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10-01-2007, 08:18 AM   #1
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Flash question

K10D using 'Trailing Curtain Flash' and on-camera flash, the flash fires twice on long exposures. Say a 1 second exposure is set, the flash will fire at the beginning of the exposure and again before the shutter closes. How can I make it stop with the damn first flash? It completely defeats the purpose of trailing curtain.

10-01-2007, 08:21 AM   #2
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i think that if you do some experimentation you will find that it is a true trailing curtain synch. i dont know. i feel like you do occasionally, but i can still shoot with trailing curtain and get the effect i want....
10-01-2007, 08:28 AM   #3
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The first flash is just the p-ttl preflash and it does not effect the exposure, it is done before the picture is taken, normally the real flash follows right after, but since the camera is set to trailing sync, the flash does not fire untill near the end of the exposure (when the second shutter curtain starts to move) so the camera will wait for that.

if you had an external flash then you could set it to something other than p-ttl and there would only be 1 flash. pure ttl is also only 1 flash, but p-ttl is 2. but this is not affecting your exposure as mentioned. I guess you can test this if you want by letting the flash fire that first flash, and then before it fires at the end of the exposure, put something in front of the flash to block it so the light wont reach your subject and then see if any flash light is visible in the image.
10-01-2007, 08:40 AM   #4
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Can I turn off the P-TTL? I really would prefer one flash right before the shutter closes.

It seems I can not get the effect I'm looking for. I should be able to compensate for any blur across the subject with a good strong flash right before the shutter closes but it appears that the first flash is strong enough to expose the subject significantly so that I end up with a light subject profile, some blur, then the subject again. It should only be blur and the subject captured in the last flash.

and, I'll try your suggestion. Maybe it's just me but I swear I'm getting two subjects with the two flashes. Another possibility I guess could be my own fault. I could be pressing the shutter release while not in motion and that's creating the second 'first' subject - because the camera is too still.

10-01-2007, 09:02 AM   #5
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Ha! Hey and, I found this reply of yours in another thread..

"Another walkaround if your lens has an aperature ring, ie non-DA, then you can turn the aperature ring to something else than A, then the built in flash will fire a single flash at full power"

Right there, that is my problem! I'm trying this with the DA 16-45 and without a shoe-mount I'm always going to get the pre-flash. I'll try this again with my SMC-A 28/f2.8 and see what results I get.

Sort of related, One thing that's a bit frustrating as well is the fact you lose so much flash functionality when using non-A lenses, or, full manual. I understand why but, I'd like it if I could tell the camera what aperture the lens is set to. You know? Set it on the lens and then enter in the camera what you set it to.
10-01-2007, 09:10 AM   #6
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The thing about turning the aperature ring away doesnt really help so much, because the flash will fire at full power every time. But according to everything I know, that first flash should not affect the exposure, it should have finished before the first shutter curtain starts to move, ie before the exposure starts.

I do wish they had included some manual controls for the built in flash but alas they dont... With an external one of course you have more options (A and M modes).

Yea I suggest you try this out yourself by setting a long shutter time and trailing sync and then blocking the flash as best as you can, try to underexpose the picture too so that if the flash doesnt fire then you will notice it very easilly in the picture.
10-01-2007, 09:13 AM   #7
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QuoteOriginally posted by Tom M Quote
Sort of related, One thing that's a bit frustrating as well is the fact you lose so much flash functionality when using non-A lenses, or, full manual. I understand why but, I'd like it if I could tell the camera what aperture the lens is set to. You know? Set it on the lens and then enter in the camera what you set it to.
Tom

I have addressed this several times with pentax, but so far, no luck.

I have 2 DSLRs, and *istD and the K10D. The *istD has true TTL flash and works great with all lenses because it is taking reflected light right off the sensor.

The justification for moving to P-TTL is that there have been some claims that the reflectivity of the sensor is not the same as film. (I don't know all the details but, we are where we are...)

While you might be thinking great, I bought the wrong camera, you need to have a go yourself at pentax, and ask them the same thing I have been asking for. Specifically the question below:

For green button functions on K mount lenses, can the camera calculate the F stop when the flash is operating, by measuring only the change in light before and after the green button is pressed and not change the shutter speed.

