This is incorrect. The DS body performs P-TTL flash metering with Pentax-A lenses.Thank you for the clarification. I thought it meant, "Program-TTL"... sorta like Matrix Metering, and it needed not only the auto aperture info, but also the current zoom and focus info.
Perhaps you misunderstand what P-TTL is vs TTL, and how they operate. It has nothing to do with focus distance information, which A lenses do not provide.
P-TTL means "pre-flash through the lens" flash metering. What happens is that the camera fires the flash once before exposure, takes both a flash reading and an ambient light reading, integrates them, and then makes the actual exposure with flash a moment later. All it requires of the lens to accomplish this is information regarding the set aperture and maximum aperture, which is available with all A, F and FA lenses, because it does not stop the lens down during the pre-flash phase of this operation.
TTL flash metering, on the other hand, means that the camera measures flash illumination "through the lens" at the time of exposure. No information from the lens is necessary for this function, all you need is a flash unit that supports TTL operation and a lens stopped down to working aperture at the time of exposure: the sensor allows the flash to quench output when the desired exposure has been reached, without regard to ambient light illumination. The DS' flash metering sensors allow for TTL operation, but the control circuitry is not included in the DS body or built-in flash; it has to be supported by a compatible dedicated flash unit.Interesting. My understanding of TTL is basically the same... that there's a sensor in the camera that can read the light *during* the shot. A single additional pin from the body to the external flash triggers the external flash to stop firing when operating in TTL. I thought that pin was a binary threshold, though... not direct sensor reading.
The DS body and built-in flash are only equipped for P-TTL flash metering. When a lens that does not transmit aperture information to the body is used (all K/M bayonet and M42 screw mount lenses, or whenever the aperture ring of any lens with an "A" position is set to anything other than "A"), the DS reverts to full power, non-metered flash operation: set the aperture according to the guide number for proper exposure.
The part that confused me was that for the flash, P-TTL is a superset of the requirements of TTL... they both have to be able to quench the flash at less than maximum. I guess the difference is that under P-TTL they body can use the *regular* meter to determine exposure, whereas for TTL it must use the TTL (mirror up) sensor.
A quick clarification here... not only one that transmits aperture information, but one that is also set to use it? In other words, an 'A' lens with the ring set to other than 'A' still sends min/max aperture via the sens pins. You said the preflash operation hasn't yet stopped down the lens, though, so without knowing what the aperture is *currently* set to, it cannot work P-TTL.So, (finally)... if I understand everything correctly, if one gets an *external* flash that does TTL, it will function in TTL with an 'A' lens. How about an even more manual lens? You'd have to choose an aperture that's reasonable, but does the camera lock you out from doing it?
... with the DS body:
- The built-in flash unit will default to P-TTL flash metering whenever a lens that transmits aperture information is fitted to the camera. Whenever a lens which does not support aperture information is fitted, the built-in flash unit will operate at full power in non-metered mode.
(I'm thinking of modifying an old K-lens to add the min/max aperture holes... can't make a true 'A' lens because of linear/log movement, but can fake an 'A' lens in non-'A' mode)
- If you use a Pentax-dedicated external flash unit that supports P-TTL flash metering, A/F/FA series lenses will be able to use P-TTL mode.Yes, I discovered this. I figured that it would at least operate the aperture in other modes, but it doesn't even do that... just leaves it wide open.
- If you use a Pentax-dedicated external flash unit that supports TTL flash metering, all lenses will use TTL when the flash is set to TTL metering.
- If you use a flash that supports both P-TTL and TTL flash metering, and does NOT provide an explicit control to switch between the two modes, it will most likely use P-TTL by default and switch to TTL mode when lenses that do not transmit aperture information are fitted (this is the behavior of the Sigma EF 500 DG Super).
- M/K bayonet lenses, as well as screw mount lenses, never send aperture information to the DS body. The only flash mode when using these lenses with the built-in flash is nonmetered full power output, regardless of whether you have set M or any other exposure mode.
- Screw mount lenses are always used stopped down manually, the body cannot operate the M42 autodiaphragm mechanism. With the built in flash, as long as you have set the correct aperture according to the flash guide number (since the flash will always be operating at full power, non metered), you will obtain proper exposure with these lenses.
- The DS body ONLY stops down M/K bayonet lenses when in Manual exposure mode IF the "Using Aperture Ring Permitted" custom function is set. The only way to obtain use of the set aperture, when using flash or otherwise, is to use Manual Exposure mode.
- All A, F and FA lenses operate like M/K bayonet lenses when their aperture ring is set to anything other than "A".Strictly speaking, there's more information to be had, since the camera knows the absolute aperture of what it's looking at due to the sens pins. The only thing it's missing is what it will be stopped down to (unless you preview).
- The DS never locks you out of making an exposure, regardless of whether the flash unit is metered (P-TTL or TTL) or not, or whether you're using the internal or an external flash unit. It *will* block the shutter release if the internal flash is activated AND in the process of charging up, UNLESS you set the custom function to allow exposure while charging.It will also *not* fire an external (dumb) flash if the shutter speed is set above the sync speed of 1/180. Protect you from yourself, I guess.
I'm sure there are more questions that can be had out of this... but I hope that clearer.Thanks a lot for the clarifications. The engineer in me sees opportunities to make improvements and workarounds for lots of these limitations. I'm sure that marketing had something to do with removing some of them.
Godfrey
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
