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04-25-2010, 02:53 AM   #1
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Difference between TTL and P-TTL flash.

Hi,

What is the difference between TTL and P-TTL flash?
I'm using a Sunpak MZ440AF-PT (a legacy from my Konica Autoreflex and Pentax MZ50 days) with my *ist DS. I understand that I'm using TTL, but with later models P-TTL is the standard. What difference will it make to me if I upgrade to Kx or K7?

04-25-2010, 03:09 AM   #2
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on a purely technical basis TTL is better than P-TTL due to the fact that P-TTL needs a preflash to get the metering right, TTL flash doesn't need this because the metering is done literally, through the lens.
04-25-2010, 03:19 AM - 2 Likes   #3
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QuoteOriginally posted by Digitalis Quote
on a purely technical basis TTL is better than P-TTL due to the fact that P-TTL needs a preflash to get the metering right, TTL flash doesn't need this because the metering is done literally, through the lens.
By that do you mean the P-TTL is not?

OP, the TTL measured light reflected off the surface of the film in order to determine when an appropriate amount of light had been received and cut off the flash.

Due to some reflectivity/glare/whatever technical crap I will never understand, this seems not to work as well with digital sensors, so they changed the thing around so the incoming flash gets measured by the camera's metering system (I think)....meaning the mirror has to be down and that the flash is not hitting the sensor. That's why the flash has to go twice. Once to get the exposure set and once to actually shoot the shot.

The short version of that is that they are two entirely different and incompatible systems which the nimrods at Pentax had no more sense than to give confusingly similar names.

Last edited by Mike Cash; 04-25-2010 at 03:25 AM.
04-25-2010, 05:03 AM - 1 Like   #4
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there are two parts to this.

forst of all TTL stands for through the lens. Both flash systems do measure light throuh the lens.

TTL flash measures the light reflected off the sensor / flim directly and terminates the flash at the correct exposure.

P-TTL measures the light with the lens wide open, both the flash light and ambient light, and calculates the flash required when the lens is stopped down,

Some have argued that TTL was not consistent with digital sensors because the reflection was different than from film, but I doubt this is correct. My *istD is TTL capable, and the flash works very well in TTL mode.

I think the issue is that in certain exposure conditions TTL is not as accurate because after the flash there is still ambient light which can lead do over exposure in some cases. Also TTL requires a separate flash sensor, and inout channel. P-TTL uses the main metering system and is therefore cheaper.

P-TTL because of the way it works, needs to know the aperture range of the lens (though the lens contacts) and therefore can only work with A series lenses.

TTL because it can measure light directly off the sensor plane does not need to know the aperture.

In my opinion both are good if you know how to use them, and I keep my *istD around because of the TT: flash capability. I can shoot flash with older lenses.

04-25-2010, 05:31 AM   #5
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Lowell Goudge provides the answer for that Mike "TTL because it can measure light directly off the sensor plane does not need to know the aperture."

It would also make TTL wireless flash a bit easier if you were able to do it with non-A series lenses. I have to use Elinchrom triggers to use wireless flash with my non-A series lenses.
04-25-2010, 06:08 AM   #6
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QuoteOriginally posted by Lowell Goudge Quote

Some have argued that TTL was not consistent with digital sensors because the reflection was different than from film, but I doubt this is correct. My *istD is TTL capable, and the flash works very well in TTL mode.

I think the issue is that in certain exposure conditions TTL is not as accurate because after the flash there is still ambient light which can lead do over exposure in some cases. Also TTL requires a separate flash sensor, and inout channel. P-TTL uses the main metering system and is therefore cheaper.

In my opinion both are good if you know how to use them, and I keep my *istD around because of the TT: flash capability. I can shoot flash with older lenses.
I have experienced always a poor performance of the old TTL (with Pentax AFT 280 and several Metz flash guns) with my istDS. It was never better than P-TTL. The best mode anyway is Auto-mode on the flash, which gives a better "hit rate", than either TTL or P-TTL with a Pentax DSLR.

