TTL v p-TTL

Can anyone explain what's the difference.

I can find anything from the sticky post.

TTL measures the flash reflection off the film while the shutter is open, and cuts the flash off when it reckons there's been enough exposure. So only one flash is needed.

The P in P-TTL is generally thought to stand for 'Pre'. There's a pre-flash, which bounces off the subject and comes back. From that, the amount of 'actual' flash required is calculated, so that the right amount of light is sent in the main flash. Hence two flashes are needed.

P-TTL is a pain when taking pictures of animals and many people. The pre-flash often makes them blink, and the eyes are half shut for the actual flash.

More details in this article on Wikipedia.

TTL equipped camera bodies have a light sensor inside the mirror box which measures the light reflected from the film or sensor. This measurement is used to turn off the flash when enough light has reached the film or sensor for a correct exposure.

In TTL, the flash starts the process of illuminating the scene, the camera stops it.

P-TTL, or Pre-Flash TTL is subtely different in that the camera instructs the flash to emit some light for the metering system to measure prior to the camera making the exposure. Once measured, the camera then sends intensity/duration data to the flash and the camera then makes the exposure and the flash fires with the given intensity/duration.

Early Pentax DSLR's (*istD & *istDS) had both TTL and P-TTL capabilities, but TTL was soon dropped, never to reappear.

So current bodies ie kx kr k7 and K5 are only p TTL?

Yes, P-TTL only for any Pentax body newer than the *istDS, and maybe the *istDS2. No newer Pentax DSLR's can use TTL flashes unless the flash has manual and/or auto modes. If you fit, say an AF280T to a modern Pentax DSLR and set the flash to TTL mode, it'll fully discharge every time with no control over its output.

The main issue with TTL and DSLR's is that a DSLR sensor is highly reflective causing lighting inaccuracies in certain circumstances. A film surface is more matt than a DSLR sensor and doesn't suffer from the same problem.

This is, of course, why you need to be careful buying flashguns.

The AF200/280/400T will all work perfectly with your DSLR in their non-TTL modes, but the AF220T will not as it only has TTL mode.

Also note that TTL requires a separate sensor for picking up the reflected light off the film.

P-TTL, on the other hand, uses the same light metering that you use for non-flash work. For the K5+K7 that means 77 segment metering (which wasn't possible with TTL).

That said, don't discount "A" mode flash, or "M" mode flash (for studio work) Depending on the environment and situation these modes can actually be easier that P-TTL and TTL

Matt

For 'bounce flash' in normal sized rooms, it can be quite easy setting up your camera and flash in M mode... the light is so dispersed the distance from camera to subject hardly matters, so you don't have to change the settings much at all.