John
On Fri, 13 May 2005 01:16:00 +0100, Godfrey DiGiorgi <[EMAIL PROTECTED]> wrote:
Yes, the manual is incorrect. A lenses work exactly the same as F/FA lenses.Ah... I see my confusion now. I had assumed that if a flash was capable of P-TTL, then it would also do TTL. But...
The DS' built-in flash will ONLY operate in P-TTL or full power manual mode.... since the internal flash will apparently NOT do TTL, it *does* fire fully with an 'A' lens. Seems like an artificial limitation. TTL should do exactly that... TTL with whatever it sees.
You use a dedicated external flash unit that supports P-TTL, TTL, and other modes, to get traditional TTL flash metering. The built-in flash is not used when you're using an external flash unit.
This is incorrect. The DS body performs P-TTL flash metering with Pentax-A lenses.
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.
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.
I'm new to this whole TTL/P-TTL flash thing. My old 35mm P-30T had an auto nondedicated that I barely figured out how to use. I've used it with the -DS when I need a little more flash poop. Since most of my lenses are MF (half 'A', half not), it's apparently no more manual than the internal flash.
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.
- 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.
- 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".
- 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.
I'm sure there are more questions that can be had out of this... but I hope that clearer.
Godfrey
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