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08-18-2010, 01:24 PM   #16
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QuoteOriginally posted by Kendrick Quote
It still would not explain why my Pentax Km will not synchronise with an external slave-flash triggered by the built-in flash even at exposure settings as long as 1/30 second. My old Fuji S5500 synchronised with the slave-flash at exposure times as short as 1/1000 second but of course that was not an SLR, only an SLR look-alike. When that was lost on a trip abroad I replaced it with the Pentax Km, not dreaming that it would not do the job just as well.
The reason you are having optical sync problems, I suspect, has nothing to do with shutter synchronization.

If you are using a standard (e.g., non-modern-Pentax) optical slave, you will have problems. Here's why:

The Pentax automatic flash exposure system, called P-TTL, uses a pre-exposure flash to determine the effect the flash has on exposure, followed by a second flash DURING the exposure that is properly calibrated (based on the pre-flash effects) to give the intended result.

The two flashes occur in pretty quick succession, so it's difficult to notice unless you are really looking for it.

A generic optical slave will trigger on the initial pre-exposure flash, and miss the exposure duration entirely.

You can avoid this, I think, by using ONLY manual flash settings to trigger generic optical slaves, thereby avoiding the P-TTL pre-flash entirely. I don't know how (or even if) you can do this using the built-in flash on the K-m, though.

09-05-2010, 07:51 AM   #17
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Re: Marc Sabatella's comments:
I wrote: "I feel sure that this is how it is done with my compact point-and-shoot digital. How would the continuous image be formed in the view-finder if the sensor is covered except during an "exposure".
Note that my rhetorical question about the continuously visible image in the viewfinder related to COMPACT digital cameras, not digital SLR's. I know that the viewfinder of an SLR gets its light from the mirror which also gets it its classification as a "reflex". This is obvious as soon as one removes the lens to substitute another. On the other hand, it is impossible for an ordinary user to tell whether, out of sight behind the mirror, is a conventional focal plane shutter or some electrical simulation of the effect of a shutter. Since the latter is obviously what is provided in a compact point-&-shoot digital camera it would seem surprising that more advanced equipment should use a more old fashioned and less efficient method with moving mechanical parts. However, no matter - I gather that this is what the manufacturers are in fact doing.
But no one has addressed my essential perplexity i.e.
". . . why my Pentax Km will not synchronise with an external slave-flash triggered by the built-in flash even at exposure settings as long as 1/30 second".
I DON'T want to use flash in synch with extremely short exposure times and mentioned the 1/1000 sec synch afforded by my old Fujifilm 5500 in reasoning that that camera simulated the effect of a shutter, and did not have a real shutter. The Fujifilm 5500 would synch well with a slave flash even at 1/1000. So why won't the Pentax km synch even at 1/30 (or, in fact at 1/15) with a slave-flash?
09-05-2010, 08:56 AM   #18
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QuoteOriginally posted by Kendrick Quote
So why won't the Pentax km synch even at 1/30 (or, in fact at 1/15) with a slave-flash?
I think you are refering to whether the built-in flash on K-m can be the "master" or "controller" to the external slave flash. I know my K100D does not support this feature, but my K10D/K-7 can.
09-05-2010, 11:05 AM   #19
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QuoteOriginally posted by Kendrick Quote
Note that my rhetorical question about the continuously visible image in the viewfinder related to COMPACT digital cameras, not digital SLR's.
Ah. Then presumably you meant "LCD" or perhaps "EVF" when you used the term "viewfinder". I assumed you were asking about DSLR's.

Anyhow, I'm not sure what specific problem you are seeing with syncing your slave flash at 1/30", but maybe your slave is getting confused by the pre-flash?


Last edited by Marc Sabatella; 09-05-2010 at 09:54 PM.
09-05-2010, 08:10 PM   #20
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QuoteOriginally posted by Kendrick Quote
". . . why my Pentax Km will not synchronise with an external slave-flash triggered by the built-in flash even at exposure settings as long as 1/30 second".
You need to read up on the Pentax p-TTL flash system. In short, the reason why your K-m (or any recent Pentax dslr, for that matter) won't sync to an optically triggered external slave flash, is that the camera's built-in flash goes off twice. The first flash is done before the mirror flips up. This allows the light sensors in the viewfinder to "read" the flash and calculate the necessary aperture.

After the first flash, the mirror flips up, the camera stops the lens down to the calculated aperture and the shutter opens to take the picture.

Unfortunately, your optical trigger saw the pre-flash and fired the slave while the mirror was still down. I've heard that there are some optical triggers that can ignore the pre-flash, but I've heard that they are not very reliable.

Try this. Set the shutter speed at 1 second. Set the camera for force it to use the flash, regardless of ambient light. If your K-m has the option of firing the flash on the trailing shutter, select that option. Trip the shutter. You should see the preflash, then, a little less than a second later, the main flash will go off.

Setting the camera in manual mode MAY turn off the preflash and fire the onboard flash at full power. However, the downside is that you will have to calculate the aperture yourself. Of course, if you're using off-camera flashes and they're not p-ttl flashes, you'd have to calculate it yourself.

BTW, Nikon and Canon dslrs do essentially the same thing. They have different names for the system. One is i-ttl and one is e-ttl. I can never keep straight which is which.

If you're interested in off-camera flash, check out The Strobist website at Strobist
09-06-2010, 12:51 PM   #21
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QuoteOriginally posted by aleonx3 Quote
I think you are refering to whether the built-in flash on K-m can be the "master" or "controller" to the external slave flash. I know my K100D does not support this feature, but my K10D/K-7 can.
The K-m onboard flash can be set as a controller, would that solve the pre-flash problem when using optical slaves? I will try to test it out tonight and see what happens.
09-07-2010, 07:34 AM   #22
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QuoteOriginally posted by veegish Quote
The K-m onboard flash can be set as a controller, would that solve the pre-flash problem when using optical slaves? I will try to test it out tonight and see what happens.
No that will not prevent the pre flash.

The best and cheapest way to trigger remote flashes is to get a cheap radio controlled set from ebay. You can than fire additional flashes with an optical slave which is triggered from the first radio controlled flash.
10-29-2010, 09:09 PM   #23
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Thanks, noblepa. This suggestion posted by yourself and others does seem to explain why the slave-flash does not synchronise. I guess the only way around this would be IF there is a way provided by the maker to turn off the pre-flash.

I had no idea that the flash-gun goes off twice. I thought the pre-flash was an infra-red flash issuing from somewhere else on the camera whether the flash-gun is popped up or not.

I wonder, does anyone know if the pre-flash can be turned off and how it can be done?


Last edited by Kendrick; 10-29-2010 at 09:11 PM. Reason: clerical error
10-29-2010, 10:44 PM   #24
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You can use a ND filter or flash with HSS.
10-30-2010, 01:46 PM   #25
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QuoteOriginally posted by Kendrick Quote
Thanks, noblepa. This suggestion posted by yourself and others does seem to explain why the slave-flash does not synchronise. I guess the only way around this would be IF there is a way provided by the maker to turn off the pre-flash.

I had no idea that the flash-gun goes off twice. I thought the pre-flash was an infra-red flash issuing from somewhere else on the camera whether the flash-gun is popped up or not.

I wonder, does anyone know if the pre-flash can be turned off and how it can be done?
If you are using an external hotshoe flash (like the Pentax 360 or 530), you can put the flash in MANUAL mode (rather than PTTL) and there will be no preflash. If you set the power really low (1/64), you might still be able to trip the slave flash and not have the hotshoe flash contribute (much) to the exposure.
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