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08-08-2010, 07:55 PM   #1
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Kornfused about P-TTL and TTL

Could anybody enlighten me as to what happens when a TTL flash (pentax) is used on a P-TTL (pentax) camera? I'm fairly sure I understand the difference between the two technologies but I can't quite grasp what the ramifications would be when using this combination.
I've got 4 TTL flashes and I'm concerned about what to do with them if they don't work with a K20 or a K-x.

08-08-2010, 08:16 PM   #2
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QuoteOriginally posted by jonhock Quote
Could anybody enlighten me as to what happens when a TTL flash (pentax) is used on a P-TTL (pentax) camera? I'm fairly sure I understand the difference between the two technologies but I can't quite grasp what the ramifications would be when using this combination.
I've got 4 TTL flashes and I'm concerned about what to do with them if they don't work with a K20 or a K-x.
Basically, nothing happens. A TTL flash simply won't function (at least in TTL mode) on a P-TTL camera. It can only be used in full manual mode, or auto-flash mode, if the flash itself has the necessary built-in light sensor. Most TTL flashes do not have such a sensor, because their design is based on the assumption that the camera does.

A TTL camera has a flash sensor built into the mirror box of the camera. It reads the reflection of light off the sensor during the exposure.

A P-TTL camera has no such sensor in the mirror box. Rather, a pre-flash (the P in P-TTL) is fired while the mirror is still down. The same light sensors in the viewfinder that measure daylight are used to measure the pre-flash and calculate the necessary exposure. The mirror then flips up, the shutter opens and the camera tells the flash how long to flash.

Take a picture with the built-in flash on your camera. Pay attention in the viewfinder. You should be able to see the pre-flash through the viewfinder. If you have a TTL flash, set it in manual mode and take a picture. You won't be able to see the flash through the viewfinder, because the mirror is up.

As for what to do with your old TTL flashes, you can always use them off-camera. Take a look at The Strobist website at http://www.strobist.blogspot.com/. You will need some optical slave triggers or radio triggers.
08-09-2010, 07:28 AM   #3
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It's not really that nothing happens when you use old TTL flashes on your new K-x digital camera.
First of all, if the flash is an old TTL and it has a high trigger voltage, your camera electronics will fry! Yes, fry!

Find out about what is the trigger voltage of your TTL flash first before mounting and trying it out on your K-x.

The other technical differences has been covered on the previous post by noblepa.
08-09-2010, 07:34 AM   #4
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Just to be really really clear,

TTL means through the lens metering, where the flash fires when the shutter is open and the flash duration is controlled by a sensor reading reflected light directly off the film or sensor (depending on film or DSLR)

P-TTL uses a preflash, with the mirror down, and measures the proportional combination of ambient light and flash intensity of the preflash to calculate a pre determined duration for the flash when the shutter is open.

TTL is no longer supported by pentax, with the last body to support it the *istDS.

On a P-TTL camera, the flash will trigger at the correct oment, but with full power if in TTL mode or at what ever power setting you set it to in manual mode.

As a result, the most typical result is completely over exposed shots due to the flash going at full intensity

Some flashes, as others have noted, may have high trigger voltages, and you can test for this using a high impedance (Greater than 10 Meg Ohm) volt meter. Using cheap meters that state 10K or 20K ohms per volt will cause metering errors that may lead to under estimating the flash trigger voltage.

There is a thread regarding flash compatibility and trigger voltage in the accessories forum

08-09-2010, 07:35 AM   #5
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...And there are only three Pentax DSLR that can use TTL-only flashes.
They are *istD, *istDS and *istDS2.
08-09-2010, 06:28 PM   #6
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Thanks guys!

I was afraid of that. I wasn't looking forward to buying a new flash but I will. The choice being between the AF360FGZ or AF540FGZ.

I guess a little info on my part might be helpful....

