[gr8wings]

Hi guys, I've got a K20D.
I'd like to share some information about 'manual focus lens' (Pentax-M) and the use of built-in flash, ansd also ASK some questions.

First of all, according to manual, with 'manual focus lens' the built-in flash will always shot at maximum power, so that AV calibration is not possible (even if you can set it in the menu by pressing Fn button).
Of course, the maximum shutter speed with flash is 1/180s

Question 1: flash compulsory mode.
what happened to me is that I wanted to use flash in a daily shot but it didn't shot ! I was using pentax-M 50mm lens.
I wanted to take a close up picture without a tripod, even if there was quite good ambient light, I had to use F1,7--F4 aperture and 1/10--1/30 time that means blurred picture and small depth of filed.
So I tried to select F16 aperture and 1/180s exposure time, pulled on the built-in flash and selected flash-on in the menu, but the lamp didn't occur and the shot was very dark.
I don't understand why the flash didn't shot !

Question 2: I read that with a manual focus lens the built-in flash (and also other Pentax flashes) only work as TTL flashes and not P-TTL flashes.
This is reasonable because a manula focus lens didn't send the camera any information about aperture value (and also focus distance), so the calculations needed for an advanced P-TTL algorithm is not possible.
The question is: as TTL auto flash the built-in flash should be capable to regulate almost the duration of the lamp (not the power, assuming that it will always shot at maximum power). Is it true or false ?

Best regards.

[rockmaster1964]

Question 2 - It is not really doing TTL. It is just not doing the preflash of P-TTL. It just fires the flash once at full power.

[Lowell Goudge]

the flash will or should fire at full power if the flash is used.

you need to calculate the F Stop you set at manually for the GN of the flash.

I believe the K20 has a GN of something between 13 and 16 meters at ISO100.

There is a standard calculation to work out the exposure as a function of distance, here is one link, Flash Exposure Control I'm sure there are others

Note also there is no TTL flash metering on DSLRs except for the *istD and DS which have a dedicated flash sensor reading reflected light off the focal plane.

The best option for newer DSLRs is to get a flash like the AF540FGZ which have a built in light sensor and you can use the "Auto" mode with this to control flash,

[flyer]

f number = guide number divided by the distance. You round up to the closest f stop. The K10D has a guide number of 11 (metric) while the K20D has a guide number of 13 (metric). Multiply the guide number by 3.3 to convert it to american measures.

[gr8wings]

Hi,
about Q2:
it's cleared that there is no TTL flash metering on DSLRs simply because the flash sensor is missing by engineers, because DSLRs rely on P-TTL technology, starting from the built-in flash unit.
That's why with lens oldest than A series the built-in flash will only shot once at full power.

I'd like to ask if someone could test K20D with a pentax-M lens and built-in flash in daylight conditions and report here, because I've found that with my M 50mm, flash-on setting, built-in flash unit extended, time 1/180 or lower, flash didn't fire !
I don't understand why, I guess this is when it's estimated by some meter that daylight exposure didn't require a lamp. ???
This sounds strange because flsh is supposed to shot in any case at max power and user must find the correct exposition using aperture and time settings.

Thanks for your precious attention.

[gr8wings]

up !
Still waiting for a feedback test: pentax-M and built-in flash in manual mode.
Thanks.

[Just1MoreDave]

Hi,
about Q2:
it's cleared that there is no TTL flash metering on DSLRs simply because the flash sensor is missing by engineers, because DSLRs rely on P-TTL technology, starting from the built-in flash unit.
That's why with lens oldest than A series the built-in flash will only shot once at full power.

I'd like to ask if someone could test K20D with a pentax-M lens and built-in flash in daylight conditions and report here, because I've found that with my M 50mm, flash-on setting, built-in flash unit extended, time 1/180 or lower, flash didn't fire !
I don't understand why, I guess this is when it's estimated by some meter that daylight exposure didn't require a lamp. ???
This sounds strange because flsh is supposed to shot in any case at max power and user must find the correct exposition using aperture and time settings.

Thanks for your precious attention.

I'd also like someone to do the same test with a K20D. I did it with my *ist DS and a Pentax-M 50mm f1.7, and it worked like the manual says. When the flash is up, shutter speed can only be set at 1/180 or slower. I used M mode so I could vary the aperture setting. The flash fired at full power. Changing the aperture changed the exposure. It worked in dim light (~Ev6) and bright (~Ev14).

The DS does have TTL sensors but they don't do anything with an M lens. I can't think of anything else that would affect the flash firing. Maybe you have a camera setting that keeps the flash from firing? I don't see a setting that would do that. though.

[gr8wings]

Well, I made additional tests using pentax-A 50mm (I've bought it just 2 days ago) and pentax-M 50mm.
Now it seems to me that the problem does not exist.

With the A lens I got in the flash menu 4 options (flash-on, flash-on red eyes, flash slow sinc second curtain, wireless). In flash-on position and the built-in flash extended, it always shot (at max power I presume).

With M lens, in the flash menu you have only 2 options (flash-on, flash-on red eyes). In flash-on position and the built-in flash extended, it always shot (at max power I presume).

I also made some study on P-TTL and TTL pentax flash system (built-in flash only) that I'd like to share with you:
1) the built-in flash is P-TTL (the last camera using TTL auto flash was *ist DS). That means the 'environment light' sensor is used, with a pre-lamp method, and also several algorhytms are used to determine the best exposure and lamp power and duration. These algorythms use several data (aperture, focusing distance, etc).
2) a TTL flash mode is not possible on DSLRs because the suitable 'flash sensor' it's not yet present. No pre-lamp is used.
3) because of P-TTL requirements, with a M lens the P-TTL won't work and flash will always shot at maximum power.
4) because of P-TTL requirements, also with a A lens the P-TTL won't work and flash will always shot at maximum power. I think P-TTL could also work (with some limitations) with A lens (even if AF informations are missing), but maybe this is a limitation decided by Pentax engineers. According to the manual, with A lens (A position used), P-TTL functionality is available when the built-in flash is used together with external AF---FG- pentax flash units.

For more infos see:
P-TTL Vs TTL

I'd like to thank you all, I hope this 3d has been useful.

[mattdm]

There's some good information there, but overall it's not necessarily as dire as RiceHigh's guide makes it sound. I recommend *isteve's slides on Pentax flash modes:

http://www.pbase.com/steve_jacob/flashmodes