash to calculate flash exposure, and that pre flash can play hell with non-Canon slaves.

 

[Ray Heath]

G'day all,

Silver, as pointed out by me earlier my DSLR, a Canon 400D, does have a "real" shutter.

Your post above re: "The issue seems to be that the EOS will first send out an initial flash to measure exposure, and this initial flash fires the slaves, so when the "real" flash happens next, the slaves ignore that "real" flash. During the pre-flash the picture is NOT taken. Yet." is not correct. My camera does not do this.

Modern cameras do a pre-flash when they can't "see" to auto-focus. They do not need to pre-flash to set flash exposure. Why would they, if they can focus they know the distance to subject, they know the flash strength, that's all that is required to calculate exposure.

So, no, the problem is not pre-flash.

 

[pellicle]

ray

why not use one of the cheapie wireless flash sync guys?

I use them and paid about AU$15 for mine on eBay

 

[pellicle]

G'day all,
They do not need to pre-flash to set flash exposure.

unfortunatly this is wrong, they do preflash about 60 milliseconds before the main flash. The sensor can not be used for "off the film" exposure determination during the exposure like film can.

Some helpful reading on wikipedia.

E-TTL (Evaluative-Through The Lens) is a Canon EOS flash exposure system that uses a brief pre-flash before the main flash in order to obtain a more correct exposure.

so as to:

Why would they, if they can focus they know the distance to subject, they know the flash strength, that's all that is required to calculate exposure.

well, that's a really good question and I've asked if for decades starting back on rec.photo

firstly its false that you can calculate flash exposure reliably only knowing distance. the GN formula itself refers to a typical reflectivity room. You know, walls ceiling... even with direct flash (at the speed of light) stuff spills back onto the subject from all directions from your flash ... try this outdoors at night and you'll notice that manually set flash is underexposed when compared to indoor in a typical living room.

also it seems that some patents prevent this was the only decent answer I could get back then as to why its not a main driver

But my ancient EOS 630 knows based on focus when the subject is out of flash coupling range even before I take the shot as soon as its completed AF I get a warning. So the camera must have focus distance. This feature was removed from subsequent EOS cameras apparently due to a complaint from Metz who owned the patent.

I know that Nikon at least have Distance information available in their lenses but not all of them. This information if available can be used by the system to assist calculations of flash in chosen situations (by the camera I presume)

of course none of this helps when the flash is off camera as the flash to subject distance would not be know ...

So, no, the problem is not pre-flash.

it may still be.

 

[Ray Heath]

thnx Chris,

I could use other devices to trigger remotely, but I'd rather understand the problem first.

Re your second post, I still disagree, I'll conduct some experiments.

 

[pellicle]

Hi

thnx Chris,

I could use other devices to trigger remotely, but I'd rather understand the problem first.

Re your second post, I still disagree, I'll conduct some experiments.

disagreement is the basis by which all science and engineering has advanced, by all means test and see.

but if you are using optical slaves then if the theory fits the situation perhaps its right

let me know how you go

ps ... to assist your experiement, assuming you're triggering off the built in flash perhaps you could disable the internal flash and use a PC sync cable to trigger a simple flash by your camera ... or on the hot shoe if you have a flash which can be set to manual (not using the E-TTL 2 system) I used to set my 430-EZ series flash on my EOS to manual 1/16th to trigger my strobes back when I was doing studio product stuff (as I didn't want the light from it on camera anyway)

 

[Ray Heath]

i've actually already done all this Chris, re-read my previous

consider:
the camera does not pre-flash - i can see that it doesn't

the camera's built in flash will fire a remote optically triggered flash, that is not in anyway dedicated to anything, but the light from that flash is not recorded

when i mount a simple non-dedicated flash in the camera's hotshoe that flash will trigger the remote and the light from the remote is recorded

the camera does have a "real" shutter - i can see it open if i fire the camera with no lens attached

first or second curtain sync makes no difference

a longer shutter speed makes no difference

it is perplexing and doesn't make any kind of sense or gel with any experience i've had in 25 years of photography

my next step is to retest all the above with the DSLR and then repeat the same sequence with a film SLR

thnx for your interest Chris

 

[pellicle]

Hi

i've actually already done all this Chris, re-read my previous

yes, I thought I read it (though may not pass an exam) my point is that the preflash is actually too close for your eye to detect ... you need equipment to test for it

 

[pellicle]

Ray

I thought I would go back to your first post:

I have a puzzling problem in regard to synching an external flash to the built in flash on my Canon 400D DSLR.

ok

When I set up a remote flash with a simple slave device, the in built flash will fire the remote unit but the camera will not record the light produced by the remote unit.

ok ....

However, when I mount a small flash unit on the camera's hot shoe that flash will also fire the remote but now light from the remote is recorded.

did you mean to write :
and now light from the remote is recorded

I expected a negative with the but ... you know

I put sugar in my tea but its not sweet

makes sence while:

I put sugar in my tea and its not sweet

has a different meaning to me at least

so to continue as if you wrote and

However, when I mount a small flash unit on the camera's hot shoe that flash will also fire the remote and now light from the remote is recorded.