I'm currently (and have for quite some time) shooting a ZX-50. The four flashes I have are-AF280T--AF240FT--AF330FTZ--and the AF400FTZ. I have a wireless set with 2 receivers (hongkong) a generic flash cord that I use with my stroboframe, 5 hot shoe F's (camera mount, not the "off camera" tripod socket) and four
5P cords (3').
Next question-If I use a P-TTL flash for my main flash and hook up a cord and a hotshoe to a TTL flash, would the P-TTL flash fire the TTL in accordance with it's settings to get a proper exposure?
I have no problem using these 4 units in manual or auto (non-TTL), and I would still use them for "studio" type shots but having the flashes "co-ordinate would be cool.





































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Last edited by jonhock; 08-09-2010 at 06:37 PM.
08-10-2010, 02:23 AM   #7
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none of the flashes you list would damage your camera.

the only problem is functional

If I recall correctly however, at least one of your flashes can work in auto mode, and perhaps you can use it to control the rest, using it's auto sensor.

the P-TTL system will not be able to control your setup, directly.

You could use manual, as you indicated as well.
08-10-2010, 02:53 AM   #8
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Is there any actual theoretical technical advantage of P-TTL over TTL?

08-10-2010, 11:38 AM   #9
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Oooops

Oops meaning that I just bought (won?) a Pentax K10D on not-always-cheap-bay for $296.90 so I guess I'll be able to figure a lot of this stuff first hand. Since, as Lowell just posted, none of my flashes will hurt the camera, I'll be able to check them out for under/over exposure when in auto or manual.
It looks like I'll be shopping for a P-TTL flash sooner than I thought!
Christopher, doing a "quick" search for P-TTL vs TTL I did find this at least-
"Talking to a tech here in Australia who repairs Pentax & Sigma & he said he see's that a lot where film would ignore slight 1/3 stop variances in older flash units & the newer flashs designed for digital were designed with a higher tolerance to suit the CCD sensors." at this address: <http://www.flickr.com/groups/pentaxstrobists/discuss/72157622522678982/>
Now would be a Perfect time to find out what these differences are

also, Lowell, my 280 has an "auto" setting and the 400 and 240 have "ttl auto" but the 330 only has ttl and manual.
I'm glad this is only a major hobby for me or I'd be a starving "professional" (professional=one who gets paid for his/her work)
08-10-2010, 05:45 PM   #10
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QuoteOriginally posted by Christopher M.W.T Quote
Is there any actual theoretical technical advantage of P-TTL over TTL?
The technical advantage comes with fill flash. Using TTL when the flash ends the shutter remains open and additional exposure from natural light leads to over exposure because the flash metering cannot alter the shutter speed mid exposure

With P-TTL the cameras metering can evaluate the mix of natural light and flash from the pre flash and calculate the correct duration which should lead to correct exposure

That's the theory any way
08-10-2010, 06:23 PM   #11
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The AF-280 does have a built in sensor. I've used it successfully on my K7 using the built in sensor.
08-11-2010, 08:15 AM   #12
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I've used the AF-280T on my K20D with no issues - works great. In fact, a lot of the time I use it instead of my P-TTL flash.
08-11-2010, 11:38 AM   #13
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eccs19 & Chip B,
Do you guys set it in "auto" mode and if so do you use the green auto or the red auto?
08-11-2010, 11:46 AM   #14
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I know one is low and the other high, I guess it depends on the situation.
I posted the post above about the auto mode before I went back and looked at the manual. I'm used to just using TTL.
AND, as long as you are here,(heh heh) can you tell me what sdhc card class you use? I'm looking at class 6 and 10 and I probably have a week or so to get it before my K10D gets here.
08-11-2010, 02:08 PM   #15
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I mostly use it in Green mode, but sometimes I'll switch to Red, but not that often. Mostly if I'm playing around, just to see. My card is a class 4. I don't shoot burst very often, but the couple times I have, I've not had an issue with the card not keeping up. I've done video 1 or 2 times, just to see, and again no issues.
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