P-TTL and M42 Lenses

Hello there,

Can I use P-TTL when shooting with M42 lenses? How about High Speed Sync?

Cheers

Hello, no, P-TTL is only compatible with autofocus lenses. Your best bet for automatic flash exposure with Manual Focus lenses is 'auto-thyristor' mode, where the flash measures the returned light itself. The older 'MK1' AF-540FGZ flash has this mode ('A'), but you should find some older third party models that will be OK on Pentax DSLRs .... Vivitar is a well used type over the years. But you should specifically check the voltage is OK for your camera whenever using old film era flashes.

HSS is normally associated with P-TTL mode, so its a no no again with standard Pentax Dedicated System flashes and old M42 lenses ...... however, in theory, using a radio trigger system that offers Manual Mode HSS (such as the Cactus V6II) should work OK with manual lenses. However I've not seen any specific reports from actual use (most High Speed Syncers have got all the most modern gear!), so it is worth asking more widely about this..... the Cactus Community Forum would be a good place to try.....

link

Any idea why?
AF has nothing to do with the calculations needed for TTL. If the pre-flash is done with the aperture fully open then it wouldn't be accurate for lenses before the A series, but I can't think of any reason it shouldn't be available on any lenses with the A setting.
If the pre-flash is done with the aperture closed it should work for ANY lens.

The AF lenses have extra contacts which may influence exposure.

If
If you wish to experiment some suggestions here:
https://www.pentaxforums.com/forums/58-troubleshooting-beginner-help/83348-p-ttl...

Mike, not everything is known about P-TTL exposure metering as there's no publically available technical documents .... but for a practical operational viewpoint there's some information in the Pentax Flash Manuals ..... eg, in the AF-540FGZII Manual :

1) (Page 24) ..."P-TTL Auto Flash (P-TTL)
A pre-flash is discharged before the main flash so that the 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 with the conventional TTL mode".

2) (Page 25) .... "The correct flash output is obtained in P-TTL mode only when the flash unit is used with autofocus lenses"

This was looked at in detail at PentaxForums when I was working on the 2nd Edition of my 'Pentax Flash Guide', and the consensus is that the active autofocus points plus the focal length information is used to determine the subject distance component .... this is added to the known aperture and ISO values to produce the "more accurate results" claimed by Pentax, over the earlier TTL mode.

Check out the following 'Development Thread' at PentaxForums ... link

Whether there are any exact mount contacts used specifically for flash related information exchange is not known .... the difficulty is that the KAF2 mount and introduction of the FA and FAJ series lenses did not coincide with the introduction of P-TTL flash mode. In any case, the focus point and focal length info is 'known' by the camera already with AF lenses, so the data may just be used internally by the metering system and applied to the flash output calculation.

The main difference with P-TTL and the older TTL and Auto-Thyristor modes is that the main output is pre-determined and simply delivered during the main exposure. On the older modes the output is actually measured during the actual exposure, and quenched when the flash sensor sees enough light (either the flash sensor for Thyristor mode, or the internal camera one 'off the film plane' for TTL).

The extra distance information is needed for the P-TTL metering to determine the required output, and the reflected light returning through the lens from the pre-flash is used to make minor adjustments dependent on the subjects surface reflectivity. As the Pentax Flash Manual states, without this extra distance information, which requires an autofocus lens, then correct flash output cannot be obtained.

What the "official Line" is and what people might like to experiment with are two different things of course. People have been using A series lenses with P-TTL for a long time. Some are happy with the reliability and some are not. For the purpose of guidance I adopted and clarified the "official line" on the compatibility of P-TTL and the different lens types ...... in fact originally I didn't even include F series lenses as compatible, due to my own poor experiences with my F35-70 and P-TTL, and the fact that they were made for the original KAF mount. The jury is still out on that really .....

As a result of the discussions at the time, the PF staff carried out some detailed testing with different lens types, and the results were published in this article ..... link

The author assumed full compatibility with A types and P-TTL, but in my interpretation of the results the reliability and inconsistency rules it out. The F series was proven to be closer in performance to the FA types.