Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-10-2008, 12:54 PM   #1
Inactive Account




Join Date: Sep 2007
Location: Belgium
Posts: 18
Problem: external slave flash too early ?

Hi all,

Any help on this is would be greatly appreciated.
I know it's a long post, but I think this puzzle is worth reading :-)

I'm experimenting with the K10D and an external flash I got from my dad.
The flash is a Canon Speedlite 380 EX, and when I mount it on the camera,
this works fine !
But now I also happen to have a slave flash trigger, which is a small cube
with a light sensor in the front and back, and a hot-shoe on top.
On the hot-shoe, I mounted the Speedlite.
Whenever the trigger "sees" a flash, it triggers the Speedlite.

Now here is the problem:
When I take a picture at F8, 1/60s with the built-in flash only, I have a perfectly exposed picture.
Now when I activate the Speedlite, and take the same picture, I can see the Speedlite reacting instantaneously to the built in flash. However, the picture is seriously UNDERexposed !
I don't get this at all, so here is my reasoning:

- The Speedlite goes off too late : not possible because then I would have the same result as
taking the picture without the Speedlite (with built-in flash only which would give a perfect result).
- The speedlite goes off too early : this must be the case, otherwise I can't explain the underexposere at all. But even in this case, why don't I get the same result as I get when not using the Speedlite at all ??
The only thing that I can think of, is that the camera somehow measures the incoming light before it takes the picture (and hence before the built-in flash fires), and that during this measuring-interval, the Speedlite goes off, causing the K10D to decrease the power of the built-in flash...
But then again, the Speedlite cannot fire before the built-in flash does, because it reacts to the light from the built-in flash ??
So then the final conclusion I can think of is that the Pentax actually has a double flash-action of some kind (man, this is getting really weird :-) and that the events happen as follows:

1. The built-in flash fires a first time to measure the light. At the same time, the Speedlite flashes, causing the K10D to think it's own flash is far too bright.
2. The shutter opens.
3. The built-in flash fires a second time, with reduced power.

Does this make ANY sense ?? :-)
If so, how do I solve this problem ?
If not, can anyone tell me how it really works ?

Thanks,

Ziggy

01-10-2008, 01:05 PM   #2
Veteran Member




Join Date: Mar 2007
Location: U.K.
Posts: 685
You have the 'pre-flash' of the K10D triggering the external slave flash trigger. So yes, there is a double flash like most digital cameras have these days. It's used to get the exposure more accurate. I don't have a K10D so I can't try this out for you, but it's possible that there is no pre-flash with the camera in M mode. Otherwise, you may be able to get a trigger that skips over the pre-flash, I know there are flashguns with this ability but they won't cost peanuts. Electronics types have made circuits for next to nothing that can do this 'first flash skipping' and I've seen the circuits on the internet a while ago.
01-10-2008, 01:22 PM   #3
and
Veteran Member




Join Date: Mar 2007
Photos: Gallery
Posts: 2,471
There are relatively inexpensive optical slaves that will not be triggered by the preflash, such as the wein peanut slave. I dont know if that will work with the ex though since i think it plugs into a sync port and no the hotshoe. Flashes like the old Nikon sb26 have a delay mode on their slaves that also wont be fooled by the preflash, I have two of those and they work fine.

afaik the only way to cut out the preflash is for example to use the aperture ring on a lens that has one, but till will cause the built in flash to fire at full power which can be annoying...
01-10-2008, 01:32 PM   #4
Inactive Account




Join Date: Sep 2007
Location: Belgium
Posts: 18
Original Poster
"but it's possible that there is no pre-flash with the camera in M mode"

Nope, I always shoot in M mode.
I also tried using the "trailing curtain sync" flash, but it has the same result.

But when you say, "to get the exposure more accurate", what do you mean ?
In M mode, the ISO, aperture and shutterspeed are fixed by me, so the only thing that is
left is reducing the power of the (actual) flash.
But then I completely don't understand why you can set the intensity of the flash manually ranging from -2 to +1 in 0.5 increments, if the camera has the ability to overrule this.
(Especially in M mode I hope that this isn't the case !)

In short, the camera shouldn't interfere at all when shooting in M mode, but then I don't understand why I get a different exposure in the two cases.
If it does interfere, I would consider this to be a major design flaw (imho) !

Z.

01-10-2008, 01:37 PM   #5
Inactive Account




Join Date: Sep 2007
Location: Belgium
Posts: 18
Original Poster
"the only way to cut out the preflash is for example to use the aperture ring on a lens that has one, but till will cause the built in flash to fire at full power which can be annoying..."