This makes no sense to me.

well it does to me, assuming that you accept that the inbuilt flash is doing a preflash for metering (no, not evaluative, but the normal E-TTL preflash which can not be disabled as far as I know on the inbuilt flash)

this evidence seems to support my previous assertions.

My flash units are all simple, non-dedicated types and in fact this problem also occurs with my Elinchrom studio lights.

if the flash in the hot shoe is also non-dedicated then it fits with the fact that the inbuilt strobe is pre-flashing (triggering your strobes early thus not being recorded)

This problem of not recording the remote flash even occurs when I set a very long shutter speed.

Any ideas?

irrespective of any shutter speed you select the preflash happens before any curtain opens. If you have a 2 second shutter even if you set second curtain sync the preflash will have fired your strobes 50 milli seconds earlier than the first curtain

over to you

 

[SilverGlow]

Modern cameras do a pre-flash when they can't "see" to auto-focus. They do not need to pre-flash to set flash exposure. Why would they, if they can focus they know the distance to subject, they know the flash strength, that's all that is required to calculate exposure.

So, no, the problem is not pre-flash.

Ray, it seems you don't know how EOS flashing works.

ALL EOS cameras will issue a pre-flash to calculate exposure. Is it possible you confuse the ifra-red AF helper light with pre-flash? The EOS flashing system will pre-flash and it has NOTHING to do with auto-focus. The distance data used to calculate flash exposure is NOT enough...the distance does not tell the flash how bright the scene is. Using distance as part of flash exposure calculation is a recent component of exposure calculation; for Canon this was brought forward as a feature just a few years ago; I think with the advent of E-TTL2 or maybe later, I forget.

The pre-flash happens so fast that to the photographer and subjects, it seems like just one flash. I find it amusing when you write that since you "can't see it" it most not happen.

I still think your issues have something to do with the pre-flash, and again I'm not talking about the infra-red AF helper light.

I find it interesting that you come across like someone that knows how EOS flashing works, but in reality you don't even know the basics. Throttle back a bit and consider other posts and maybe you might learn something. Here we are trying to help you, but your arrogance (25 years in photography) attitude is holding you back from truly learning and that's sad.

 

[Ray Heath]

I'm sorry for any apparent arrogance SilverGlow, I at no time intended to infer that I am some kind of expert on this subject. I don't know how modern flash systems work, I freely acknowledge that fact.

I thank you, Chris and the other posters, you have helped me to better understand the concepts.

In regard to "seeing" the actual pre-flash I was thinking about how one can see the AF assist pre-flashes. So when I wrote that I could not see a pre-flash that was my reference.

How does the camera control flash exposure?

How can I better use slaves?

How can I do fill flash with the inbuilt flash?

 

[pellicle]

Good Morning Ray (well it is both here)

I thank you, Chris and the other posters, you have helped me to better understand the concepts.

your totally welcome .. happy to help.

In regard to "seeing" the actual pre-flash I was thinking about how one can see the AF assist pre-flashes. So when I wrote that I could not see a pre-flash that was my reference.

good question. I'm not sure, but as one with a background in physics (and chem) I would propose this experiment:

Have a darkened room and a second camera (digital would be ideal, quality irrelevant). Set the second camera on a tripod and set it for a shutter speed of something over 1 second (longer is easier to coordinate) and the widest aperture it has

now, you need something which moves, a record player is pretty ideal. Set it for 45 (or 78 if it has such) and put a single small white object on the platter.

now, with the camera in Manual Focus (so it won't emit much light for focus assist) take a shot of the turntable with the camera you are testing (a 40D right?) while the shutter is open on the other camera.

Because the flash duration is so fast (less than 1/5000th) if you see two images (they may be quite closely overlapping) you know you had a preflash. The faster the object is moving the more chance you have of seeing the separation, perhaps something like a fan with a mark (piece of masking tape ought to do it) on one blade would do well too

other than that you may need to resort to stuff like a CdS cell and a CRO to see what the peaks are if they are too close together. I would have this sort of thing lying around on my bench at home (always handy stuff a cro ... specially if youre a HAM radio person)

How does the camera control flash exposure?

it uses the sensors to measure the flash then allows the main exposure. I guess you have found this: Dead Link Removed

How can I better use slaves?

personally I prefer the RF wireless ones ... you can then trigger 4 or more flashes from the same unit. I've got a yumCha brand one from China I bought for a few bucks ... simmilar to this

How can I do fill flash with the inbuilt flash?

there should be a menu on the camera to control the flash, there was on my 10 and 20D. I could dial in up to -2EV compensation specific to the flash exposure (not the compensation on the top deck, that does all together) ... should be something in the manual for this.

 

[SilverGlow]

Here is your salvation, and as already posted, and I agree:

Dead Link Removed