Ben
04-25-2010, 06:51 AM   #7
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I think the hit rate is a function of using the flash.

I find TTL is the best for long lenses and wildlife with flash, And that is where I use it, but I have never had an issue with either P-TTL or TTL when taking shots at partise, usually bounce, but even with direct.
I think the more difficule situation is fill flash not full flash esposure. And with fill flash, this really is where user problems come in.
04-25-2010, 10:11 AM   #8
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QuoteOriginally posted by Digitalis Quote
Lowell Goudge provides the answer for that Mike "TTL because it can measure light directly off the sensor plane does not need to know the aperture."
I know it measures off the film/sensor plane; I said that myself.

What I asked was whether you meant to say that with P-TTL the camera is measuring the light from someplace other than inside the camera, as the way you worded it makes it sound like the light does not travel through the lens in the case of P-TTL.

04-25-2010, 10:32 AM   #9
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QuoteOriginally posted by Mike Cash Quote
I know it measures off the film/sensor plane; I said that myself.

What I asked was whether you meant to say that with P-TTL the camera is measuring the light from someplace other than inside the camera, as the way you worded it makes it sound like the light does not travel through the lens in the case of P-TTL.
I think his worsing was not great because it sort of does not imply that P-TTL goes through the lens.

But, as I said, P-TTL measures the light with the camera's metering system off the focusing screen, and measures the impact of the preflash to total exposure, with the lens wide open, and then calculates the flash needed, based upon the combination of ambient light and flash, and the aperture selected. It is a predictive flash metering system, where as TTL is reactive, cutting flash power when exposure is reached, not considering that the shutter is still open and ambient light is still coming through the lens
04-25-2010, 12:01 PM   #10
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QuoteOriginally posted by Lowell Goudge Quote
I think the hit rate is a function of using the flash.

I find TTL is the best for long lenses and wildlife with flash, And that is where I use it, but I have never had an issue with either P-TTL or TTL when taking shots at partise, usually bounce, but even with direct.
I think the more difficule situation is fill flash not full flash esposure. And with fill flash, this really is where user problems come in.
Yes, bouncing in general gives more reliable results, as it circumvents those direct reflections, which seem to bring P-TTL out of step. P-TTL seems in my own experience especially unreliable at short distances, whereas the flash's Auto-mode gets more unreliable at farther distances - so both, technioques can be used to substitute each other nicely.

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04-25-2010, 01:10 PM   #11
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QuoteOriginally posted by Ben_Edict Quote
Yes, bouncing in general gives more reliable results, as it circumvents those direct reflections, which seem to bring P-TTL out of step. P-TTL seems in my own experience especially unreliable at short distances, whereas the flash's Auto-mode gets more unreliable at farther distances - so both, technioques can be used to substitute each other nicely.

Ben
I think the issue for p-ttl at short distances may be more to do with the mode you shoot in, and settings.

Many times the first notch of flash power might be in excess of what is needed for the shot.

this is especially true when you consider that in auto modes, pentax attempts to match exposure first without flash, i.e. it will adjust shutter and ISO to the limits in AV mode and aperture and ISO to the limits in Tv Mode first, then fill the remainder with flash.

I always shoot manyual with flash, and force the solution upon flash to use reasonable power with maximum shutter speed and a reasonable aperture 1-2 stops down from wide open.
04-25-2010, 01:31 PM   #12
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QuoteOriginally posted by Lowell Goudge Quote
I think the issue for p-ttl at short distances may be more to do with the mode you shoot in, and settings.

Many times the first notch of flash power might be in excess of what is needed for the shot.
.
I'm sure that happens some of the time. I do notice that P-TTL is much more easily fooled by a near white shirt. I did some tests with this a few months ago. Avoiding center weighted metering helped P-TTL greatly in these situations, but the auto mode on the flash still consistently outperformed P-TTL at close range.

