Originally posted by Lowell Goudge aperture is absolutely important because you state you need a "false aperture" [...] it can ignore distance, but not aperture, it needs one to know how to scale the power for the full exposure.
Well, I should have written "a false aperture is needed to fool the camera and enable P-TTL with m42".
But P-TTL could perfectly work with preset lenses (m42) without any aperture information, if only Pentax would make the tiny firmware fix needed for this.
And, moreover, P-TTL could be made fully functional with PK-m lenses, with the simple algorithm change here : do the metering flash once the lens is stopped-down, not before. So simple it's a wonder they didn't made it right from the *istD days.
In fact, PK-m lenses could be fully supported (meaning Av and P-TTL) by the following behavior (could be added to the "Permit A ring" custom function):
- if a non-A manual lens is mounted (either m42 or PK-m, doesn't matter), then stop it down in permanence (so, the PK-m would behave just like a m42, enabling Av mode).
- DoF preview could open the lens for ease of focusing.
- P-TTL can now be enabled, and the metering flash will always occur while the lens is stopped down, so the reading will be accurate.
This is just a matter of moving the aperture actuator in the right place, so this should not drain the battery (except if a constant force is needed for this, but I don't think so).
In this case, the camera does not need to know the actual aperture used and the lens' aperture range, as both metering and actual recording will occur at the same physical aperture. That's what we do when tricking the camera with m42 lenses by selecting the greatest aperture available.
Another solution would be to stop down the lens on shutter half-press only, at metering time, but if the aperture is slow to close (oil on blade, etc), this could lead to exposure problems.
OT : in the same time, it would be great if Pentax added the following tweaks (not P-TTL related):
- Enable trap focus with PK-m and m42 manual lenses when the body is set to AF (currently need to shorten the mount contact pins).
- Enable trap focus with all lenses when the body is set to MF and the user presses the AF button.