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10-26-2010, 02:47 PM   #1
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Difference between P-TTL and Auto?

I've been playing around with a manual bounce flash from my newspaper class, and it has sparked an interest in getting my own. I've done research though, and I still can't quite understand the difference between P-TTL flashes and non P-TTL ones that have "auto" modes. In the AF 280T Review, bdery said "In auto mode, it works flawlessly. The camera detects the flash, adjusts the exposure time and aperture accordingly (in P mode), and syncs as expected."

What would the difference between this auto mode and a P-TTL flash be? I only plan on using a flash on-camera or handheld/bracketed, not with any other flashes, if that matters.


Thank you

10-26-2010, 04:42 PM   #2
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There are a few things different between P-TTL, TTL and Auto

Fundamentally TTL and auto are the same where a sensor measures the light back from the subject and turns the flash off when the light level is sufficient forth exposure. The difference is that Auto uses a sensor on the flash and TTL uses a sensor in the camera body taking light reflected off the film or sensor TTL is more accurate because it takes light off the film and not through a wide angle sensor on the flash that may include light off out of frame subjects. Also TTL accounts for transmission losses of the lens and filters where auto does not

Both have one drawback specifically with fill flash and mixed flash and natural light. When the flash stops the exposure is not over and for the rest of the shutter time more light comes through the lens leading to potential over exposure

P-TTL is different. Flaw duration is not a reactive process where flash is turned off when there has been enough light. P-TTL is predictive. It measures ambient light and the contribution of the flash through the camera's metering by using a pre-flash and with this and knowledge of the lens maximum aperture it calculates the required flash duration to balance perfectly for the residual natural light The use of the preflash is also cheaper because it uses the cameras normal metering sensor as opposed to a dedicated metering sensor off the film/sensor

Last edited by Lowell Goudge; 10-27-2010 at 04:09 AM.
10-26-2010, 05:02 PM   #3
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QuoteOriginally posted by Lowell Goudge Quote
and for the rest of the shitter time
No, that is just so wrong.
10-26-2010, 05:15 PM   #4
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QuoteOriginally posted by Ex Finn. Quote
No, that is just so wrong.
Actually it is correct. Neither TTL or Auto change the shutter timing. When the flash ends the shutter stays open for the remainder of the selected shutter speed

P-TTL as I said is different it sets the flash duration to add to to exposure set BUT still the shutter can remain open long after the flash stops

Remember. Flash power is roughly constant light at a high level for a very short time

10-26-2010, 05:25 PM   #5
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QuoteOriginally posted by Lowell Goudge Quote
Actually it is correct.
Actually, he was being your spell checker.
10-27-2010, 04:10 AM   #6
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QuoteOriginally posted by Ex Finn. Quote
No, that is just so wrong.
QuoteOriginally posted by chalion Quote
Actually, he was being your spell checker.
funny, I missed that the first time. Must be posting from my Ipod that did it
10-27-2010, 05:17 AM   #7
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Thanks for quoting me! Glad to know I'm being read :P

Another way to see this is that, in auto mode, the flash will use its light sensor to decide when it has illuminated the scene enough. The flash communicates its recommended exposure parameters to the camera, which will adapt itself to expose according to the flash settings (with the AF280T there are two power outputs, with associated apertures). But it is the flawh which decides when exposure should be good.

In P-TTL, the camera measures the exposure and informs the flash of the recommended settings. The main differences are that you can control the way the camera meters exposure (spot, etc) and you could be using filters or other stuff, which could change the required exposure. The flash has no way of knowing if you put a polarizer in front of your lens, for instance.

I'd love, all things being equal, to have a more powerful flash, and I'd like it to send AF assist bursts. But since I use my flash indoors in low light, I refrain from using filters and I use lenses that are fast enough to focus in low light. That way the AF280T is an inexpensive and very reliable alternative.
10-27-2010, 05:33 AM   #8
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QuoteOriginally posted by bdery Quote
Thanks for quoting me! Glad to know I'm being read :P