I think Ben pretty much nailed the best uses of the modes from my experience as well. I did not become fully aware of the weaker performance at greater distance of the flash auto mode until recently. However, to be fair, that comparison was with the AF400T and a film camera using TTL.
04-25-2010, 02:25 PM   #13
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Well, P-TTL was here before pentax went digital, so the "sensor high reflection" argument is certainly false. My MZ6 has it, as does the MZS, and *ist, *istD, *istDS and *istDS2...
I think Pentax went this road because the multi-segment metering is (supposedly) better than the old TTL cell, and it can take the Spot/CW/matrix settings into account, and it's the only way you can do wireless synch.

Now, P-TTL surely uses the lens' aperture for its computing, but it also works pretty well with m42 lenses with a false aperture, so aperture/focus distance are not that important for it.
I even found the PTTL algorithms in Pentax' patents, and it's clearly defined that if either one of these information are missing, the system continues, ignoring the related variable.
04-25-2010, 03:29 PM   #14
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QuoteOriginally posted by dlacouture Quote
Well, P-TTL was here before pentax went digital, so the "sensor high reflection" argument is certainly false. My MZ6 has it, as does the MZS, and *ist,
I don't disagree, I never believed this to be the case, it is just what is reported as "justification". Note that P-TTL is much cheaper to implement because it removes a sensor, and an analog channel and D/A conversion as well
QuoteQuote:
*istD, *istDS and *istDS2...
these are digital, and supported both ttl and p-ttl, i.e. the best of both worlds
QuoteQuote:
I think Pentax went this road because the multi-segment metering is (supposedly) better than the old TTL cell, and it can take the Spot/CW/matrix settings into account, and it's the only way you can do wireless synch.
no disagreement
QuoteQuote:

Now, P-TTL surely uses the lens' aperture for its computing, but it also works pretty well with m42 lenses with a false aperture, so aperture/focus distance are not that important for it.
aperture is absolutly important because you state you need a "false aperture" but I agree focus distance is irrelevant because it is measuring the impact of reflected light off the subject to the sensor, so distance is not needed.
QuoteQuote:
I even found the PTTL algorithms in Pentax' patents, and it's clearly defined that if either one of these information are missing, the system continues, ignoring the related variable.
As stated above it can ignore distance, but not aperture, it needs one to know how to scale the power for the full exposure.
04-26-2010, 01:40 AM   #15
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QuoteOriginally posted by Lowell Goudge Quote
aperture is absolutely important because you state you need a "false aperture" [...] it can ignore distance, but not aperture, it needs one to know how to scale the power for the full exposure.
Well, I should have written "a false aperture is needed to fool the camera and enable P-TTL with m42".
But P-TTL could perfectly work with preset lenses (m42) without any aperture information, if only Pentax would make the tiny firmware fix needed for this.
And, moreover, P-TTL could be made fully functional with PK-m lenses, with the simple algorithm change here : do the metering flash once the lens is stopped-down, not before. So simple it's a wonder they didn't made it right from the *istD days.

In fact, PK-m lenses could be fully supported (meaning Av and P-TTL) by the following behavior (could be added to the "Permit A ring" custom function):
- if a non-A manual lens is mounted (either m42 or PK-m, doesn't matter), then stop it down in permanence (so, the PK-m would behave just like a m42, enabling Av mode).
- DoF preview could open the lens for ease of focusing.
- P-TTL can now be enabled, and the metering flash will always occur while the lens is stopped down, so the reading will be accurate.

This is just a matter of moving the aperture actuator in the right place, so this should not drain the battery (except if a constant force is needed for this, but I don't think so).
In this case, the camera does not need to know the actual aperture used and the lens' aperture range, as both metering and actual recording will occur at the same physical aperture. That's what we do when tricking the camera with m42 lenses by selecting the greatest aperture available.

Another solution would be to stop down the lens on shutter half-press only, at metering time, but if the aperture is slow to close (oil on blade, etc), this could lead to exposure problems.

OT : in the same time, it would be great if Pentax added the following tweaks (not P-TTL related):
- Enable trap focus with PK-m and m42 manual lenses when the body is set to AF (currently need to shorten the mount contact pins).
- Enable trap focus with all lenses when the body is set to MF and the user presses the AF button.
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