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04-26-2010, 02:29 AM   #16
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The other thing that ticks me off about P-TTL is the fact that for second curtain synch it needs to fire the preflash...which ruins the effect. - TTL doesn't do this.

04-26-2010, 03:57 AM   #17
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TTL works well with film as it meters off the film surface real time. Since the CCD/CMOS is highly reflective, TTL doesn't work quite as well. P-TTL is based on pre-flash but never quite as reliable as TTL, not even close. If HSS is not needed, plain A flash is wonderful.
04-26-2010, 04:37 AM   #18
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QuoteOriginally posted by Digitalis Quote
The other thing that ticks me off about P-TTL is the fact that for second curtain synch it needs to fire the preflash...which ruins the effect. - TTL doesn't do this.
the preflash happens with the mirror down, before the shutter is open This should not impact rear curtain sync
04-26-2010, 04:37 AM   #19
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QuoteOriginally posted by wlachan Quote
TTL works well with film as it meters off the film surface real time. Since the CCD/CMOS is highly reflective, TTL doesn't work quite as well. P-TTL is based on pre-flash but never quite as reliable as TTL, not even close. If HSS is not needed, plain A flash is wonderful.
I think this is a myth, or at best an issue with shooting directly into a reflective surface. As pointed out, P-TTL existed before digital. There was no issue then, it was a logical advancement in flash metering and control.

04-26-2010, 04:45 AM   #20
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QuoteOriginally posted by dlacouture Quote
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.
I don't disagree, with M42 lenses do the preflash and assume nothing changes
QuoteQuote:
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.
this will make an unreasonable shutter delay because it requires the lens to stop down fully before the mirror moves, presently they both take place at the same time
QuoteQuote:

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).
except that the stop down function takes power, the camera is designed to wide open meter not stop down
QuoteQuote:
- 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.
while it can work the camera is not designed this way, you are implying a reversal of logic where the camera does not hold the lens open while focusing, this would make the camera incompatible with A series lenses
QuoteQuote:

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).
i believe you are wrong here, if you consider that the priority must be to deal with A and not non A lenses, For A lenses it must hold the aperture open with no power, for non A lenses it is the opposite
QuoteQuote:
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.
actually the best way is to enter the max and min aperture, and let the camera re-scale the activation lever to deal with the diameter linear with lever movement as opposed to area linear with lever movement, issue
QuoteQuote:
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.
04-26-2010, 06:25 AM   #21
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QuoteOriginally posted by Lowell Goudge Quote
I think this is a myth, or at best an issue with shooting directly into a reflective surface. As pointed out, P-TTL existed before digital. There was no issue then, it was a logical advancement in flash metering and control.
Ofcourse P-TTL was there even in film days. But that does in no way rule out, that the old TTL is hindered by the different reflectivity of the sensor compared to the film surface. Even in film days some film material with an odd reflectivity caused problems with TTL flash control, like the good old Velvia, which was well-known for these problems.

Whether P-TTL worked more reliable in film days, I don't know. I never used it back then.

Ben
04-26-2010, 07:32 AM   #22
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QuoteOriginally posted by Lowell Goudge Quote
this will make an unreasonable shutter delay because it requires the lens to stop down fully before the mirror moves.
That's why I proposed to stop down the lens as soon as it's mounted (I knew you would counterattack with this line ). I'd bet the aperture actuator is a kind of linear motor, and so once moved, does not require any additional power to hold its place.

QuoteOriginally posted by Lowell Goudge Quote
this would make the camera incompatible with A series lenses
No it doesn't : the camera knows when there is an A lens and when there is a non-A lens. So what I proposed (which applies to non-A lenses only) does not impact A lenses functionalities.

QuoteOriginally posted by Lowell Goudge Quote
actually the best way is to enter the max and min aperture, and let the camera re-scale the activation lever to deal with the diameter linear with lever movement as opposed to area linear with lever movement, issue
Except that some PK-m lenses are not necessarily diameter-driven... I have a 55/1.2 PK lens that is surface-related (so it works quite well as an A lens once the conversion was done, I'm lucky!).
I'm not sure there was really a diameter-lever relation back then. I think it's pretty much whatever worked at the time...

Last edited by dlacouture; 04-26-2010 at 07:38 AM.
04-26-2010, 08:07 AM   #23
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One last thing : M mode with PK-m lenses would still work exactly as before, so nothing is lost here. What I propose only concerns non-A lenses in Av mode (not supported yet!).
PK-A lenses behavior is absolutely unchanged (except that you can now use the A ring in Av mode!!!).


Last edited by dlacouture; 04-26-2010 at 08:12 AM.
04-26-2010, 09:10 AM   #24
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Lowell and others, this is very good info for flash photography. Thanks,
04-27-2010, 02:56 AM   #25
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Original Poster
Thanks all.
I didn't realise that my simple question was not so simple!
I expected a couple of "when you buy a K7 or Kx, buy a new flash as well" type answers.
there is obviously more to it than that - including issues with different lens generations.
All my lenses are FA or DA series so I won't encounter the issues relating to Ms and As.
I guess regardless of the quality of the above comments, there is still a bit of 'suck it and see' to see what works for me, for my flash, lenses, and whatever camera I end up with.
Thanks again to all who added something to the discussion.
12-21-2012, 03:36 AM   #26
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Just resurrecting an old thread, can't understand why a dSLR targeted to professionals like the K5(s) keeps sporting a metering system way inferior than that adopted by old flagship LX.