This would allow the camera to calculate an F stop so that preflash works properly.
10-01-2007, 09:17 AM   #8
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QuoteOriginally posted by Tom M Quote
Can I turn off the P-TTL? I really would prefer one flash right before the shutter closes.

It seems I can not get the effect I'm looking for. I should be able to compensate for any blur across the subject with a good strong flash right before the shutter closes but it appears that the first flash is strong enough to expose the subject significantly so that I end up with a light subject profile, some blur, then the subject again. It should only be blur and the subject captured in the last flash.
Tom Something is not right here. With P-TTL the preflash is before the shutter is opened, and the lens stopped down. The preflash is taken with the lens wide open, Shutter closed, to measure the wide open light into the camera, full flash is calculated based upon the F Stop set, and the preflash reading, to give you correct exposure.

You should not have 2 flash images in the photo.

Are you using a second flash unit with an optical slave that is triggering with the preflash?

10-01-2007, 09:21 AM   #9
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Good points andreas and Lowell, I'll send off a letter to the folks at Pentax as that simple change would allow much more functionality to it all..

Also, this seems more rumor than anything as I've only been able to find a little about it on flickr.. It's claimed that you can switch the internal flash from 'auto' to 'manual' and that doing so eliminates the pre-flash.. They claim it can be done on the K100 and should be able to be done on the K10D.. Is any of this true? I don't see anywhere where this can be changed.
10-01-2007, 09:27 AM   #10
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QuoteOriginally posted by Lowell Goudge Quote
Tom Something is not right here. With P-TTL the preflash is before the shutter is opened, and the lens stopped down. The preflash is taken with the lens wide open, Shutter closed, to measure the wide open light into the camera, full flash is calculated based upon the F Stop set, and the preflash reading, to give you correct exposure.

You should not have 2 flash images in the photo.

Are you using a second flash unit with an optical slave that is triggering with the preflash?
No, just the internal flash is being used.

I did come up with a second scenerio as to why I keep getting the first image. Maybe, just maybe, I'm not moving fast enough and it's simply freezing the image enough to screw up my exposures. Tonight when I get home I'll do some more testing.
10-01-2007, 09:29 AM   #11
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Well as mentioned you can trick the flash into manual, but you cannot adjust it, its full power and normally thats not what you want.

But I still dont think you really have a problem here, the first flash is not causing you any trouble, you think its affecting the exposure but it shouldnt be, you have to test this properly to verify the behaviour, my k10d certainly works fine if I do this.
10-01-2007, 09:32 AM   #12
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QuoteOriginally posted by Tom M Quote
It seems I can not get the effect I'm looking for. I should be able to compensate for any blur across the subject with a good strong flash right before the shutter closes but it appears that the first flash is strong enough to expose the subject significantly so that I end up with a light subject profile, some blur, then the subject again. It should only be blur and the subject captured in the last flash.
Got any samples? Because as others have stated, the pre-flash should not be captured in any way. I'd just like to see what you mean.

There's a manual flash option with the hotshoe flash accessory. My built in flash is always PTTL unless the camera's using an unknown aperture.
10-01-2007, 09:39 AM   #13
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and, I really think you're correct and for the sake of doing a 1 minute test, I'll double-check it.

The reason for all of this is because I was recently looking through some photos and noticed a very unique 'look' to a few that one individual took. I recall taking film photos that had the identical effect and, they're quite nice. Recalling how I took the photos 15 years ago I decided to duplicate them with the K10D and, well, not so much luck.

Also, that pre-flash really confuses the subjects. They see the first flash and think, that's it!
10-01-2007, 09:49 AM   #14
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QuoteOriginally posted by Tom M Quote
Also, that pre-flash really confuses the subjects. They see the first flash and think, that's it!
I agree about that, especially using mirror-up self timer, the preflash will go off, then the mirror is raised and then the real shot is made, I have to tell people "thats not it! that not it! wait for it!!"

I know the effect you are going for, I havent tried it myself in real shots because I pretty much dont shoot any motion. But its not all plug and play, and certainly easier to do in manual mode with an external flash.
10-01-2007, 11:45 AM   #15
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Yes.. I'm looking to have the background in motion with the subject sharp. It looks like crap until you get it just right. Done correctly, it looks like a sharp photo of the subject was photoshopped over a blurred background photo.. Talk about 'pop'.. This technique is king of pop.
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