Hmm, I must be missing something... In manual mode, don't you WANT the built in flash to fire at full power (or at least, the power you have set in the flash menu, ranging from -2 to +1) ?
Why would the camera behave differently with a DA lens (without aperture ring) compared to a lens with aperture ring ? This is exactly what I don't want to happen in M mode !
Damn, this really seems to be a design flaw... :-(
01-10-2008, 01:38 PM   #6
and
Veteran Member




Join Date: Mar 2007
Photos: Gallery
Posts: 2,471
I think your original assumption makes sense. That is whats happening.

as for M mode and auto flash well thats how it is, the flash on the k10d does not have manual mode, so its automatic, so whatever you set the shutter, iso and aperture to, as long as you dont go over the max sync speed, the flash will fire the appropriate amount of light, essentally making it auto mode. to get full manual you need to use a manual flash, which your canon flash can do.

I dont think that is a design flaw, its just that the internal flash doesnt do manual. What would you expect it to do? the only alternative would be for it to fire at full power, but that is not really usefull. I would love some manual controls on it and that should NOT be hard to do but alas I dont know any camera maker that has it.
01-10-2008, 01:40 PM   #7
and
Veteran Member




Join Date: Mar 2007
Photos: Gallery
Posts: 2,471
QuoteOriginally posted by ZigDaPig Quote
"the only way to cut out the preflash is for example to use the aperture ring on a lens that has one, but till will cause the built in flash to fire at full power which can be annoying..."

Hmm, I must be missing something... In manual mode, don't you WANT the built in flash to fire at full power (or at least, the power you have set in the flash menu, ranging from -2 to +1) ?
Why would the camera behave differently with a DA lens (without aperture ring) compared to a lens with aperture ring ? This is exactly what I don't want to happen in M mode !
Damn, this really seems to be a design flaw... :-(
Its a P-TTL only flash and P-TTL is auto exposure so thats how it works... I wouldnt say the design of it is flawed, just complain that it doesnt also have Manual mode.

The -2 +1 are not power settings, just compensation setting, compensating the automatically calculated exposure. Manual settings would be in power steps 1/1, 1/2, 1/4 etc.
01-10-2008, 01:45 PM   #8
Inactive Account




Join Date: Sep 2007
Location: Belgium
Posts: 18
Original Poster
Hi Andreas,

Thanks already for the tip on the lenses with aperture rings !
I just tested it with a lens with aperture ring, and this indeed cuts out the preflash.
Now I get the result I wanted... but not with my DA lens !
Is there really now way to cut out this pre-flash behaviour ?
Isn't anyone else bothered by the pre-flash (more particularly, by the automatic adjustment of the second flash) in M mode ?

Z.

01-10-2008, 01:51 PM   #9
Inactive Account




Join Date: Sep 2007
Location: Belgium
Posts: 18
Original Poster
Ok, not being a DSLR expert, I just assumed that manual mode also implied manual (or unadapted) flash...
Thanks for clearing this out.

But when you say:
"The -2 +1 are not power settings, just compensation setting, compensating the automatically calculated exposure."
does this mean that the flash always fires at the same power, and the exposure is altered in a way that corresponds somewhat to changing ISO-values ?

Z.
01-10-2008, 02:18 PM   #10
and
Veteran Member




Join Date: Mar 2007
Photos: Gallery
Posts: 2,471
The flash does not always fire at the same power, it calculates what it assumes is the correct exposure.

If we assume that the flash system was perfect and never made a mistake then that value would be the correct exposure for a given combination of iso, aperture and subject distance. setting this to -1 would cause a 1 stop underexposure and +1 a 1 stop overexposure.

that they are compensations mean that you can alter an automatically calculated value, since that value changes all the time it is not a fixed power setting.

this works the same as exposure compensation in Av Tv etc modes, where the camera calculates exposure and you can alter that calculated exposure to make it darker or brighter by adding compensation.
01-10-2008, 02:21 PM   #11
Inactive Account




Join Date: Sep 2007
Location: Belgium
Posts: 18
Original Poster
Ok, thanks !
I learned a lot this past hour :-)

Cheers !

Z.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
built-in, camera, dslr, fires, flash, k10d, light, photography, picture, result, slave, speedlite, time
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
In what situation can you use the external flash that the built in flash couldnt? rustynail925 Flashes, Lighting, and Studio 25 01-13-2010 01:26 PM
For Sale - Sold: Flash + optical slave kibipod Sold Items 4 04-06-2009 05:33 AM
Slave flash setup Nesster Flashes, Lighting, and Studio 8 12-31-2008 08:22 PM
DIY slave flash problem gamgee Flashes, Lighting, and Studio 4 04-03-2008 09:56 AM
Slave Flash fun ronald_durst Post Your Photos! 4 11-09-2007 11:01 PM



All times are GMT -7. The time now is 12:29 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top