Pentax LX - How does its metering works

"The IDM system measures the actual amount of light reaching the film plane during exposure, regardless of aperture setting or which focusing screen or viewfinder is being used. Even if light conditions change during the brief moment of exposure, shutter speed is adjusted instantaneously to compensate. When the camera is operated in the automatic exposure mode, the IDM system switches on the instant the mirror flips up and the front shutter curtain begins to move (see illustrations). It calculates the amount of light hitting the film plane and instantaneously controls rear shutter curtain motion for correct exposure. Light values, metered through the actual aperture, are monitored for the duration of the exposure - not the instant before the shutter is released. Thus, shutter speed is correct, even in changing light."

and

"For automatic exposure in the TTL Auto Flash mode, the IDM system measures only light reflecting off the film plane since the flash goes off after the shutter is fully open. The rear shutter curtain closes after the IDM system determines that enough light has reached the film and terminates the flash."

Aside from auto-everything, I would say that this feature would matter.
12-21-2012, 03:13 PM   #27
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QuoteOriginally posted by seppstefano Quote
Just resurrecting an old thread, can't understand why a dSLR targeted to professionals like the K5(s) keeps sporting a metering system way inferior than that adopted by old flagship LX.

Pentax LX - How does its metering works

"The IDM system measures the actual amount of light reaching the film plane during exposure, regardless of aperture setting or which focusing screen or viewfinder is being used. Even if light conditions change during the brief moment of exposure, shutter speed is adjusted instantaneously to compensate. When the camera is operated in the automatic exposure mode, the IDM system switches on the instant the mirror flips up and the front shutter curtain begins to move (see illustrations). It calculates the amount of light hitting the film plane and instantaneously controls rear shutter curtain motion for correct exposure. Light values, metered through the actual aperture, are monitored for the duration of the exposure - not the instant before the shutter is released. Thus, shutter speed is correct, even in changing light."

and

"For automatic exposure in the TTL Auto Flash mode, the IDM system measures only light reflecting off the film plane since the flash goes off after the shutter is fully open. The rear shutter curtain closes after the IDM system determines that enough light has reached the film and terminates the flash."

Aside from auto-everything, I would say that this feature would matter.
It would solve the problem of over exposure for high ambient light
12-22-2012, 05:19 AM   #28
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A simple way would be to buy a Vivitar 285 HV. It is non dedicated, very powerful, incredibly easy to use and cheap to buy on Amazon.
I have been using them for simple bounce flash work and macro. On the manual setting, there is full power - 1/2 - 1/4 - 1/16. I use two units set up on tripods and triggered by camera pop-up flash set on lowest power for portrait work.
12-22-2012, 01:52 PM   #29
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QuoteOriginally posted by seppstefano Quote
Just resurrecting an old thread, can't understand why a dSLR targeted to professionals like the K5(s) keeps sporting a metering system way inferior than that adopted by old flagship LX.

Pentax LX - How does its metering works

"The IDM system measures the actual amount of light reaching the film plane during exposure, regardless of aperture setting or which focusing screen or viewfinder is being used. Even if light conditions change during the brief moment of exposure, shutter speed is adjusted instantaneously to compensate. When the camera is operated in the automatic exposure mode, the IDM system switches on the instant the mirror flips up and the front shutter curtain begins to move (see illustrations). It calculates the amount of light hitting the film plane and instantaneously controls rear shutter curtain motion for correct exposure. Light values, metered through the actual aperture, are monitored for the duration of the exposure - not the instant before the shutter is released. Thus, shutter speed is correct, even in changing light."

and

"For automatic exposure in the TTL Auto Flash mode, the IDM system measures only light reflecting off the film plane since the flash goes off after the shutter is fully open. The rear shutter curtain closes after the IDM system determines that enough light has reached the film and terminates the flash."

Aside from auto-everything, I would say that this feature would matter.
IDM works off the film surface. That is not possible with the sensor surface of a DSLR - otherwise the old players with these film-based measurement systems would also have stuck to it, like Sony (Minolta held the original patent for this technology), Olympus (the first on the market) or even Nikon (F3 and later).

Sensors have completely different reflective properties, which are obviously not so easy to manage and create a whole set of problems, which did not exist in film days. The istD was the last Pentax DSLR, which sported flash-TTL metering and it was of not really much use - at least I did not find it any more realiable than P-TTL. Simple A flashes work best anyway...

Ben
12-22-2012, 02:26 PM   #30
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Ben, my old tank (a Canon 30d) even when used with pentax lenses and an adaptor ring (I have neither EF or AF lenses ) allows both TTL flash and consistent metering and exposures without any additional "A" pin trick, without any glitch and with 100% hit ratio.

The 30d does have a sensor (although way inferior to that of my K-x) AND is an old camera.

Can't find excuse to Pentax decision on carrying on an obsolete, absurd idea as P-TTL and preventing TTL metering to work without headaches on any lens.

I think its name is just cost-cutting (for P-TTL) and marketing (to effectively avoid people using older lenses).

Richard, I have something like 4 Metz flashes around (or maybe more). Yet the point is that TTL flash was bynow a standard and some marketing genius is trying to let us forget it.

Can't agree less with Pentax on this issue and would ask Pentax to come to its senses. No FF and no TTL on flash? Come on! Look at what other brands are doing!

Stefano
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