Can I switch off the preflash on K20D P-TTL onboard flash?

Hi all,
I'd like to use the onboard flash with a DA Pentax lens so that it fires without the preflash so that I can use my old flash guns with slave units.
I thought setting the exposure mode to manual would do it but that still preflashes, setting off my old flashguns, I've been through most of the modes unsuccessfully (stopped when the cat started batting the whiney flashguns off the sofa!).

I realise I can use manual lenses as I use my Tamron 90 with an old flash on a bracket with onboard flash triggering the slave unit.

Both my old flashes are too high a voltage (THANK YOU to whoever first posted the photo strobe trigger voltages link - I would have plugged 270v flashes in otherwise).
I assume that applies to PC socket as well?

Am I missing anything?

I can't afford new flashes; especially as I only normally want to use them close to the subject.

Sorry if this has been covered before but couldn't find anything on a quick search.

thanks.

Are you trying to say that you want the onboard flash to fire but whenever you do it fires a preflash? And is that (preflash) that you want it cancelled is it?
If that is the case I think there is a way of doing it, but can you confirm that this is precisely what you want?

I have tried this and found that the pre-flash only turned off when I was using a lens with an aperture rind and it was turned from the 'A' setting. Worked fine, was even able to set flash output power. Another good reason to keep older lens.

You cannot turn preflash off on the in-camera flash.

I think the hotshoe and PC port are good up to 25V, but there are warnings in the manual about using higher X sync voltages so I would not risk it.

Get one of these
http://www.theflashcentre.com/wein-safe-sync-hotshoe-to-hotshoe-i358.html

This will allow you to use any flash either on the shoe or using a PC cord.

Alternatively if you want to fire the flash remotely at a distance, you can always get an IR trigger to replace the camera flash. I use this one in a studio all the time and its very reliable (if bulky).

http://www.hiltonphoto.co.uk/products/details/3151.html

You can pick up an old Pentax AF160 flashgun for around £10 (or less).
When you connect a flash gun that has "A" and "M" modes only onto the hot-shoe of the camera THEN you definitely get just one flash

HTH!
Matt

Are there not 2 types of preflash?

the first which occurs before the exposure and is for red eye reduction and can be turned off in the K20D's Fn menu.

the second is the PTTL preflash which I thought occurs during the exposure and is very small and used for assessing how big the actual flash should be.

If I have understood correctly I would have thought the PTTL preflash would have made no practical difference to actuating a slave flash and it's only the red eye flash which would set of the slaves before the shutter is open.

Also to get the PTTL preflash but no main flash from the popup you can set it to wireless commander mode...

I'm sure someone will tell me if I'm wrong

AAArgh, just spent 45 mins responding and got logged out! lunch over. will respond tonight if can get g/f off computer.

Yes, there are two types of preflash. However, the P-TTL preflash cannot be disabled (unless you're using an old lens!)

The preflash for P-TTL definitely IS a problem for light-triggered slaves! There's only milliseconds between the pre-flash and the main flash, but it's enough to discharge the slave so it has nothing left for the main flash.

It works like this:
* Preflash fires
* Camera takes metering information
* Mirror goes up and shutter opens
* Main flash fires
* Shutter closes and mirror goes down.

When using wireless remote the pop-up flash STILL does a preflash and main-flash. In fact, it's more complex since the wireless is not radio but light! The popup flash communicates with the slave before the exposure by using bursts of light.

All this will mess up a light-triggered slave.
So, your only real options are:
1) Use an AF360 or AF540 as a remote in wireless P-TTL/Manual/A mode
2) Use an older flash on the hotshoe as a controller for the "dumb" slaves
3) Use the PC connector and use wired slaves

Does that make better sense?
Matt

Thanks Matt,

my mistake in thinking the PTTL preflash occured during the exposure rather than before it.

I've never had to worry as I've used a 360 or 540 flashes when I've wanted slaves

Hi all,
Cut and paste this time

Aminstar, that’s what I want to do which happens with my old manual lenses but not DA lenses, Mattmatic below says this can’t be done without using old lenses which I thought but was hoping there might be a sneaky way!

Beakynet, I didn’t know the flash output was adjustable with manual lenses, I was adjusting aperture so I’ll try that, thanks.

Critical-i, the safe-sync looks useful although at the price I could likely get an old flash gun that has got a safe trigger voltage – is there a safe-sync for PC cables (being lazy; I can search it myself now I know what it’s called)
The IR is too high-tech for my Eddison-era flashes.

Bob and his Pentax, I think Matt answered that pretty well, I think there is more of a delay with the red-eye flash than with the P-TTL but the cats eyes still close quicker!

Mattmatic, Many thanks, your 2 posts confirmed my queries. Also I assumed Wireless was radio rather than light


With my old camera I’d made myself a 20ft long PC cable so I could fire one via PC sync and the other by slave light trigger while keeping the camera further away.

The other thing I have is some flash brackets on my wall and I’ve got some old mobile-phone chargers powering the flashes instead of batteries.

I was hoping to use DA lenses and onboard flash instead of PC sync to fire the above combinations as the cats wander in and out , my camera is normally by my chair and if they do anything interesting I was hoping to switch on, pop up flash and go click.

My flashes are Cullman C28 and Sunpak Auto 140 so very low tech.

As I’m only a hobbyist 2x AF360s at £600+ for the pair is a little pricey for the amount of use I’d get.

Thanks everyone for the advice as I’m trying to do my photography with lots of good advice and little cash outlay – It’s a hobby that can get very pricey very quickly, and my photography is partly to record my other hobbies.