Another way to see this is that, in auto mode, the flash will use its light sensor to decide when it has illuminated the scene enough. The flash communicates its recommended exposure parameters to the camera, which will adapt itself to expose according to the flash settings (with the AF280T there are two power outputs, with associated apertures). But it is the flawh which decides when exposure should be good.
are you sure about this? I don't think the flash does any exposure calculation at all. In simplest form, there is only one communication between camera and flash in auto mode and that is the trigger. You set the lens F stop and ISO on the flash manually, and with this information, and the light it receives back through it's sensor, the flash terminates when it has decided there is enough light, that is all. depending on metering mode of the camera, the camera still decides what the exposure will be, and will attempt to make that exposure without any assistance from the flash.
QuoteQuote:
In P-TTL, the camera measures the exposure and informs the flash of the recommended settings. The main differences are that you can control the way the camera meters exposure (spot, etc) and you could be using filters or other stuff, which could change the required exposure.
close, the camera does not send recommended setting but in P-TTL mode it sets the flash duration, in advance, based upon meter reading. the main difference is that P-TTL is predictive, as opposed to auto which is reactive
QuoteQuote:
The flash has no way of knowing if you put a polarizer in front of your lens, for instance.
no, and it does not care because the camera is doing the measuring through the lens
QuoteQuote:

I'd love, all things being equal, to have a more powerful flash, and I'd like it to send AF assist bursts. But since I use my flash indoors in low light, I refrain from using filters and I use lenses that are fast enough to focus in low light. That way the AF280T is an inexpensive and very reliable alternative.
No argument there, auto capable flashes are a dime a dozen, compared to P-TTL and there are some quite good flashes around. In low lit conditions, except for the case where you have a subject close and a background further away, Auto does quite well for overall scene illumination. Auto requires much more attention in fill mode, where you need to deliberately under expose with flash ald allow the natural light to bring up the remainder of the exposure.

10-27-2010, 07:29 AM   #9
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I believe that Pentax flashes in auto mode will send a signal to the body to tell it what aperture to use, that's it.

See the definition of how the "Mode" pin is used by "Program Flash" units at Flash Systems Evolution, Features and Operation
10-27-2010, 10:30 AM   #10
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QuoteOriginally posted by Entropy Quote
I believe that Pentax flashes in auto mode will send a signal to the body to tell it what aperture to use, that's it.

See the definition of how the "Mode" pin is used by "Program Flash" units at Flash Systems Evolution, Features and Operation
this will be a function of the lens used as well as the individual flash.

for example, using manual aperture lenses (my princple reason for using Auto mode flash on the K10D or K7, you have to set the flash to match the aperture you are shooting on the lens, because the camera does not know it, and the camera cannot set it. I believe in general the flash does not signal to the body the aperture to use, if anything, the camera signals, with an A lens the aperture used. It is the camera that controls things not the flash,
10-27-2010, 12:07 PM   #11
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Lowell,

Please read this post. I think you'll understand more why Entropy writes "Pentax flashes in auto mode will send a signal to the body...."

The AF280T and AF400T have the same signal interface as the AF200T.

The AF200S is similar. The problem is that the aperture value set by the flash is incorrect.
10-27-2010, 02:05 PM   #12
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QuoteOriginally posted by SOldBear Quote
Lowell,

Please read this post. I think you'll understand more why Entropy writes "Pentax flashes in auto mode will send a signal to the body...."

The AF280T and AF400T have the same signal interface as the AF200T.

The AF200S is similar. The problem is that the aperture value set by the flash is incorrect.
OK clearer now, the only auto flashes I have are a vivitar where there is no data interface to the camera, and the AF 540, but I don't use it in auto mode with A lenses, just with K lenses.

The vivitar does have the F stop switch, in fact it has 4 positions, not just 2. and again the positions are set by the user to adjust aperture with flash power and the shooting distance.
10-27-2010, 02:15 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
the only auto flashes I have are a vivitar where there is no data interface to the camera.
When my K-7 is in Av or P mode, my Vivitar 5600 (with PM/S module) and my Sunpak flashes (with PT-1D or PT-2D module) set the sync. shutter speed (value depending on ambient light and the focal length of the lens).

The Sunpak PT-2D also tries to set the aperture depending on the flash's ISO and auto settings, but the aperture it uses is incorrect (similar to the Pentax AF200S).

That makes me wonder why the Pentax 540 can't automatically set the camera body when the flash is in A mode. It would be nice if the current 540 could do the same thing as the ancient AF280T.
10-28-2010, 05:03 AM   #14
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Soldbear's post is correct, Lowell. When my 280T is ready, the camera sets itself to the recommended aperture and shutter speed for the flash, based on the ISO setting. The "aperture" switch on the back of the flash is just a guideline for manual cameras, nothing more.

So the flash trusts that the camera will be set with a specific array of parameters (depending on "green" or "Red" mode). Knowing this, the flash will fire and turns itself off when enough light has reached it (just cover the front sensor and you'll see!). That front sensor (in the green circle) is an exposure meter. The flash will even tell you if a correct exposure was made, by lighting its green "Test" light.

So the flash does not communicate complicated exposure measurements to the camera, what it says is basically "use these settings, and I'll do my thing"...
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