[newarts]

My camera is a K100D; when it thinks the lens is an "A" type, it pre-flashes* and uses the TTL metering system to correctly set the exposure depending on the light that gets through the lens.

Isn't the 'auto' mode I'm faking a P-TTL mode? Regardless of terminology, it works; preset the aperture on the lens, pop-up the built-in flash, shoot the photo; exposure is correct.

Dave

*I can see the pre-flash fire through the view finder. I get correct flash exposures with a manual Takumar as I change the lens' preset aperture from lowest to highest f-numbers.

no, it's not P-TTL. there's no data transfer. you're simply doing an auto TTL.

P-TTL
A pre-flash is discharged before main flash so that multi-segment metering sensor can measure the subject's distance, brightness, brightness difference, backlit condition, etc. The data obtained is incorporated to set the output of the main flash. This mode obtains more accurate results than conventional TTL mode.

But there is data transfer from the lens to the camera. The camera has received what it thinks is proper data from the lens: ie. A type lens, Max & Min f-numbers, and a low logic state from the Data pin (ie. no MTF info & I don't know what else.)

So far as the camera knows the lens is a KA type lens, set to the A position. The camera's user manual says it works...


Dave

When the shutter button is pressed the preflash is fired and the camera measures the light intensity with its matrix metering system, not light reflection from the sensor.

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Last edited by newarts; 08-18-2009 at 08:52 AM.

[attack11]

But there is data transfer from the lens to the camera. The camera has received what it thinks is proper data from the lens: ie. A type lens, Max & Min f-numbers, and a low logic state from the Data pin (ie. no MTF info & I don't know what else.)

When the shutter button is pressed the preflash is fired and the camera measures the light intensity with its matrix metering system, not light reflection from the sensor.

there is no data transfer. a short will be a 0 or 1, but it's not accurate and it's not going to take any info from the lens into account.

ttl with a pre-flash is still just ttl.

think of it this way.. p-ttl lets a user shoot in av, tv or any other priority. your method of shorting the body doesn't let it work as true p-ttl. you might as well use a viv285hv with it's sensor.

[newarts]

there is no data transfer. a short will be a 0 or 1, but it's not accurate and it's not going to take any info from the lens into account.

ttl with a pre-flash is still just ttl.

think of it this way.. p-ttl lets a user shoot in av, tv or any other priority. your method of shorting the body doesn't let it work as true p-ttl. you might as well use a viv285hv with it's sensor.

Your definitions of P-TTL and Pentax' definitions seem to differ.

The accuracy of the shorted data doesn't matter. The camera thinks the lens is a KA. KA lenses have a shorted data pin.

[newarts]

there is no data transfer. a short will be a 0 or 1, but it's not accurate and it's not going to take any info from the lens into account.

ttl with a pre-flash is still just ttl.

think of it this way.. p-ttl lets a user shoot in av, tv or any other priority. your method of shorting the body doesn't let it work as true p-ttl. you might as well use a viv285hv with it's sensor.

Your definitions of P-TTL and Pentax' definitions seem to differ.

The accuracy of the shorted data doesn't matter. The camera thinks the lens is a KA. KA lenses have a shorted data pin.

Does the camera know if an AF540FGZ or AF360FGZ is actually being used - ie. do these external flashes communicate anything to the camera?

[attack11]

Does the camera know if an AF540FGZ or AF360FGZ is actually being used - ie. do these external flashes communicate anything to the camera?

i'm not sure if the flash sends data back. the camera sends generally the same data for a ttl setup. however, if ya read the manual you'd know this timbit..

The correct flash output is obtained in P-TTL auto mode only when the flash unit is used with autofocus lenses. - Pg. 28, same manual.

p-ttl requires data from the lens for distance calcs. have you even rtfm?

[attack11]

Your definitions of P-TTL and Pentax' definitions seem to differ.

The accuracy of the shorted data doesn't matter. The camera thinks the lens is a KA. KA lenses have a shorted data pin.

my definitions come from the flash manual, download it from the pentax website if you want to understand how p-ttl works.

[richard64]

Easiest way is to use the camera in 2 second mirror lock up mode. The pre-flash should happen with the mirror lock up, and the main flash should fire when the shutter opens two seconds latter.

I don't see the pre-flash. However,

Then set the viewfinder's f-stop indicator to the lowest setting it'll go to using the ewheel. The camera than thinks you are shooting with the aperture wide open (regardless of what it is actually preset to using the lens' aperture ring.)

I see F-- and so I cannot change the f-stop in the viewfinder. So clearly I'm not doing something right. I have a small metal shim (the thin metal you get in the anti-theft tags) wedged under the lens by the contacts, so it is shorting all of them. I'll have to try it some more times.

Richard

[Russell-Evans]

I don't see the pre-flash. However,

I see F-- and so I cannot change the f-stop in the viewfinder. So clearly I'm not doing something right. I have a small metal shim (the thin metal you get in the anti-theft tags) wedged under the lens by the contacts, so it is shorting all of them. I'll have to try it some more times.

The A contact is recessed, so a flat metal isn't going to short it.

Thank you
Russell

[Big I]

Very complicated, thorough answer. Thanks, everyone!

To Russell-Evans: Let's say my camera is in manual mode at 1/160, f/4. I have a FA 50/1.4 on there. When the flash is in P-TTL mode, will it adjust itself accordingly, so that the exposure is correct?

[Russell-Evans]

Let's say my camera is in manual mode at 1/160, f/4. I have a FA 50/1.4 on there. When the flash is in P-TTL mode, will it adjust itself accordingly, so that the exposure is correct?

Yes to the same degree that metering needs to be tweaked for different subject matter in the same way as it would be for exposure without flash. There is also a limitation to the range of P-TTL metering both at the short and long end. You can find the charts in the manual.

This ignores the debate of whether P-TTL can deliver correct exposure, and do it consistently. Some people, with a lot more experience than me, have opinions that P-TTL isn't able to perform adequately. I'm just commenting on the technical "is this the way it should work", not does it work according to how other people think/know it should. As you say, it is complicated.

Thank you
Russell

[attack11]

Yes to the same degree that metering needs to be tweaked for different subject matter in the same way as it would be for exposure without flash. There is also a limitation to the range of P-TTL metering both at the short and long end. You can find the charts in the manual.

This ignores the debate of whether P-TTL can deliver correct exposure, and do it consistently. Some people, with a lot more experience than me, have opinions that P-TTL isn't able to perform adequately. I'm just commenting on the technical "is this the way it should work", not does it work according to how other people think/know it should. As you say, it is complicated.

Thank you
Russell

for that scenario you'd want to set your camera to Av with your iso set to whatever will give you adequate light based on the flash coverage. p-ttl will only work if the flash is at 0d. it's auto-fill based on distance of the camera to the subject with the scene metered accordingly. putting the body into manual kinda defeats the whole point of p-ttl.

it's very consistent when used properly; aka no bounce.

[attack11]

i think almost everyone in this thread should read this:

Digital SLR and P-TTL flash guide

[richard64]

The A contact is recessed, so a flat metal isn't going to short it.

OK, so what is the best way to short it?